



Internet Engineering Task Force (IETF)                    J. Richer, Ed.
Request for Comments: 9635                           Bespoke Engineering
Category: Standards Track                                     F. Imbault
ISSN: 2070-1721                                                 acert.io
                                                            October 2024


          Grant Negotiation and Authorization Protocol (GNAP)

Abstract

   The Grant Negotiation and Authorization Protocol (GNAP) defines a
   mechanism for delegating authorization to a piece of software and
   conveying the results and artifacts of that delegation to the
   software.  This delegation can include access to a set of APIs as
   well as subject information passed directly to the software.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9635.

Copyright Notice

   Copyright (c) 2024 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   include Revised BSD License text as described in Section 4.e of the
   Trust Legal Provisions and are provided without warranty as described
   in the Revised BSD License.

Table of Contents

   1.  Introduction
     1.1.  Terminology
     1.2.  Roles
     1.3.  Elements
     1.4.  Trust Relationships
     1.5.  Protocol Flow
     1.6.  Sequences
       1.6.1.  Overall Protocol Sequence
       1.6.2.  Redirect-Based Interaction
       1.6.3.  User Code Interaction
       1.6.4.  Asynchronous Authorization
       1.6.5.  Software-Only Authorization
       1.6.6.  Refreshing an Expired Access Token
       1.6.7.  Requesting Subject Information Only
       1.6.8.  Cross-User Authentication
   2.  Requesting Access
     2.1.  Requesting Access to Resources
       2.1.1.  Requesting a Single Access Token
       2.1.2.  Requesting Multiple Access Tokens
     2.2.  Requesting Subject Information
     2.3.  Identifying the Client Instance
       2.3.1.  Identifying the Client Instance by Reference
       2.3.2.  Providing Displayable Client Instance Information
       2.3.3.  Authenticating the Client Instance
     2.4.  Identifying the User
       2.4.1.  Identifying the User by Reference
     2.5.  Interacting with the User
       2.5.1.  Start Mode Definitions
       2.5.2.  Interaction Finish Methods
       2.5.3.  Hints
   3.  Grant Response
     3.1.  Request Continuation
     3.2.  Access Tokens
       3.2.1.  Single Access Token
       3.2.2.  Multiple Access Tokens
     3.3.  Interaction Modes
       3.3.1.  Redirection to an Arbitrary URI
       3.3.2.  Launch of an Application URI
       3.3.3.  Display of a Short User Code
       3.3.4.  Display of a Short User Code and URI
       3.3.5.  Interaction Finish
     3.4.  Returning Subject Information
       3.4.1.  Assertion Formats
     3.5.  Returning a Dynamically Bound Client Instance Identifier
     3.6.  Error Response
   4.  Determining Authorization and Consent
     4.1.  Starting Interaction with the End User
       4.1.1.  Interaction at a Redirected URI
       4.1.2.  Interaction at the Static User Code URI
       4.1.3.  Interaction at a Dynamic User Code URI
       4.1.4.  Interaction through an Application URI
     4.2.  Post-Interaction Completion
       4.2.1.  Completing Interaction with a Browser Redirect to the
               Callback URI
       4.2.2.  Completing Interaction with a Direct HTTP Request
               Callback
       4.2.3.  Calculating the Interaction Hash
   5.  Continuing a Grant Request
     5.1.  Continuing after a Completed Interaction
     5.2.  Continuing during Pending Interaction (Polling)
     5.3.  Modifying an Existing Request
     5.4.  Revoking a Grant Request
   6.  Token Management
     6.1.  Rotating the Access Token Value
       6.1.1.  Binding a New Key to the Rotated Access Token
     6.2.  Revoking the Access Token
   7.  Securing Requests from the Client Instance
     7.1.  Key Formats
       7.1.1.  Key References
       7.1.2.  Key Protection
     7.2.  Presenting Access Tokens
     7.3.  Proving Possession of a Key with a Request
       7.3.1.  HTTP Message Signatures
       7.3.2.  Mutual TLS
       7.3.3.  Detached JWS
       7.3.4.  Attached JWS
   8.  Resource Access Rights
     8.1.  Requesting Resources by Reference
   9.  Discovery
     9.1.  RS-First Method of AS Discovery
     9.2.  Dynamic Grant Endpoint Discovery
   10. IANA Considerations
     10.1.  HTTP Authentication Scheme Registration
     10.2.  Media Type Registration
       10.2.1.  application/gnap-binding-jwsd
       10.2.2.  application/gnap-binding-jws
       10.2.3.  application/gnap-binding-rotation-jwsd
       10.2.4.  application/gnap-binding-rotation-jws
     10.3.  GNAP Grant Request Parameters
       10.3.1.  Registration Template
       10.3.2.  Initial Contents
     10.4.  GNAP Access Token Flags
       10.4.1.  Registration Template
       10.4.2.  Initial Contents
     10.5.  GNAP Subject Information Request Fields
       10.5.1.  Registration Template
       10.5.2.  Initial Contents
     10.6.  GNAP Assertion Formats
       10.6.1.  Registration Template
       10.6.2.  Initial Contents
     10.7.  GNAP Client Instance Fields
       10.7.1.  Registration Template
       10.7.2.  Initial Contents
     10.8.  GNAP Client Instance Display Fields
       10.8.1.  Registration Template
       10.8.2.  Initial Contents
     10.9.  GNAP Interaction Start Modes
       10.9.1.  Registration Template
       10.9.2.  Initial Contents
     10.10. GNAP Interaction Finish Methods
       10.10.1.  Registration Template
       10.10.2.  Initial Contents
     10.11. GNAP Interaction Hints
       10.11.1.  Registration Template
       10.11.2.  Initial Contents
     10.12. GNAP Grant Response Parameters
       10.12.1.  Registration Template
       10.12.2.  Initial Contents
     10.13. GNAP Interaction Mode Responses
       10.13.1.  Registration Template
       10.13.2.  Initial Contents
     10.14. GNAP Subject Information Response Fields
       10.14.1.  Registration Template
       10.14.2.  Initial Contents
     10.15. GNAP Error Codes
       10.15.1.  Registration Template
       10.15.2.  Initial Contents
     10.16. GNAP Key Proofing Methods
       10.16.1.  Registration Template
       10.16.2.  Initial Contents
     10.17. GNAP Key Formats
       10.17.1.  Registration Template
       10.17.2.  Initial Contents
     10.18. GNAP Authorization Server Discovery Fields
       10.18.1.  Registration Template
       10.18.2.  Initial Contents
   11. Security Considerations
     11.1.  TLS Protection in Transit
     11.2.  Signing Requests from the Client Software
     11.3.  MTLS Message Integrity
     11.4.  MTLS Deployment Patterns
     11.5.  Protection of Client Instance Key Material
     11.6.  Protection of Authorization Server
     11.7.  Symmetric and Asymmetric Client Instance Keys
     11.8.  Generation of Access Tokens
     11.9.  Bearer Access Tokens
     11.10. Key-Bound Access Tokens
     11.11. Exposure of End-User Credentials to Client Instance
     11.12. Mixing Up Authorization Servers
     11.13. Processing of Client-Presented User Information
     11.14. Client Instance Pre-registration
     11.15. Client Instance Impersonation
     11.16. Client-Hosted Logo URI
     11.17. Interception of Information in the Browser
     11.18. Callback URI Manipulation
     11.19. Redirection Status Codes
     11.20. Interception of Responses from the AS
     11.21. Key Distribution
     11.22. Key Rotation Policy
     11.23. Interaction Finish Modes and Polling
     11.24. Session Management for Interaction Finish Methods
     11.25. Calculating Interaction Hash
     11.26. Storage of Information during Interaction and Continuation
     11.27. Denial of Service (DoS) through Grant Continuation
     11.28. Exhaustion of Random Value Space
     11.29. Front-Channel URIs
     11.30. Processing Assertions
     11.31. Stolen Token Replay
     11.32. Self-Contained Stateless Access Tokens
     11.33. Network Problems and Token and Grant Management
     11.34. Server-Side Request Forgery (SSRF)
     11.35. Multiple Key Formats
     11.36. Asynchronous Interactions
     11.37. Compromised RS
     11.38. AS-Provided Token Keys
   12. Privacy Considerations
     12.1.  Surveillance
       12.1.1.  Surveillance by the Client
       12.1.2.  Surveillance by the Authorization Server
     12.2.  Stored Data
     12.3.  Intrusion
     12.4.  Correlation
       12.4.1.  Correlation by Clients
       12.4.2.  Correlation by Resource Servers
       12.4.3.  Correlation by Authorization Servers
     12.5.  Disclosure in Shared References
   13. References
     13.1.  Normative References
     13.2.  Informative References
   Appendix A.  Comparison with OAuth 2.0
   Appendix B.  Example Protocol Flows
     B.1.  Redirect-Based User Interaction
     B.2.  Secondary Device Interaction
     B.3.  No User Involvement
     B.4.  Asynchronous Authorization
     B.5.  Applying OAuth 2.0 Scopes and Client IDs
   Appendix C.  Interoperability Profiles
     C.1.  Web-Based Redirection
     C.2.  Secondary Device
   Appendix D.  Guidance for Extensions
   Appendix E.  JSON Structures and Polymorphism
   Acknowledgements
   Authors' Addresses

1.  Introduction

   GNAP allows a piece of software, the client instance, to request
   delegated authorization to resource servers and subject information.
   The delegated access to the resource server can be used by the client
   instance to access resources and APIs on behalf a resource owner, and
   delegated access to subject information can in turn be used by the
   client instance to make authentication decisions.  This delegation is
   facilitated by an authorization server, usually on behalf of a
   resource owner.  The end user operating the software can interact
   with the authorization server to authenticate, provide consent, and
   authorize the request as a resource owner.

   The process by which the delegation happens is known as a grant, and
   GNAP allows for the negotiation of the grant process over time by
   multiple parties acting in distinct roles.

   This specification focuses on the portions of the delegation process
   facing the client instance.  In particular, this specification
   defines interoperable methods for a client instance to request,
   negotiate, and receive access to information facilitated by the
   authorization server.  This specification additionally defines
   methods for the client instance to access protected resources at a
   resource server.  This specification also discusses discovery
   mechanisms that enable the client instance to configure itself
   dynamically.  The means for an authorization server and resource
   server to interoperate are discussed in [GNAP-RS].

   The focus of this protocol is to provide interoperability between the
   different parties acting in each role, not to specify implementation
   details of each.  Where appropriate, GNAP may make recommendations
   about internal implementation details, but these recommendations are
   to ensure the security of the overall deployment rather than to be
   prescriptive in the implementation.

   This protocol solves many of the same use cases as OAuth 2.0
   [RFC6749], OpenID Connect [OIDC], and the family of protocols that
   have grown up around that ecosystem.  However, GNAP is not an
   extension of OAuth 2.0 and is not intended to be directly compatible
   with OAuth 2.0.  GNAP seeks to provide functionality and solve use
   cases that OAuth 2.0 cannot easily or cleanly address.  Appendix A
   further details the protocol rationale compared to OAuth 2.0.  GNAP
   and OAuth 2.0 will likely exist in parallel for many deployments, and
   considerations have been taken to facilitate the mapping and
   transition from existing OAuth 2.0 systems to GNAP.  Some examples of
   these can be found in Appendix B.5.

1.1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   This document contains non-normative examples of partial and complete
   HTTP messages, JSON structures, URIs, query components, keys, and
   other elements.  Whenever possible, the document uses URI as a
   generic term, since it aligns with the recommendations in [RFC3986]
   and better matches the intent that the identifier may be reachable
   through various/generic means (compared to URLs).  Some examples use
   a single trailing backslash (\) to indicate line wrapping for long
   values, as per [RFC8792].  The \ character and leading spaces on
   wrapped lines are not part of the value.

   This document uses the term "mutual TLS" as defined by [RFC8705].
   The shortened form "MTLS" is used to mean the same thing.

   For brevity, the term "signature" on its own is used in this document
   to refer to both digital signatures (which use asymmetric
   cryptography) and keyed Message Authentication Codes (MACs) (which
   use symmetric cryptography).  Similarly, the verb "sign" refers to
   the generation of either a digital signature or a keyed MAC over a
   given signature base.  The qualified term "digital signature" refers
   specifically to the output of an asymmetric cryptographic signing
   operation.

1.2.  Roles

   The parties in GNAP perform actions under different roles.  Roles are
   defined by the actions taken and the expectations leveraged on the
   role by the overall protocol.

   +-------------+            +------------+
   |             |            |            |
   |Authorization|            |  Resource  |
   |   Server    |            |   Server   |
   |             |<--+   +--->|            |
   +-----+-------+   |   |    +------------+
         ║           |   |
         ║        +--+---+---+
         ║        |  Client  |
         ║        | Instance |
         ║        +----+-----+
         ║             ║
    .----+----.        ║      .----------.
   |           |       +=====+            |
   |  Resource |             |    End     |
   |   Owner   | ~ ~ ~ ~ ~ ~ |    User    |
   |           |             |            |
    `---------`               `----------`

   Legend:
   ===== indicates interaction between a human and computer
   ----- indicates interaction between two pieces of software
   ~ ~ ~ indicates a potential equivalence or out-of-band
           communication between roles

                          Figure 1: Roles in GNAP

   Authorization Server (AS):  Server that grants delegated privileges
      to a particular instance of client software in the form of access
      tokens or other information (such as subject information).  The AS
      is uniquely defined by the grant endpoint URI, which is the
      absolute URI where grant requests are started by clients.

   Client:  Application that consumes resources from one or several
      resource servers, possibly requiring access privileges from one or
      several ASes.  The client is operated by the end user, or it runs
      autonomously on behalf of a resource owner.

      For example, a client can be a mobile application, a web
      application, a backend data processor, etc.

      Note: This specification differentiates between a specific
      instance (the client instance, identified by its unique key) and
      the software running the instance (the client software).  For some
      kinds of client software, there could be many instances of that
      software, each instance with a different key.

   Resource Server (RS):  Server that provides an API on protected
      resources, where operations on the API require a valid access
      token issued by a trusted AS.

   Resource Owner (RO):  Subject entity that may grant or deny
      operations on resources it has authority upon.

      Note: The act of granting or denying an operation may be manual
      (i.e., through an interaction with a physical person) or automatic
      (i.e., through predefined organizational rules).

   End user:  Natural person that operates a client instance.

      Note: That natural person may or may not be the same entity as the
      RO.

   The design of GNAP does not assume any one deployment architecture
   but instead attempts to define roles that can be fulfilled in a
   number of different ways for different use cases.  As long as a given
   role fulfills all of its obligations and behaviors as defined by the
   protocol, GNAP does not make additional requirements on its structure
   or setup.

   Multiple roles can be fulfilled by the same party, and a given party
   can switch roles in different instances of the protocol.  For
   example, in many instances, the RO and end user are the same person,
   where a user authorizes the client instance to act on their own
   behalf at the RS.  In this case, one party fulfills the roles of both
   RO and end user, but the roles themselves are still defined
   separately from each other to allow for other use cases where they
   are fulfilled by different parties.

   As another example, in some complex scenarios, an RS receiving
   requests from one client instance can act as a client instance for a
   downstream secondary RS in order to fulfill the original request.  In
   this case, one piece of software is both an RS and a client instance
   from different perspectives, and it fulfills these roles separately
   as far as the overall protocol is concerned.

   A single role need not be deployed as a monolithic service.  For
   example, a client instance could have frontend components that are
   installed on the end user's device as well as a backend system that
   the frontend communicates with.  If both of these components
   participate in the delegation protocol, they are both considered part
   of the client instance.  If there are several copies of the client
   software that run separately but all share the same key material,
   such as a deployed cluster, then this cluster is considered a single
   client instance.  In these cases, the distinct components of what is
   considered a GNAP client instance may use any number of different
   communication mechanisms between them, all of which would be
   considered an implementation detail of the client instances and out
   of scope of GNAP.

   As another example, an AS could likewise be built out of many
   constituent components in a distributed architecture.  The component
   that the client instance calls directly could be different from the
   component that the RO interacts with to drive consent, since API
   calls and user interaction have different security considerations in
   many environments.  Furthermore, the AS could need to collect
   identity claims about the RO from one system that deals with user
   attributes while generating access tokens at another system that
   deals with security rights.  From the perspective of GNAP, all of
   these are pieces of the AS and together fulfill the role of the AS as
   defined by the protocol.  These pieces may have their own internal
   communications mechanisms, which are considered out of scope of GNAP.

1.3.  Elements

   In addition to the roles above, the protocol also involves several
   elements that are acted upon by the roles throughout the process.

   Access Token:  A data artifact representing a set of rights and/or
      attributes.

      Note: An access token can be first issued to a client instance
      (requiring authorization by the RO) and subsequently rotated.

   Grant:  (verb): To permit an instance of client software to receive
      some attributes at a specific time and with a specific duration of
      validity and/or to exercise some set of delegated rights to access
      a protected resource.

      (noun): The act of granting permission to a client instance.

   Privilege:  Right or attribute associated with a subject.

      Note: The RO defines and maintains the rights and attributes
      associated to the protected resource and might temporarily
      delegate some set of those privileges to an end user.  This
      process is referred to as "privilege delegation".

   Protected Resource:  Protected API that is served by an RS and that
      can be accessed by a client, if and only if a valid and sufficient
      access token is provided.

      Note: To avoid complex sentences, the specification document may
      simply refer to "resource" instead of "protected resource".

   Right:  Ability given to a subject to perform a given operation on a
      resource under the control of an RS.

   Subject:  Person or organization.  The subject decides whether and
      under which conditions its attributes can be disclosed to other
      parties.

   Subject Information:  Set of statements and attributes asserted by an
      AS about a subject.  These statements can be used by the client
      instance as part of an authentication decision.

1.4.  Trust Relationships

   GNAP defines its trust objective as follows: the RO trusts the AS to
   ensure access validation and delegation of protected resources to end
   users, through third party clients.

   This trust objective can be decomposed into trust relationships
   between software elements and roles, especially the pairs end user/
   RO, end user/client, client/AS, RS/RO, AS/RO, and AS/RS.  Trust of an
   agent by its pair can exist if the pair is informed that the agent
   has made a promise to follow the protocol in the past (e.g., pre-
   registration and uncompromised cryptographic components) or if the
   pair is able to infer by indirect means that the agent has made such
   a promise (e.g., a compliant client request).  Each agent defines its
   own valuation function of promises given or received.  Examples of
   such valuations can be the benefits from interacting with other
   agents (e.g., safety in client access and interoperability with
   identity standards), the cost of following the protocol (including
   its security and privacy requirements and recommendations), a ranking
   of promise importance (e.g., a policy decision made by the AS), the
   assessment of one's vulnerability or risk of not being able to defend
   against threats, etc.  Those valuations may depend on the context of
   the request.  For instance, depending on the specific case in which
   GNAP is used, the AS may decide to either take into account or
   discard hints provided by the client, or the RS may refuse bearer
   tokens.  Some promises can be affected by previous interactions
   (e.g., repeated requests).

   Below are details of each trust relationship:

   end user/RO:  This relationship exists only when the end user and the
      RO are different, in which case the end user needs some out-of-
      band mechanism of getting the RO consent (see Section 4).  GNAP
      generally assumes that humans can be authenticated, thanks to
      identity protocols (for instance, through an id_token assertion as
      described in Section 2.2).

   end user/client:  The client acts as a user agent.  Depending on the
      technology used (browser, single-page application (SPA), mobile
      application, Internet of Things (IoT) device, etc.), some
      interactions may or may not be possible (as described in
      Section 2.5.1).  Client developers implement requirements and
      generally some recommendations or best practices, so that the end
      users may confidently use their software.  However, end users
      might also face an attacker's client software or a poorly
      implemented client without even realizing it.

   end user/AS:  When the client supports the interaction feature (see
      Section 3.3), the end user interacts with the AS through an AS-
      provided interface.  In many cases, this happens through a front-
      channel interaction through the end user's browser.  See
      Section 11.29 for some considerations in trusting these
      interactions.

   client/AS:  An honest AS may face an attacker's client (as discussed
      just above), or the reverse, and GNAP aims to make common attacks
      impractical.  This specification makes access tokens opaque to the
      client and defines the request/response scheme in detail,
      therefore avoiding extra trust hypotheses from this critical piece
      of software.  Yet, the AS may further define cryptographic
      attestations or optional rules to simplify the access of clients
      it already trusts, due to past behavior or organizational policies
      (see Section 2.3).

   RS/RO:  On behalf of the RO, the RS promises to protect its resources
      from unauthorized access and only accepts valid access tokens
      issued by a trusted AS.  In case tokens are key bound, proper
      validation of the proofing method is expected from the RS.

   AS/RO:  The AS is expected to follow the decisions made by the RO,
      through either interactive consent requests, repeated
      interactions, or automated rules (as described in Section 1.6).
      Privacy considerations aim to reduce the risk of an honest but
      too-curious AS or the consequences of an unexpected user data
      exposure.

   AS/RS:  The AS promises to issue valid access tokens to legitimate
      client requests (i.e., after carrying out appropriate due
      diligence, as defined in the GNAP).  Some optional configurations
      are covered by [GNAP-RS].

   A global assumption made by GNAP is that authorization requests are
   security and privacy sensitive, and appropriate measures are detailed
   in Sections 11 and 12, respectively.

   A formal trust model is out of scope of this specification, but one
   could be developed using techniques such as the Promise Theory
   [promise-theory].

1.5.  Protocol Flow

   GNAP is fundamentally designed to allow delegated access to APIs and
   other information, such as subject information, using a multi-stage,
   stateful process.  This process allows different parties to provide
   information into the system to alter and augment the state of the
   delegated access and its artifacts.

   The underlying requested grant moves through several states as
   different actions take place during the protocol, as shown in
   Figure 2.

                                                       .-----.
                                                      |       |
                                               +------+--+    | Continue
                      .---Need Interaction---->|         |    |
                     /                         | Pending |<--`
                    /   .--Finish Interaction--+         |
                   /   /     (approve/deny)    +----+----+
                  /   /                             |
                 /   /                              | Cancel
                /   v                               v
             +-+----------+                   +===========+
             |            |                   ║           ║
---Request-->| Processing +------Finalize---->║ Finalized ║
             |            |                   ║           ║
             +-+----------+                   +===========+
                \    ^                              ^
                 \    \                             | Revoke or
                  \    \                            | Finalize
                   \    \                     +-----+----+
                    \    `-----Update---------+          |
                     \                        | Approved |<--.
                      `-----No Interaction--->|          |    |
                                              +-------+--+    | Continue
                                                      |       |
                                                       `-----`

          Figure 2: State Diagram of a Grant Request in GNAP

   The state of the grant request is defined and managed by the AS,
   though the client instance also needs to manage its view of the grant
   request over time.  The means by which these roles manage their state
   are outside the scope of this specification.

   _Processing_:  When a request for access (Section 2) is received by
      the AS, a new grant request is created and placed in the
      _processing_ state by the AS.  This state is also entered when an
      existing grant request is updated by the client instance and when
      interaction is completed.  In this state, the AS processes the
      context of the grant request to determine whether interaction with
      the end user or RO is required for approval of the request.  The
      grant request has to exit this state before a response can be
      returned to the client instance.  If approval is required, the
      request moves to the _pending_ state, and the AS returns a
      continuation response (Section 3.1) along with any appropriate
      interaction responses (Section 3.3).  If no such approval is
      required, such as when the client instance is acting on its own
      behalf or the AS can determine that access has been fulfilled, the
      request moves to the _approved_ state where access tokens for API
      access (Section 3.2) and subject information (Section 3.4) can be
      issued to the client instance.  If the AS determines that no
      additional processing can occur (such as a timeout or an
      unrecoverable error), the grant request is moved to the
      _finalized_ state and is terminated.

   _Pending_:  When a request needs to be approved by an RO, or
      interaction with the end user is required, the grant request
      enters a state of _pending_. In this state, no access tokens can
      be granted, and no subject information can be released to the
      client instance.  While a grant request is in this state, the AS
      seeks to gather the required consent and authorization (Section 4)
      for the requested access.  A grant request in this state is always
      associated with a continuation access token bound to the client
      instance's key (see Section 3.1 for details of the continuation
      access token).  If no interaction finish method (Section 2.5.2) is
      associated with this request, the client instance can send a
      polling continuation request (Section 5.2) to the AS.  This
      returns a continuation response (Section 3.1) while the grant
      request remains in this state, allowing the client instance to
      continue to check the state of the pending grant request.  If an
      interaction finish method (Section 2.5.2) is specified in the
      grant request, the client instance can continue the request after
      interaction (Section 5.1) to the AS to move this request to the
      _processing_ state to be re-evaluated by the AS.  Note that this
      occurs whether the grant request has been approved or denied by
      the RO, since the AS needs to take into account the full context
      of the request before determining the next step for the grant
      request.  When other information is made available in the context
      of the grant request, such as through the asynchronous actions of
      the RO, the AS moves this request to the _processing_ state to be
      re-evaluated.  If the AS determines that no additional interaction
      can occur, e.g., all the interaction methods have timed out or a
      revocation request (Section 5.4) is received from the client
      instance, the grant request can be moved to the _finalized_ state.

   _Approved_:  When a request has been approved by an RO and no further
      interaction with the end user is required, the grant request
      enters a state of _approved_. In this state, responses to the
      client instance can include access tokens for API access
      (Section 3.2) and subject information (Section 3.4).  If
      continuation and updates are allowed for this grant request, the
      AS can include the continuation response (Section 3.1).  In this
      state, post-interaction continuation requests (Section 5.1) are
      not allowed and will result in an error, since all interaction is
      assumed to have been completed.  If the client instance sends a
      polling continuation request (Section 5.2) while the request is in
      this state, new access tokens (Section 3.2) can be issued in the
      response.  Note that this always creates a new access token, but
      any existing access tokens could be rotated and revoked using the
      token management API (Section 6).  The client instance can send an
      update continuation request (Section 5.3) to modify the requested
      access, causing the AS to move the request back to the
      _processing_ state for re-evaluation.  If the AS determines that
      no additional tokens can be issued and that no additional updates
      are to be accepted (e.g., the continuation access tokens have
      expired), the grant is moved to the _finalized_ state.

   _Finalized_:  After the access tokens are issued, if the AS does not
      allow any additional updates on the grant request, the grant
      request enters the _finalized_ state.  This state is also entered
      when an existing grant request is revoked by the client instance
      (Section 5.4) or otherwise revoked by the AS (such as through out-
      of-band action by the RO).  This state can also be entered if the
      AS determines that no additional processing is possible, for
      example, if the RO has denied the requested access or if
      interaction is required but no compatible interaction methods are
      available.  Once in this state, no new access tokens can be
      issued, no subject information can be returned, and no
      interactions can take place.  Once in this state, the grant
      request is dead and cannot be revived.  If future access is
      desired by the client instance, a new grant request can be
      created, unrelated to this grant request.

   While it is possible to deploy an AS in a stateless environment, GNAP
   is a stateful protocol, and such deployments will need a way to
   manage the current state of the grant request in a secure and
   deterministic fashion without relying on other components, such as
   the client software, to keep track of the current state.

1.6.  Sequences

   GNAP can be used in a variety of ways to allow the core delegation
   process to take place.  Many portions of this process are
   conditionally present depending on the context of the deployments,
   and not every step in this overview will happen in all circumstances.

   Note that a connection between roles in this process does not
   necessarily indicate that a specific protocol message is sent across
   the wire between the components fulfilling the roles in question or
   that a particular step is required every time.  For example, for a
   client instance interested in only getting subject information
   directly and not calling an RS, all steps involving the RS below do
   not apply.

   In some circumstances, the information needed at a given stage is
   communicated out of band or is pre-configured between the components
   or entities performing the roles.  For example, one entity can
   fulfill multiple roles, so explicit communication between the roles
   is not necessary within the protocol flow.  Additionally, some
   components may not be involved in all use cases.  For example, a
   client instance could be calling the AS just to get direct user
   information and have no need to get an access token to call an RS.

1.6.1.  Overall Protocol Sequence

   The following diagram provides a general overview of GNAP, including
   many different optional phases and connections.  The diagrams in the
   following sections provide views of GNAP under more specific
   circumstances.  These additional diagrams use the same conventions as
   the overall diagram below.

    .----------.           .----------.
   |  End user  | ~ ~ ~ ~ |  Resource  |
   |            |         | Owner (RO) |
    `----+-----`           `-----+----`
         ║                       ║
         ║                       ║
        (A)                     (B)
         ║                       ║
         ║                       ║
   +-----+--+                    ║           +------------+
   | Client | (1)                ║           |  Resource  |
   |Instance|                    ║           |   Server   |
   |        |        +-----------+---+       |    (RS)    |
   |        +--(2)-->| Authorization |       |            |
   |        |<-(3)---+     Server    |       |            |
   |        |        |      (AS)     |       |            |
   |        +--(4)-->|               |       |            |
   |        |<-(5)---+               |       |            |
   |        |        |               |       |            |
   |        +---------------(6)------------->|            |
   |        |        |               |   (7) |            |
   |        |<--------------(8)------------->|            |
   |        |        |               |       |            |
   |        +--(9)-->|               |       |            |
   |        |<-(10)--+               |       |            |
   |        |        |               |       |            |
   |        +---------------(11)------------>|            |
   |        |        |               |  (12) |            |
   |        +--(13)->|               |       |            |
   |        |        |               |       |            |
   +--------+        +---------------+       +------------+

   Legend:
   ===== indicates a possible interaction with a human
   ----- indicates an interaction between protocol roles
   ~ ~ ~ indicates a potential equivalence or out-of-band
           communication between roles

                     Figure 3: Overall Sequence of GNAP

   *  (A) The end user interacts with the client instance to indicate a
      need for resources on behalf of the RO.  This could identify the
      RS that the client instance needs to call, the resources needed,
      or the RO that is needed to approve the request.  Note that the RO
      and end user are often the same entity in practice, but GNAP makes
      no general assumption that they are.

   *  (1) The client instance determines what access is needed and which
      AS to approach for access.  Note that for most situations, the
      client instance is pre-configured with which AS to talk to and
      which kinds of access it needs, but some more dynamic processes
      are discussed in Section 9.1.

   *  (2) The client instance requests access at the AS (Section 2).

   *  (3) The AS processes the request and determines what is needed to
      fulfill the request (see Section 4).  The AS sends its response to
      the client instance (Section 3).

   *  (B) If interaction is required, the AS interacts with the RO
      (Section 4) to gather authorization.  The interactive component of
      the AS can function using a variety of possible mechanisms,
      including web page redirects, applications, challenge/response
      protocols, or other methods.  The RO approves the request for the
      client instance being operated by the end user.  Note that the RO
      and end user are often the same entity in practice, and many of
      GNAP's interaction methods allow the client instance to facilitate
      the end user interacting with the AS in order to fulfill the role
      of the RO.

   *  (4) The client instance continues the grant at the AS (Section 5).
      This action could occur in response to receiving a signal that
      interaction has finished (Section 4.2) or through a periodic
      polling mechanism, depending on the interaction capabilities of
      the client software and the options active in the grant request.

   *  (5) If the AS determines that access can be granted, it returns a
      response to the client instance (Section 3), including an access
      token (Section 3.2) for calling the RS and any directly returned
      information (Section 3.4) about the RO.

   *  (6) The client instance uses the access token (Section 7.2) to
      call the RS.

   *  (7) The RS determines if the token is sufficient for the request
      by examining the token.  The means of the RS determining this
      access are out of scope of this specification, but some options
      are discussed in [GNAP-RS].

   *  (8) The client instance calls the RS (Section 7.2) using the
      access token until the RS or client instance determines that the
      token is no longer valid.

   *  (9) When the token no longer works, the client instance rotates
      the access token (Section 6.1).

   *  (10) The AS issues a new access token (Section 3.2) to the client
      instance with the same rights as the original access token
      returned in (5).

   *  (11) The client instance uses the new access token (Section 7.2)
      to call the RS.

   *  (12) The RS determines if the new token is sufficient for the
      request, as in (7).

   *  (13) The client instance disposes of the token (Section 6.2) once
      the client instance has completed its access of the RS and no
      longer needs the token.

   The following sections and Appendix B contain specific guidance on
   how to use GNAP in different situations and deployments.  For
   example, it is possible for the client instance to never request an
   access token and never call an RS, just as it is possible to have no
   end user involved in the delegation process.

1.6.2.  Redirect-Based Interaction

   In this example flow, the client instance is a web application that
   wants access to resources on behalf of the current user, who acts as
   both the end user and the RO.  Since the client instance is capable
   of directing the user to an arbitrary URI and receiving responses
   from the user's browser, interaction here is handled through front-
   channel redirects using the user's browser.  The redirection URI used
   for interaction is a service hosted by the AS in this example.  The
   client instance uses a persistent session with the user to ensure the
   same user that is starting the interaction is the user that returns
   from the interaction.

 +--------+                                  +--------+          .----.
 | Client |                                  |   AS   |         | End  |
 |Instance|                                  |        |         | User |
 |        |<=(1)== Start Session ===============================+      |
 |        |                                  |        |         |      |
 |        +--(2)--- Request Access --------->|        |         |      |
 |        |                                  |        |         |      |
 |        |<-(3)-- Interaction Needed -------+        |         |      |
 |        |                                  |        |         |      |
 |        +==(4)== Redirect for Interaction ===================>|      |
 |        |                                  |        |         +------+
 |        |                                  |        |<==(5)==>|      |
 |        |                                  |        |  AuthN  |  RO  |
 |        |                                  |        |         |      |
 |        |                                  |        |<==(6)==>|      |
 |        |                                  |        |  AuthZ  +------+
 |        |                                  |        |         | End  |
 |        |<=(7)== Redirect for Continuation ===================+ User |
 |        |                                  |        |          `----`
 |        +--(8)--- Continue Request ------->|        |
 |        |                                  |        |
 |        |<-(9)----- Grant Access ----------+        |
 |        |                                  |        |
 |        |                                  |        |     +--------+
 |        +--(10)-- Access API ---------------------------->|   RS   |
 |        |                                  |        |     |        |
 |        |<-(11)-- API Response ---------------------------|        |
 |        |                                  |        |     +--------+
 +--------+                                  +--------+

           Figure 4: Diagram of a Redirect-Based Interaction

   *  (1) The client instance establishes a session with the user, in
      the role of the end user.

   *  (2) The client instance requests access to the resource
      (Section 2).  The client instance indicates that it can redirect
      to an arbitrary URI (Section 2.5.1.1) and receive a redirect from
      the browser (Section 2.5.2.1).  The client instance stores
      verification information for its redirect in the session created
      in (1).

   *  (3) The AS determines that interaction is needed and responds
      (Section 3) with a URI to send the user to (Section 3.3.1) and
      information needed to verify the redirect (Section 3.3.5) in (7).
      The AS also includes information the client instance will need to
      continue the request (Section 3.1) in (8).  The AS associates this
      continuation information with an ongoing request that will be
      referenced in (4), (6), and (8).

   *  (4) The client instance stores the verification and continuation
      information from (3) in the session from (1).  The client instance
      then redirects the user to the URI (Section 4.1.1) given by the AS
      in (3).  The user's browser loads the interaction redirect URI.
      The AS loads the pending request based on the incoming URI
      generated in (3).

   *  (5) The user authenticates at the AS, taking on the role of the
      RO.

   *  (6) As the RO, the user authorizes the pending request from the
      client instance.

   *  (7) When the AS is done interacting with the user, the AS
      redirects the user back (Section 4.2.1) to the client instance
      using the redirect URI provided in (2).  The redirect URI is
      augmented with an interaction reference that the AS associates
      with the ongoing request created in (2) and referenced in (4).
      The redirect URI is also augmented with a hash of the security
      information provided in (2) and (3).  The client instance loads
      the verification information from (2) and (3) from the session
      created in (1).  The client instance calculates a hash
      (Section 4.2.3) based on this information and continues only if
      the hash validates.  Note that the client instance needs to ensure
      that the parameters for the incoming request match those that it
      is expecting from the session created in (1).  The client instance
      also needs to be prepared for the end user never being returned to
      the client instance and handle timeouts appropriately.

   *  (8) The client instance loads the continuation information from
      (3) and sends the interaction reference from (7) in a request to
      continue the request (Section 5.1).  The AS validates the
      interaction reference, ensuring that the reference is associated
      with the request being continued.

   *  (9) If the request has been authorized, the AS grants access to
      the information in the form of access tokens (Section 3.2) and
      direct subject information (Section 3.4) to the client instance.

   *  (10) The client instance uses the access token (Section 7.2) to
      call the RS.

   *  (11) The RS validates the access token and returns an appropriate
      response for the API.

   An example set of protocol messages for this method can be found in
   Appendix B.1.

1.6.3.  User Code Interaction

   In this example flow, the client instance is a device that is capable
   of presenting a short, human-readable code to the user and directing
   the user to enter that code at a known URI.  The user enters the code
   at a URI that is an interactive service hosted by the AS in this
   example.  The client instance is not capable of presenting an
   arbitrary URI to the user, nor is it capable of accepting incoming
   HTTP requests from the user's browser.  The client instance polls the
   AS while it is waiting for the RO to authorize the request.  The
   user's interaction is assumed to occur on a secondary device.  In
   this example, it is assumed that the user is both the end user and
   RO.  Note that since the user is not assumed to be interacting with
   the client instance through the same web browser used for interaction
   at the AS, the user is not shown as being connected to the client
   instance in this diagram.

 +--------+                                  +--------+          .----.
 | Client |                                  |   AS   |         | End  |
 |Instance+--(1)--- Request Access --------->|        |         | User |
 |        |                                  |        |         |      |
 |        |<-(2)-- Interaction Needed -------+        |         |      |
 |        |                                  |        |         |      |
 |        +==(3)==== Display User Code ========================>|      |
 |        |                                  |        |         |      |
 |        |                                  |        |<==(4)===+      |
 |        |                                  |        |Open URI |      |
 |        |                                  |        |         +------+
 |        |                                  |        |<==(5)==>|  RO  |
 |        |                                  |        |  AuthN  |      |
 |        +--(9)--- Continue Request (A) --->|        |         |      |
 |        |                                  |        |<==(6)==>|      |
 |        |<-(10)-- Not Yet Granted (Wait) --+        |  Code   |      |
 |        |                                  |        |         |      |
 |        |                                  |        |<==(7)==>|      |
 |        |                                  |        |  AuthZ  |      |
 |        |                                  |        |         |      |
 |        |                                  |        |<==(8)==>|      |
 |        |                                  |        |Complete |      |
 |        |                                  |        |         +------+
 |        +--(11)-- Continue Request (B) --->|        |         | End  |
 |        |                                  |        |         | User |
 |        |<-(12)----- Grant Access ---------+        |          `----`
 |        |                                  |        |
 |        |                                  |        |     +--------+
 |        +--(13)-- Access API ---------------------------->|   RS   |
 |        |                                  |        |     |        |
 |        |<-(14)-- API Response ---------------------------+        |
 |        |                                  |        |     +--------+
 +--------+                                  +--------+

           Figure 5: Diagram of a User-Code-Based Interaction

   *  (1) The client instance requests access to the resource
      (Section 2).  The client instance indicates that it can display a
      user code (Section 2.5.1.3).

   *  (2) The AS determines that interaction is needed and responds
      (Section 3) with a user code to communicate to the user
      (Section 3.3.3).  The AS also includes information the client
      instance will need to continue the request (Section 3.1) in (8)
      and (10).  The AS associates this continuation information with an
      ongoing request that will be referenced in (4), (6), (8), and
      (10).

   *  (3) The client instance stores the continuation information from
      (2) for use in (8) and (10).  The client instance then
      communicates the code to the user (Section 4.1.2) given by the AS
      in (2).

   *  (4) The user directs their browser to the user code URI.  This URI
      is stable and can be communicated via the client software's
      documentation, the AS documentation, or the client software
      itself.  Since it is assumed that the RO will interact with the AS
      through a secondary device, the client instance does not provide a
      mechanism to launch the RO's browser at this URI.

   *  (5) The end user authenticates at the AS, taking on the role of
      the RO.

   *  (6) The RO enters the code communicated in (3) to the AS.  The AS
      validates this code against a current request in process.

   *  (7) As the RO, the user authorizes the pending request from the
      client instance.

   *  (8) When the AS is done interacting with the user, the AS
      indicates to the RO that the request has been completed.

   *  (9) Meanwhile, the client instance loads the continuation
      information stored at (3) and continues the request (Section 5).
      The AS determines which ongoing access request is referenced here
      and checks its state.

   *  (10) If the access request has not yet been authorized by the RO
      in (6), the AS responds to the client instance to continue the
      request (Section 3.1) at a future time through additional polled
      continuation requests.  This response can include updated
      continuation information as well as information regarding how long
      the client instance should wait before calling again.  The client
      instance replaces its stored continuation information from the
      previous response (2).  Note that the AS may need to determine
      that the RO has not approved the request in a sufficient amount of
      time and return an appropriate error to the client instance.

   *  (11) The client instance continues to poll the AS (Section 5.2)
      with the new continuation information in (9).

   *  (12) If the request has been authorized, the AS grants access to
      the information in the form of access tokens (Section 3.2) and
      direct subject information (Section 3.4) to the client instance.

   *  (13) The client instance uses the access token (Section 7.2) to
      call the RS.

   *  (14) The RS validates the access token and returns an appropriate
      response for the API.

   An example set of protocol messages for this method can be found in
   Appendix B.2.

1.6.4.  Asynchronous Authorization

   In this example flow, the end user and RO roles are fulfilled by
   different parties, and the RO does not interact with the client
   instance.  The AS reaches out asynchronously to the RO during the
   request process to gather the RO's authorization for the client
   instance's request.  The client instance polls the AS while it is
   waiting for the RO to authorize the request.

 +--------+                                  +--------+          .----.
 | Client |                                  |   AS   |         |  RO  |
 |Instance+--(1)--- Request Access --------->|        |         |      |
 |        |                                  |        |         |      |
 |        |<-(2)-- Not Yet Granted (Wait) ---+        |         |      |
 |        |                                  |        |<==(3)==>|      |
 |        |                                  |        |  AuthN  |      |
 |        +--(6)--- Continue Request (A) --->|        |         |      |
 |        |                                  |        |<==(4)==>|      |
 |        |<-(7)-- Not Yet Granted (Wait) ---+        |  AuthZ  |      |
 |        |                                  |        |         |      |
 |        |                                  |        |<==(5)==>|      |
 |        |                                  |        |Completed|      |
 |        |                                  |        |         |      |
 |        +--(8)--- Continue Request (B) --->|        |          `----`
 |        |                                  |        |
 |        |<-(9)------ Grant Access ---------+        |
 |        |                                  |        |
 |        |                                  |        |     +--------+
 |        +--(10)-- Access API ---------------------------->|   RS   |
 |        |                                  |        |     |        |
 |        |<-(11)-- API Response ---------------------------+        |
 |        |                                  |        |     +--------+
 +--------+                                  +--------+

    Figure 6: Diagram of an Asynchronous Authorization Process, with
                        No End-User Interaction

   *  (1) The client instance requests access to the resource
      (Section 2).  The client instance does not send any interaction
      modes to the server, indicating that it does not expect to
      interact with the RO.  The client instance can also signal which
      RO it requires authorization from, if known, by using the subject
      request field (Section 2.2) and user request field (Section 2.4).
      It's also possible for the AS to determine which RO needs to be
      contacted by the nature of what access is being requested.

   *  (2) The AS determines that interaction is needed, but the client
      instance cannot interact with the RO.  The AS responds (Section 3)
      with the information the client instance will need to continue the
      request (Section 3.1) in (6) and (8), including a signal that the
      client instance should wait before checking the status of the
      request again.  The AS associates this continuation information
      with an ongoing request that will be referenced in (3), (4), (5),
      (6), and (8).

   *  (3) The AS determines which RO to contact based on the request in
      (1), through a combination of the user request (Section 2.4), the
      subject request (Section 2.2), the access request (Section 2.1),
      and other policy information.  The AS contacts the RO and
      authenticates them.

   *  (4) The RO authorizes the pending request from the client
      instance.

   *  (5) When the AS is done interacting with the RO, the AS indicates
      to the RO that the request has been completed.

   *  (6) Meanwhile, the client instance loads the continuation
      information stored at (2) and continues the request (Section 5).
      The AS determines which ongoing access request is referenced here
      and checks its state.

   *  (7) If the access request has not yet been authorized by the RO in
      (6), the AS responds to the client instance to continue the
      request (Section 3.1) at a future time through additional polling.
      Note that this response is not an error message, since no error
      has yet occurred.  This response can include refreshed credentials
      as well as information regarding how long the client instance
      should wait before calling again.  The client instance replaces
      its stored continuation information from the previous response
      (2).  Note that the AS may need to determine that the RO has not
      approved the request in a sufficient amount of time and return an
      appropriate error to the client instance.

   *  (8) The client instance continues to poll the AS (Section 5.2)
      with the new continuation information from (7).

   *  (9) If the request has been authorized, the AS grants access to
      the information in the form of access tokens (Section 3.2) and
      direct subject information (Section 3.4) to the client instance.

   *  (10) The client instance uses the access token (Section 7.2) to
      call the RS.

   *  (11) The RS validates the access token and returns an appropriate
      response for the API.

   An example set of protocol messages for this method can be found in
   Appendix B.4.

   Additional considerations for asynchronous interactions like this are
   discussed in Section 11.36.

1.6.5.  Software-Only Authorization

   In this example flow, the AS policy allows the client instance to
   make a call on its own behalf, without the need for an RO to be
   involved at runtime to approve the decision.  Since there is no
   explicit RO, the client instance does not interact with an RO.

   +--------+                            +--------+
   | Client |                            |   AS   |
   |Instance+--(1)--- Request Access --->|        |
   |        |                            |        |
   |        |<-(2)---- Grant Access -----+        |
   |        |                            |        |  +--------+
   |        +--(3)--- Access API ------------------->|   RS   |
   |        |                            |        |  |        |
   |        |<-(4)--- API Response ------------------+        |
   |        |                            |        |  +--------+
   +--------+                            +--------+

      Figure 7: Diagram of a Software-Only Authorization, with No End
                      User or Explicit Resource Owner

   *  (1) The client instance requests access to the resource
      (Section 2).  The client instance does not send any interaction
      modes to the server.

   *  (2) The AS determines that the request has been authorized based
      on the identity of the client instance making the request and the
      access requested (Section 2.1).  The AS grants access to the
      resource in the form of access tokens (Section 3.2) to the client
      instance.  Note that direct subject information (Section 3.4) is
      not generally applicable in this use case, as there is no user
      involved.

   *  (3) The client instance uses the access token (Section 7.2) to
      call the RS.

   *  (4) The RS validates the access token and returns an appropriate
      response for the API.

   An example set of protocol messages for this method can be found in
   Appendix B.3.

1.6.6.  Refreshing an Expired Access Token

   In this example flow, the client instance receives an access token to
   access an RS through some valid GNAP process.  The client instance
   uses that token at the RS for some time, but eventually the access
   token expires.  The client instance then gets a refreshed access
   token by rotating the expired access token's value at the AS using
   the token management API.

   +--------+                                          +--------+
   | Client |                                          |   AS   |
   |Instance+--(1)--- Request Access ----------------->|        |
   |        |                                          |        |
   |        |<-(2)--- Grant Access --------------------+        |
   |        |                                          |        |
   |        |                             +--------+   |        |
   |        +--(3)--- Access Resource --->|   RS   |   |        |
   |        |                             |        |   |        |
   |        |<-(4)--- Success Response ---+        |   |        |
   |        |                             |        |   |        |
   |        |       ( Time Passes )       |        |   |        |
   |        |                             |        |   |        |
   |        +--(5)--- Access Resource --->|        |   |        |
   |        |                             |        |   |        |
   |        |<-(6)--- Error Response -----+        |   |        |
   |        |                             +--------+   |        |
   |        |                                          |        |
   |        +--(7)--- Rotate Token ------------------->|        |
   |        |                                          |        |
   |        |<-(8)--- Rotated Token -------------------+        |
   |        |                                          |        |
   +--------+                                          +--------+

      Figure 8: Diagram of the Process of Refreshing an Expired Access
                                   Token

   *  (1) The client instance requests access to the resource
      (Section 2).

   *  (2) The AS grants access to the resource (Section 3) with an
      access token (Section 3.2) usable at the RS.  The access token
      response includes a token management URI.

   *  (3) The client instance uses the access token (Section 7.2) to
      call the RS.

   *  (4) The RS validates the access token and returns an appropriate
      response for the API.

   *  (5) Time passes and the client instance uses the access token to
      call the RS again.

   *  (6) The RS validates the access token and determines that the
      access token is expired.  The RS responds to the client instance
      with an error.

   *  (7) The client instance calls the token management URI returned in
      (2) to rotate the access token (Section 6.1).  The client instance
      uses the access token (Section 7.2) in this call as well as the
      appropriate key; see Section 6.1 for details.

   *  (8) The AS validates the rotation request, including the signature
      and keys presented in (7), and refreshes the access token
      (Section 3.2.1).  The response includes a new version of the
      access token and can also include updated token management
      information, which the client instance will store in place of the
      values returned in (2).

1.6.7.  Requesting Subject Information Only

   In this scenario, the client instance does not call an RS and does
   not request an access token.  Instead, the client instance only
   requests and is returned direct subject information (Section 3.4).
   Many different interaction modes can be used in this scenario, so
   these are shown only in the abstract as functions of the AS here.

 +--------+                                  +--------+          .----.
 | Client |                                  |   AS   |         | End  |
 |Instance|                                  |        |         | User |
 |        +--(1)--- Request Access --------->|        |         |      |
 |        |                                  |        |         |      |
 |        |<-(2)-- Interaction Needed -------+        |         |      |
 |        |                                  |        |         |      |
 |        +==(3)== Facilitate Interaction =====================>|      |
 |        |                                  |        |         +------+
 |        |                                  |        |<==(4)==>|  RO  |
 |        |                                  |        |  AuthN  |      |
 |        |                                  |        |         |      |
 |        |                                  |        |<==(5)==>|      |
 |        |                                  |        |  AuthZ  +------+
 |        |                                  |        |         | End  |
 |        |<=(6)== Signal Continuation =========================+ User |
 |        |                                  |        |          `----`
 |        +--(7)--- Continue Request ------->|        |
 |        |                                  |        |
 |        |<-(8)----- Grant Access ----------+        |
 |        |                                  |        |
 +--------+                                  +--------+

  Figure 9: Diagram of the Process of Requesting and Releasing Subject
                  Information apart from Access Tokens

   *  (1) The client instance requests access to subject information
      (Section 2).

   *  (2) The AS determines that interaction is needed and responds
      (Section 3) with appropriate information for facilitating user
      interaction (Section 3.3).

   *  (3) The client instance facilitates the user interacting with the
      AS (Section 4) as directed in (2).

   *  (4) The user authenticates at the AS, taking on the role of the
      RO.

   *  (5) As the RO, the user authorizes the pending request from the
      client instance.

   *  (6) When the AS is done interacting with the user, the AS returns
      the user to the client instance and signals continuation.

   *  (7) The client instance loads the continuation information from
      (2) and calls the AS to continue the request (Section 5).

   *  (8) If the request has been authorized, the AS grants access to
      the requested direct subject information (Section 3.4) to the
      client instance.  At this stage, the user is generally considered
      "logged in" to the client instance based on the identifiers and
      assertions provided by the AS.  Note that the AS can restrict the
      subject information returned, and it might not match what the
      client instance requested; see Section 3.4 for details.

1.6.8.  Cross-User Authentication

   In this scenario, the end user and RO are two different people.
   Here, the client instance already knows who the end user is, likely
   through a separate authentication process.  The end user, operating
   the client instance, needs to get subject information about another
   person in the system, the RO.  The RO is given an opportunity to
   release this information using an asynchronous interaction method
   with the AS.  This scenario would apply, for instance, when the end
   user is an agent in a call center and the RO is a customer
   authorizing the call-center agent to access their account on their
   behalf.

  .----.                                                         .----.
 | End  |                                                       |  RO  |
 | User |<=================(1)== Identify RO ==================>|      |
 |      |                                                       |      |
 |      |        +--------+                  +--------+         |      |
 |      +==(2)==>| Client |                  |   AS   |         |      |
 |      | RO ID  |Instance|                  |        |         |      |
 |      |        |        |                  |        |         |      |
 |      |        |        +--(3)-- Req. ---->|        |         |      |
 |      |        |        |                  |        |         |      |
 |      |        |        |<-(4)-- Res. -----+        |         |      |
 |      |        |        |                  |        |<==(5)==>|      |
 |      |        |        |                  |        |  AuthN  |      |
 |      |        |        |                  |        |         |      |
 |      |        |        |                  |        |<==(6)==>|      |
 |      |        |        |                  |        |  AuthZ  |      |
 |      |        |        |                  |        |         |      |
 |      |        |        |                  |        |<==(7)==>|      |
 |      |        |        |<-(8)--- Finish --+        |Completed|      |
 |      |        |        |                  |        |         |      |
 |      |        |        +--(9)--- Cont. -->|        |         |      |
 |      |        |        |                  |        |         |      |
 |      |        |        |<-(10)-- Subj. ---+        |         |      |
 |      |<=(11)==+        |         Info     |        |         |      |
 |      | Return |        |                  |        |         |      |
 |      | RO     |        |                  |        |         |      |
 |      | Info   |        |                  |        |         |      |
  `----`         +--------+                  +--------+          `----`

     Figure 10: Diagram of Cross-User Authorization, Where the End
                       User and RO Are Different

   Precondition: The end user is authenticated to the client instance,
   and the client instance has an identifier representing the end user
   that it can present to the AS.  This identifier should be unique to
   the particular session with the client instance and the AS.  The
   client instance is also known to the AS and allowed to access this
   advanced functionality where the information of someone other than
   the end user is returned to the client instance.

   *  (1) The RO communicates a human-readable identifier to the end
      user, such as an email address or account number.  This
      communication happens out of band from the protocol, such as over
      the phone between parties.  Note that the RO is not interacting
      with the client instance.

   *  (2) The end user communicates the identifier to the client
      instance.  The means by which the identifier is communicated to
      the client instance are out of scope for this specification.

   *  (3) The client instance requests access to subject information
      (Section 2).  The request includes the RO's identifier in the
      sub_ids field of the subject information request (Section 2.2) and
      the end user's identifier in the user field (Section 2.4).  The
      request includes no interaction start methods, since the end user
      is not expected to be the one interacting with the AS.  The
      request does include the push-based interaction finish method
      (Section 2.5.2.2) to allow the AS to signal to the client instance
      when the interaction with the RO has concluded.

   *  (4) The AS sees that the identifiers for the end user and subject
      being requested are different.  The AS determines that it can
      reach out to the RO asynchronously for approval.  While it is
      doing so, the AS returns a continuation response (Section 3.1)
      with a finish nonce to allow the client instance to continue the
      grant request after interaction with the RO has concluded.

   *  (5) The AS contacts the RO and has them authenticate to the
      system.  The means for doing this are outside the scope of this
      specification, but the identity of the RO is known from the
      Subject Identifier sent in (3).

   *  (6) The RO is prompted to authorize the end user's request via the
      client instance.  Since the end user was identified in (3) via the
      user field, the AS can show this information to the RO during the
      authorization request.

   *  (7) The RO completes the authorization with the AS.  The AS marks
      the request as _approved_.

   *  (8) The RO pushes the interaction finish message (Section 4.2.2)
      to the client instance.  Note that in the case the RO cannot be
      reached or the RO denies the request, the AS still sends the
      interaction finish message to the client instance, after which the
      client instance can negotiate next steps if possible.

   *  (9) The client instance validates the interaction finish message
      and continues the grant request (Section 5.1).

   *  (10) The AS returns the RO's subject information (Section 3.4) to
      the client instance.

   *  (11) The client instance can display or otherwise utilize the RO's
      user information in its session with the end user.  Note that
      since the client instance requested different sets of user
      information in (3), the client instance does not conflate the end
      user with the RO.

   Additional considerations for asynchronous interactions like this are
   discussed in Section 11.36.

2.  Requesting Access

   To start a request, the client instance sends an HTTP POST with a
   JSON [RFC8259] document to the grant endpoint of the AS.  The grant
   endpoint is a URI that uniquely identifies the AS to client instances
   and serves as the identifier for the AS.  The document is a JSON
   object where each field represents a different aspect of the client
   instance's request.  Each field is described in detail in a
   subsection below.

   access_token (object / array of objects):  Describes the rights and
      properties associated with the requested access token.  REQUIRED
      if requesting an access token.  See Section 2.1.

   subject (object):  Describes the information about the RO that the
      client instance is requesting to be returned directly in the
      response from the AS.  REQUIRED if requesting subject information.
      See Section 2.2.

   client (object / string):  Describes the client instance that is
      making this request, including the key that the client instance
      will use to protect this request, any continuation requests at the
      AS, and any user-facing information about the client instance used
      in interactions.  REQUIRED.  See Section 2.3.

   user (object / string):  Identifies the end user to the AS in a
      manner that the AS can verify, either directly or by interacting
      with the end user to determine their status as the RO.  OPTIONAL.
      See Section 2.4.

   interact (object):  Describes the modes that the client instance
      supports for allowing the RO to interact with the AS and modes for
      the client instance to receive updates when interaction is
      complete.  REQUIRED if interaction is supported.  See Section 2.5.

   Additional members of this request object can be defined by
   extensions using the "GNAP Grant Request Parameters" registry
   (Section 10.3).

   A non-normative example of a grant request is below:

   {
       "access_token": {
           "access": [
               {
                   "type": "photo-api",
                   "actions": [
                       "read",
                       "write",
                       "dolphin"
                   ],
                   "locations": [
                       "https://server.example.net/",
                       "https://resource.local/other"
                   ],
                   "datatypes": [
                       "metadata",
                       "images"
                   ]
               },
               "dolphin-metadata"
           ]
       },
       "client": {
         "display": {
           "name": "My Client Display Name",
           "uri": "https://example.net/client"
         },
         "key": {
           "proof": "httpsig",
           "jwk": {
             "kty": "RSA",
             "e": "AQAB",
             "kid": "xyz-1",
             "alg": "RS256",
             "n": "kOB5rR4Jv0GMeL...."
           }
         }
       },
       "interact": {
           "start": ["redirect"],
           "finish": {
               "method": "redirect",
               "uri": "https://client.example.net/return/123455",
               "nonce": "LKLTI25DK82FX4T4QFZC"
           }
       },
       "subject": {
           "sub_id_formats": ["iss_sub", "opaque"],
           "assertion_formats": ["id_token"]
       }
   }

   Sending a request to the grant endpoint creates a grant request in
   the _processing_ state.  The AS processes this request to determine
   whether interaction or authorization are necessary (moving to the
   _pending_ state) or if access can be granted immediately (moving to
   the _approved_ state).

   The request MUST be sent as a JSON object in the content of the HTTP
   POST request with Content-Type application/json.  A key proofing
   mechanism MAY define an alternative content type, as long as the
   content is formed from the JSON object.  For example, the attached
   JSON Web Signature (JWS) key proofing mechanism (see Section 7.3.4)
   places the JSON object into the payload of a JWS wrapper, which is in
   turn sent as the message content.

2.1.  Requesting Access to Resources

   If the client instance is requesting one or more access tokens for
   the purpose of accessing an API, the client instance MUST include an
   access_token field.  This field MUST be an object (for a single
   access token (Section 2.1.1)) or an array of these objects (for
   multiple access tokens (Section 2.1.2)), as described in the
   following subsections.

2.1.1.  Requesting a Single Access Token

   To request a single access token, the client instance sends an
   access_token object composed of the following fields.

   access (array of objects/strings):  Describes the rights that the
      client instance is requesting for the access token to be used at
      the RS.  REQUIRED.  See Section 8.

   label (string):  A unique name chosen by the client instance to refer
      to the resulting access token.  The value of this field is opaque
      to the AS and is not intended to be exposed to or used by the end
      user.  If this field is included in the request, the AS MUST
      include the same label in the token response (Section 3.2).
      REQUIRED if used as part of a request for multiple access tokens
      (Section 2.1.2); OPTIONAL otherwise.

   flags (array of strings):  A set of flags that indicate desired
      attributes or behavior to be attached to the access token by the
      AS.  OPTIONAL.

   The values of the flags field defined by this specification are as
   follows:

   "bearer":  If this flag is included, the access token being requested
      is a bearer token.  If this flag is omitted, the access token is
      bound to the key used by the client instance in this request (or
      that key's most recent rotation), and the access token MUST be
      presented using the same key and proofing method.  Methods for
      presenting bound and bearer access tokens are described in
      Section 7.2.  See Section 11.9 for additional considerations on
      the use of bearer tokens.

   Flag values MUST NOT be included more than once.  If the request
   includes a flag value multiple times, the AS MUST return an
   invalid_flag error defined in Section 3.6.

   Additional flags can be defined by extensions using the "GNAP Access
   Token Flags" registry (Section 10.4).

   In the following non-normative example, the client instance is
   requesting access to a complex resource described by a pair of access
   request object.

   "access_token": {
       "access": [
           {
               "type": "photo-api",
               "actions": [
                   "read",
                   "write",
                   "delete"
               ],
               "locations": [
                   "https://server.example.net/",
                   "https://resource.local/other"
               ],
               "datatypes": [
                   "metadata",
                   "images"
               ]
           },
           {
               "type": "walrus-access",
               "actions": [
                   "foo",
                   "bar"
               ],
               "locations": [
                   "https://resource.other/"
               ],
               "datatypes": [
                   "data",
                   "pictures",
                   "walrus whiskers"
               ]
           }
       ],
       "label": "token1-23"
   }

   If access is approved, the resulting access token is valid for the
   described resource.  Since the bearer flag is not provided in this
   example, the token is bound to the client instance's key (or its most
   recent rotation).  The token is labeled "token1-23".  The token
   response structure is described in Section 3.2.1.

2.1.2.  Requesting Multiple Access Tokens

   To request that multiple access tokens be returned in a single
   response, the client instance sends an array of objects as the value
   of the access_token parameter.  Each object MUST conform to the
   request format for a single access token request, as specified in
   Section 2.1.1.  Additionally, each object in the array MUST include
   the label field, and all values of these fields MUST be unique within
   the request.  If the client instance does not include a label value
   for any entry in the array or the values of the label field are not
   unique within the array, the AS MUST return an "invalid_request"
   error (Section 3.6).

   The following non-normative example shows a request for two separate
   access tokens: token1 and token2.

   "access_token": [
       {
           "label": "token1",
           "access": [
               {
                   "type": "photo-api",
                   "actions": [
                       "read",
                       "write",
                       "dolphin"
                   ],
                   "locations": [
                       "https://server.example.net/",
                       "https://resource.local/other"
                   ],
                   "datatypes": [
                       "metadata",
                       "images"
                   ]
               },
               "dolphin-metadata"
           ]
       },
       {
           "label": "token2",
           "access": [
               {
                   "type": "walrus-access",
                   "actions": [
                       "foo",
                       "bar"
                   ],
                   "locations": [
                       "https://resource.other/"
                   ],
                   "datatypes": [
                       "data",
                       "pictures",
                       "walrus whiskers"
                   ]
               }
           ],
           "flags": [ "bearer" ]
       }
   ]

   All approved access requests are returned in the response structure
   for multiple access tokens (Section 3.2.2) using the values of the
   label fields in the request.

2.2.  Requesting Subject Information

   If the client instance is requesting information about the RO from
   the AS, it sends a subject field as a JSON object.  This object MAY
   contain the following fields.

   sub_id_formats (array of strings):  An array of Subject Identifier
      subject formats requested for the RO, as defined by [RFC9493].
      REQUIRED if Subject Identifiers are requested.

   assertion_formats (array of strings):  An array of requested
      assertion formats.  Possible values include id_token for an OpenID
      Connect ID Token [OIDC] and saml2 for a Security Assertion Markup
      Language (SAML) 2 assertion [SAML2].  Additional assertion formats
      can be defined in the "GNAP Assertion Formats" registry
      (Section 10.6).  REQUIRED if assertions are requested.

   sub_ids (array of objects):  An array of Subject Identifiers
      representing the subject for which information is being requested.
      Each object is a Subject Identifier as defined by [RFC9493].  All
      identifiers in the sub_ids array MUST identify the same subject.
      If omitted, the AS SHOULD assume that subject information requests
      are about the current user and SHOULD require direct interaction
      or proof of presence before releasing information.  OPTIONAL.

   Additional fields can be defined in the "GNAP Subject Information
   Request Fields" registry (Section 10.5).

   "subject": {
     "sub_id_formats": [ "iss_sub", "opaque" ],
     "assertion_formats": [ "id_token", "saml2" ]
   }

   The AS can determine the RO's identity and permission for releasing
   this information through interaction with the RO (Section 4), AS
   policies, or assertions presented by the client instance
   (Section 2.4).  If this is determined positively, the AS MAY return
   the RO's information in its response (Section 3.4) as requested.

   Subject Identifier types requested by the client instance serve only
   to identify the RO in the context of the AS and can't be used as
   communication channels by the client instance, as discussed in
   Section 3.4.

2.3.  Identifying the Client Instance

   When sending a new grant request to the AS, the client instance MUST
   identify itself by including its client information in the client
   field of the request and by signing the request with its unique key
   as described in Section 7.3.  Note that once a grant has been created
   and is in either the _pending_ or the _approved_ state, the AS can
   determine which client is associated with the grant by dereferencing
   the continuation access token sent in the continuation request
   (Section 5).  As a consequence, the client field is not sent or
   accepted for continuation requests.

   Client information is sent by value as an object or by reference as a
   string (see Section 2.3.1).

   When client instance information is sent by value, the client field
   of the request consists of a JSON object with the following fields.

   key (object / string):  The public key of the client instance to be
      used in this request as described in Section 7.1 or a reference to
      a key as described in Section 7.1.1.  REQUIRED.

   class_id (string):  An identifier string that the AS can use to
      identify the client software comprising this client instance.  The
      contents and format of this field are up to the AS.  OPTIONAL.

   display (object):  An object containing additional information that
      the AS MAY display to the RO during interaction, authorization,
      and management.  OPTIONAL.  See Section 2.3.2.

   "client": {
       "key": {
           "proof": "httpsig",
           "jwk": {
               "kty": "RSA",
               "e": "AQAB",
               "kid": "xyz-1",
               "alg": "RS256",
               "n": "kOB5rR4Jv0GMeLaY6_It_r3ORwdf8ci_JtffXyaSx8..."
           }
       },
       "class_id": "web-server-1234",
       "display": {
           "name": "My Client Display Name",
           "uri": "https://example.net/client"
       }
   }

   Additional fields can be defined in the "GNAP Client Instance Fields"
   registry (Section 10.7).

   Absent additional attestations, profiles, or trust mechanisms, both
   the display and class_id fields are self-declarative, presented by
   the client instance.  The AS needs to exercise caution in their
   interpretation, taking them as a hint but not as absolute truth.  The
   class_id field can be used in a variety of ways to help the AS make
   sense of the particular context in which the client instance is
   operating.  In corporate environments, for example, different levels
   of trust might apply depending on security policies.  This field aims
   to help the AS adjust its own access decisions for different classes
   of client software.  It is possible to configure a set of values and
   rules during a pre-registration and then have the client instances
   provide them later in runtime as a hint to the AS.  In other cases,
   the client runs with a specific AS in mind, so a single hardcoded
   value would be acceptable (for instance, a set-top box with a
   class_id claiming to be "FooBarTV version 4").  While the client
   instance may not have contacted the AS yet, the value of this
   class_id field can be evaluated by the AS according to a broader
   context of dynamic use, alongside other related information available
   elsewhere (for instance, corresponding fields in a certificate).  If
   the AS is not able to interpret or validate the class_id field, it
   MUST either return an invalid_client error (Section 3.6) or interpret
   the request as if the class_id were not present.  See additional
   discussion of client instance impersonation in Section 11.15.

   The client instance MUST prove possession of any presented key by the
   proofing mechanism associated with the key in the request.  Key
   proofing methods are defined in the "GNAP Key Proofing Methods"
   registry (Section 10.16), and an initial set of methods is described
   in Section 7.3.

   If the same public key is sent by value on different access requests,
   the AS MUST treat these requests as coming from the same client
   instance for purposes of identification, authentication, and policy
   application.

   If the AS does not know the client instance's public key ahead of
   time, the AS can choose how to process the unknown key.  Common
   approaches include:

   *  Allowing the request and requiring RO authorization in a trust-on-
      first-use model

   *  Limiting the client's requested access to only certain APIs and
      information

   *  Denying the request entirely by returning an invalid_client error
      (Section 3.6)

   The client instance MUST NOT send a symmetric key by value in the key
   field of the request, as doing so would expose the key directly
   instead of simply proving possession of it.  See considerations on
   symmetric keys in Section 11.7.  To use symmetric keys, the client
   instance can send the key by reference (Section 7.1.1) or send the
   entire client identity by reference (Section 2.3.1).

   The client instance's key can be pre-registered with the AS ahead of
   time and associated with a set of policies and allowable actions
   pertaining to that client.  If this pre-registration includes other
   fields that can occur in the client request object described in this
   section, such as class_id or display, the pre-registered values MUST
   take precedence over any values given at runtime.  Additional fields
   sent during a request but not present in a pre-registered client
   instance record at the AS SHOULD NOT be added to the client's pre-
   registered record.  See additional considerations regarding client
   instance impersonation in Section 11.15.

   A client instance that is capable of talking to multiple ASes SHOULD
   use a different key for each AS to prevent a class of mix-up attacks
   as described in Section 11.31, unless other mechanisms can be used to
   assure the identity of the AS for a given request.

2.3.1.  Identifying the Client Instance by Reference

   If the client instance has an instance identifier that the AS can use
   to determine appropriate key information, the client instance can
   send this instance identifier as a direct reference value in lieu of
   the client object.  The instance identifier MAY be assigned to a
   client instance at runtime through a grant response (Section 3.5) or
   MAY be obtained in another fashion, such as a static registration
   process at the AS.

   "client": "client-541-ab"

   When the AS receives a request with an instance identifier, the AS
   MUST ensure that the key used to sign the request (Section 7.3) is
   associated with the instance identifier.

   If the AS does not recognize the instance identifier, the request
   MUST be rejected with an invalid_client error (Section 3.6).

2.3.2.  Providing Displayable Client Instance Information

   If the client instance has additional information to display to the
   RO during any interactions at the AS, it MAY send that information in
   the "display" field.  This field is a JSON object that declares
   information to present to the RO during any interactive sequences.

   name (string):  Display name of the client software.  RECOMMENDED.

   uri (string):  User-facing information about the client software,
      such as a web page.  This URI MUST be an absolute URI.  OPTIONAL.

   logo_uri (string):  Display image to represent the client software.
      This URI MUST be an absolute URI.  The logo MAY be passed by value
      by using a data: URI [RFC2397] referencing an image media type.
      OPTIONAL.

   "display": {
       "name": "My Client Display Name",
       "uri": "https://example.net/client",
       "logo_uri": "data:image/png;base64,Eeww...="
   }

   Additional display fields can be defined in the "GNAP Client Instance
   Display Fields" registry (Section 10.8).

   The AS SHOULD use these values during interaction with the RO.  The
   values are for informational purposes only and MUST NOT be taken as
   authentic proof of the client instance's identity or source.  The AS
   MAY restrict display values to specific client instances, as
   identified by their keys in Section 2.3.  See additional
   considerations for displayed client information in Section 11.15 and
   for the logo_uri in particular in Section 11.16.

2.3.3.  Authenticating the Client Instance

   If the presented key is known to the AS and is associated with a
   single instance of the client software, the process of presenting a
   key and proving possession of that key is sufficient to authenticate
   the client instance to the AS.  The AS MAY associate policies with
   the client instance identified by this key, such as limiting which
   resources can be requested and which interaction methods can be used.
   For example, only specific client instances with certain known keys
   might be trusted with access tokens without the AS interacting
   directly with the RO, as in Appendix B.3.

   The presentation of a key allows the AS to strongly associate
   multiple successive requests from the same client instance with each
   other.  This is true when the AS knows the key ahead of time and can
   use the key to authenticate the client instance, but it is also true
   if the key is ephemeral and created just for this series of requests.
   As such, the AS MAY allow for client instances to make requests with
   unknown keys.  This pattern allows for ephemeral client instances
   (such as single-page applications) and client software with many
   individual long-lived instances (such as mobile applications) to
   generate key pairs per instance and use the keys within the protocol
   without having to go through a separate registration step.  The AS
   MAY limit which capabilities are made available to client instances
   with unknown keys.  For example, the AS could have a policy saying
   that only previously registered client instances can request
   particular resources or that all client instances with unknown keys
   have to be interactively approved by an RO.

2.4.  Identifying the User

   If the client instance knows the identity of the end user through one
   or more identifiers or assertions, the client instance MAY send that
   information to the AS in the user field.  The client instance MAY
   pass this information by value or by reference (see Section 2.4.1).

   sub_ids (array of objects):  An array of Subject Identifiers for the
      end user, as defined by [RFC9493].  OPTIONAL.

   assertions (array of objects):  An array containing assertions as
      objects, each containing the assertion format and the assertion
      value as the JSON string serialization of the assertion, as
      defined in Section 3.4.  OPTIONAL.

   "user": {
     "sub_ids": [ {
       "format": "opaque",
       "id": "J2G8G8O4AZ"
     } ],
     "assertions": [ {
       "format": "id_token",
       "value": "eyj..."
     } ]
   }

   Subject Identifiers are hints to the AS in determining the RO and
   MUST NOT be taken as authoritative statements that a particular RO is
   present at the client instance and acting as the end user.

   Assertions presented by the client instance SHOULD be validated by
   the AS.  While the details of such validation are outside the scope
   of this specification, common validation steps include verifying the
   signature of the assertion against a trusted signing key, verifying
   the audience and issuer of the assertion map to expected values, and
   verifying the time window for the assertion itself.  However, note
   that in many use cases, some of these common steps are relaxed.  For
   example, an AS acting as an identity provider (IdP) could expect that
   assertions being presented using this mechanism were issued by the AS
   to the client software.  The AS would verify that the AS is the
   issuer of the assertion, not the audience, and that the client
   instance is instead the audience of the assertion.  Similarly, an AS
   might accept a recently expired assertion in order to help bootstrap
   a new session with a specific end user.

   If the identified end user does not match the RO present at the AS
   during an interaction step and the AS is not explicitly allowing a
   cross-user authorization, the AS SHOULD reject the request with an
   unknown_user error (Section 3.6).

   If the AS trusts the client instance to present verifiable assertions
   or known Subject Identifiers, such as an opaque identifier issued by
   the AS for this specific client instance, the AS MAY decide, based on
   its policy, to skip interaction with the RO, even if the client
   instance provides one or more interaction modes in its request.

   See Section 11.30 for considerations for the AS when accepting and
   processing assertions from the client instance.

2.4.1.  Identifying the User by Reference

   The AS can identify the current end user to the client instance with
   a reference that can be used by the client instance to refer to the
   end user across multiple requests.  If the client instance has a
   reference for the end user at this AS, the client instance MAY pass
   that reference as a string.  The format of this string is opaque to
   the client instance.

   "user": "XUT2MFM1XBIKJKSDU8QM"

   One means of dynamically obtaining such a user reference is from the
   AS returning an opaque Subject Identifier as described in
   Section 3.4.  Other means of configuring a client instance with a
   user identifier are out of scope of this specification.  The lifetime
   and validity of these user references are determined by the AS, and
   this lifetime is not exposed to the client instance in GNAP.  As
   such, a client instance using such a user reference is likely to keep
   using that reference until it stops working.

   User reference identifiers are not intended to be human-readable user
   identifiers or structured assertions.  For the client instance to
   send either of these, the client can use the full user request object
   (Section 2.4) instead.

   If the AS does not recognize the user reference, it MUST return an
   unknown_user error (Section 3.6).

2.5.  Interacting with the User

   Often, the AS will require interaction with the RO (Section 4) in
   order to approve a requested delegation to the client instance for
   both access to resources and direct subject information.  Many times,
   the end user using the client instance is the same person as the RO,
   and the client instance can directly drive interaction with the end
   user by facilitating the process through means such as redirection to
   a URI or launching an application.  Other times, the client instance
   can provide information to start the RO's interaction on a secondary
   device, or the client instance will wait for the RO to approve the
   request asynchronously.  The client instance could also be signaled
   that interaction has concluded through a callback mechanism.

   The client instance declares the parameters for interaction methods
   that it can support using the interact field.

   The interact field is a JSON object with three keys whose values
   declare how the client can initiate and complete the request, as well
   as provide hints to the AS about user preferences such as locale.  A
   client instance MUST NOT declare an interaction mode it does not
   support.  The client instance MAY send multiple modes in the same
   request.  There is no preference order specified in this request.  An
   AS MAY respond to any, all, or none of the presented interaction
   modes (Section 3.3) in a request, depending on its capabilities and
   what is allowed to fulfill the request.

   start (array of objects/strings):  Indicates how the client instance
      can start an interaction.  REQUIRED.  See Section 2.5.1.

   finish (object):  Indicates how the client instance can receive an
      indication that interaction has finished at the AS.  OPTIONAL.
      See Section 2.5.2.

   hints (object):  Provides additional information to inform the
      interaction process at the AS.  OPTIONAL.  See Section 2.5.3.

   In the following non-normative example, the client instance is
   indicating that it can redirect (Section 2.5.1.1) the end user to an
   arbitrary URI and can receive a redirect (Section 2.5.2.1) through a
   browser request.  Note that the client instance does not accept a
   push-style callback.  The pattern of using a redirect for both
   interaction start and finish is common for web-based client software.

   "interact": {
       "start": ["redirect"],
       "finish": {
           "method": "redirect",
           "uri": "https://client.example.net/return/123455",
           "nonce": "LKLTI25DK82FX4T4QFZC"
       }
   }

   In the following non-normative example, the client instance is
   indicating that it can display a user code (Section 2.5.1.3) and
   direct the end user to an arbitrary URI (Section 2.5.1.1), but it
   cannot accept a redirect or push-style callback.  This pattern is
   common for devices that have robust display capabilities but expect
   the use of a secondary device to facilitate end-user interaction with
   the AS, such as a set-top box capable of displaying an interaction
   URL as a QR code.

   "interact": {
       "start": ["redirect", "user_code"]
   }

   In the following non-normative example, the client instance is
   indicating that it cannot start any interaction with the end user but
   that the AS can push an interaction finish message (Section 2.5.2.2)
   when authorization from the RO is received asynchronously.  This
   pattern is common for scenarios where a service needs to be
   authorized, but the RO is able to be contacted separately from the
   GNAP transaction itself, such as through a push notification or
   existing interactive session on a secondary device.

   "interact": {
       "start": [],
       "finish": {
           "method": "push",
           "uri": "https://client.example.net/return/123455",
           "nonce": "LKLTI25DK82FX4T4QFZC"
       }
   }

   If all of the following conditions are true, the AS MUST return an
   invalid_interaction error (Section 3.6) since the client instance
   will be unable to complete the request without authorization:

   *  The client instance does not provide a suitable interaction
      mechanism.

   *  The AS cannot contact the RO asynchronously.

   *  The AS determines that interaction is required.

2.5.1.  Start Mode Definitions

   If the client instance is capable of starting interaction with the
   end user, the client instance indicates this by sending an array of
   start modes under the start key.  Each interaction start mode has a
   unique identifying name.  Interaction start modes are specified in
   the array either by a string, which consists of the start mode name
   on its own, or by a JSON object with the required field mode:

   mode:  The interaction start mode.  REQUIRED.

   Interaction start modes defined as objects MAY define additional
   parameters to be required in the object.

   The start array can contain both string-type and object-type modes.

   This specification defines the following interaction start modes:

   "redirect" (string):  Indicates that the client instance can direct
      the end user to an arbitrary URI for interaction.  See
      Section 2.5.1.1.

   "app" (string):  Indicates that the client instance can launch an
      application on the end user's device for interaction.  See
      Section 2.5.1.2.

   "user_code" (string):  Indicates that the client instance can
      communicate a short, human-readable code to the end user for use
      with a stable URI.  See Section 2.5.1.3.

   "user_code_uri" (string):  Indicates that the client instance can
      communicate a short, human-readable code to the end user for use
      with a short, dynamic URI.  See Section 2.5.1.4.

   Additional start modes can be defined in the "GNAP Interaction Start
   Modes" registry (Section 10.9).

2.5.1.1.  Redirect to an Arbitrary URI

   If the client instance is capable of directing the end user to a URI
   defined by the AS at runtime, the client instance indicates this by
   including redirect in the array under the start key.  The means by
   which the client instance will activate this URI are out of scope of
   this specification, but common methods include an HTTP redirect,
   launching a browser on the end user's device, providing a scannable
   image encoding, and printing out a URI to an interactive console.
   While this URI is generally hosted at the AS, the client instance can
   make no assumptions about its contents, composition, or relationship
   to the grant endpoint URI.

   "interact": {
     "start": ["redirect"]
   }

   If this interaction mode is supported for this client instance and
   request, the AS returns a redirect interaction response
   (Section 3.3.1).  The client instance manages this interaction method
   as described in Section 4.1.1.

   See Section 11.29 for more considerations regarding the use of front-
   channel communication techniques.

2.5.1.2.  Open an Application-Specific URI

   If the client instance can open a URI associated with an application
   on the end user's device, the client instance indicates this by
   including app in the array under the start key.  The means by which
   the client instance determines the application to open with this URI
   are out of scope of this specification.

   "interact": {
     "start": ["app"]
   }

   If this interaction mode is supported for this client instance and
   request, the AS returns an app interaction response with an app URI
   payload (Section 3.3.2).  The client instance manages this
   interaction method as described in Section 4.1.4.

2.5.1.3.  Display a Short User Code

   If the client instance is capable of displaying or otherwise
   communicating a short, human-entered code to the RO, the client
   instance indicates this by including user_code in the array under the
   start key.  This code is to be entered at a static URI that does not
   change at runtime.  The client instance has no reasonable means to
   communicate a dynamic URI to the RO, so this URI is usually
   communicated out of band to the RO through documentation or other
   messaging outside of GNAP.  While this URI is generally hosted at the
   AS, the client instance can make no assumptions about its contents,
   composition, or relationship to the grant endpoint URI.

   "interact": {
       "start": ["user_code"]
   }

   If this interaction mode is supported for this client instance and
   request, the AS returns a user code as specified in Section 3.3.3.
   The client instance manages this interaction method as described in
   Section 4.1.2.

2.5.1.4.  Display a Short User Code and URI

   If the client instance is capable of displaying or otherwise
   communicating a short, human-entered code along with a short, human-
   entered URI to the RO, the client instance indicates this by
   including user_code_uri in the array under the start key.  This code
   is to be entered at the dynamic URL given in the response.  While
   this URL is generally hosted at the AS, the client instance can make
   no assumptions about its contents, composition, or relationship to
   the grant endpoint URI.

   "interact": {
       "start": ["user_code_uri"]
   }

   If this interaction mode is supported for this client instance and
   request, the AS returns a user code and interaction URL as specified
   in Section 3.3.4.  The client instance manages this interaction
   method as described in Section 4.1.3.

2.5.2.  Interaction Finish Methods

   If the client instance is capable of receiving a message from the AS
   indicating that the RO has completed their interaction, the client
   instance indicates this by sending the following members of an object
   under the finish key.

   method (string):  The callback method that the AS will use to contact
      the client instance.  REQUIRED.

   uri (string):  Indicates the URI that the AS will use to signal the
      client instance that interaction has completed.  This URI MAY be
      unique per request and MUST be hosted by or accessible to the
      client instance.  This URI MUST be an absolute URI and MUST NOT
      contain any fragment component.  If the client instance needs any
      state information to tie to the front-channel interaction
      response, it MUST use a unique callback URI to link to that
      ongoing state.  The allowable URIs and URI patterns MAY be
      restricted by the AS based on the client instance's presented key
      information.  The callback URI SHOULD be presented to the RO
      during the interaction phase before redirect.  REQUIRED for
      redirect and push methods.

   nonce (string):  Unique ASCII string value to be used in the
      calculation of the "hash" query parameter sent to the callback
      URI.  It must be sufficiently random to be unguessable by an
      attacker.  It MUST be generated by the client instance as a unique
      value for this request.  REQUIRED.

   hash_method (string):  An identifier of a hash calculation mechanism
      to be used for the callback hash in Section 4.2.3, as defined in
      the IANA "Named Information Hash Algorithm Registry" [HASH-ALG].
      If absent, the default value is sha-256.  OPTIONAL.

   This specification defines the following values for the method
   parameter; additional values can be defined in the "GNAP Interaction
   Finish Methods" registry (Section 10.10):

   "redirect":  Indicates that the client instance can receive a
      redirect from the end user's device after interaction with the RO
      has concluded.  See Section 2.5.2.1.

   "push":  Indicates that the client instance can receive an HTTP POST
      request from the AS after interaction with the RO has concluded.
      See Section 2.5.2.2.

   If interaction finishing is supported for this client instance and
   request, the AS will return a nonce (Section 3.3.5) used by the
   client instance to validate the callback.  All interaction finish
   methods MUST use this nonce to allow the client to verify the
   connection between the pending interaction request and the callback.
   GNAP does this through the use of the interaction hash, defined in
   Section 4.2.3.  All requests to the callback URI MUST be processed as
   described in Section 4.2.

   All interaction finish methods MUST require presentation of an
   interaction reference for continuing this grant request.  This means
   that the interaction reference MUST be returned by the AS and MUST be
   presented by the client as described in Section 5.1.  The means by
   which the interaction reference is returned to the client instance
   are specific to the interaction finish method.

2.5.2.1.  Receive an HTTP Callback through the Browser

   A finish method value of redirect indicates that the client instance
   will expect a request from the RO's browser using the HTTP method GET
   as described in Section 4.2.1.

   The client instance's URI MUST be protected by HTTPS, be hosted on a
   server local to the RO's browser ("localhost"), or use an
   application-specific URI scheme that is loaded on the end user's
   device.

   "interact": {
       "finish": {
           "method": "redirect",
           "uri": "https://client.example.net/return/123455",
           "nonce": "LKLTI25DK82FX4T4QFZC"
       }
   }

   Requests to the callback URI MUST be processed by the client instance
   as described in Section 4.2.1.

   Since the incoming request to the callback URI is from the RO's
   browser, this method is usually used when the RO and end user are the
   same entity.  See Section 11.24 for considerations on ensuring the
   incoming HTTP message matches the expected context of the request.
   See Section 11.29 for more considerations regarding the use of front-
   channel communication techniques.

2.5.2.2.  Receive an HTTP Direct Callback

   A finish method value of push indicates that the client instance will
   expect a request from the AS directly using the HTTP method POST as
   described in Section 4.2.2.

   The client instance's URI MUST be protected by HTTPS, be hosted on a
   server local to the RO's browser ("localhost"), or use an
   application-specific URI scheme that is loaded on the end user's
   device.

   "interact": {
       "finish": {
           "method": "push",
           "uri": "https://client.example.net/return/123455",
           "nonce": "LKLTI25DK82FX4T4QFZC"
       }
   }

   Requests to the callback URI MUST be processed by the client instance
   as described in Section 4.2.2.

   Since the incoming request to the callback URI is from the AS and not
   from the RO's browser, this request is not expected to have any
   shared session information from the start method.  See Sections 11.24
   and 11.23 for more considerations regarding the use of back-channel
   and polling mechanisms like this.

2.5.3.  Hints

   The hints key is an object describing one or more suggestions from
   the client instance that the AS can use to help drive user
   interaction.

   This specification defines the following property under the hints
   key:

   ui_locales (array of strings):  Indicates the end user's preferred
      locales that the AS can use during interaction, particularly
      before the RO has authenticated.  OPTIONAL.  Section 2.5.3.1

   The following subsection details requests for interaction hints.
   Additional interaction hints can be defined in the "GNAP Interaction
   Hints" registry (Section 10.11).

2.5.3.1.  Indicate Desired Interaction Locales

   If the client instance knows the end user's locale and language
   preferences, the client instance can send this information to the AS
   using the ui_locales field with an array of locale strings as defined
   by [RFC5646].

   "interact": {
       "hints": {
           "ui_locales": ["en-US", "fr-CA"]
       }
   }

   If possible, the AS SHOULD use one of the locales in the array, with
   preference to the first item in the array supported by the AS.  If
   none of the given locales are supported, the AS MAY use a default
   locale.

3.  Grant Response

   In response to a client instance's request, the AS responds with a
   JSON object as the HTTP content.  Each possible field is detailed in
   the subsections below.

   continue (object):  Indicates that the client instance can continue
      the request by making one or more continuation requests.  REQUIRED
      if continuation calls are allowed for this client instance on this
      grant request.  See Section 3.1.

   access_token (object / array of objects):  A single access token or
      set of access tokens that the client instance can use to call the
      RS on behalf of the RO.  REQUIRED if an access token is included.
      See Section 3.2.

   interact (object):  Indicates that interaction through some set of
      defined mechanisms needs to take place.  REQUIRED if interaction
      is expected.  See Section 3.3.

   subject (object):  Claims about the RO as known and declared by the
      AS.  REQUIRED if subject information is included.  See
      Section 3.4.

   instance_id (string):  An identifier this client instance can use to
      identify itself when making future requests.  OPTIONAL.  See
      Section 3.5.

   error (object or string):  An error code indicating that something
      has gone wrong.  REQUIRED for an error condition.  See
      Section 3.6.

   Additional fields can be defined by extensions to GNAP in the "GNAP
   Grant Response Parameters" registry (Section 10.12).

   In the following non-normative example, the AS is returning an
   interaction URI (Section 3.3.1), a callback nonce (Section 3.3.5),
   and a continuation response (Section 3.1).

   NOTE: '\' line wrapping per RFC 8792

   {
       "interact": {
           "redirect": "https://server.example.com/interact/4CF492ML\
             VMSW9MKMXKHQ",
           "finish": "MBDOFXG4Y5CVJCX821LH"
       },
       "continue": {
           "access_token": {
               "value": "80UPRY5NM33OMUKMKSKU",
           },
           "uri": "https://server.example.com/tx"
       }
   }

   In the following non-normative example, the AS is returning a bearer
   access token (Section 3.2.1) with a management URI and a Subject
   Identifier (Section 3.4) in the form of an opaque identifier.

   {
       "access_token": {
           "value": "OS9M2PMHKUR64TB8N6BW7OZB8CDFONP219RP1LT0",
           "flags": ["bearer"],
           "manage": {
               "uri": "https://server.example.com/token/PRY5NM33O",
               "access_token": {
                   "value": "B8CDFONP21-4TB8N6.BW7ONM"
               }
           }
       },
       "subject": {
           "sub_ids": [ {
             "format": "opaque",
             "id": "J2G8G8O4AZ"
           } ]
       }
   }

   In the following non-normative example, the AS is returning set of
   Subject Identifiers (Section 3.4), simultaneously as an opaque
   identifier, an email address, and a decentralized identifier (DID),
   formatted as a set of Subject Identifiers as defined in [RFC9493].

   {
       "subject": {
           "sub_ids": [ {
             "format": "opaque",
             "id": "J2G8G8O4AZ"
           }, {
             "format": "email",
             "email": "user@example.com"
           }, {
             "format": "did",
             "url": "did:example:123456"
           } ]
       }
   }

   The response MUST be sent as a JSON object in the content of the HTTP
   response with Content-Type application/json, unless otherwise
   specified by the specific response (e.g., an empty response with no
   Content-Type).

   The AS MUST include the HTTP Cache-Control response header field
   [RFC9111] with a value set to "no-store".

3.1.  Request Continuation

   If the AS determines that the grant request can be continued by the
   client instance, the AS responds with the continue field.  This field
   contains a JSON object with the following properties.

   uri (string):  The URI at which the client instance can make
      continuation requests.  This URI MAY vary per request or MAY be
      stable at the AS.  This URI MUST be an absolute URI.  The client
      instance MUST use this value exactly as given when making a
      continuation request (Section 5).  REQUIRED.

   wait (integer):  The amount of time in integer seconds the client
      instance MUST wait after receiving this request continuation
      response and calling the continuation URI.  The value SHOULD NOT
      be less than five seconds, and omission of the value MUST be
      interpreted as five seconds.  RECOMMENDED.

   access_token (object):  A unique access token for continuing the
      request, called the "continuation access token".  The value of
      this property MUST be an object in the format specified in
      Section 3.2.1.  This access token MUST be bound to the client
      instance's key used in the request and MUST NOT be a bearer token.
      As a consequence, the flags array of this access token MUST NOT
      contain the string bearer, and the key field MUST be omitted.
      This access token MUST NOT have a manage field.  The client
      instance MUST present the continuation access token in all
      requests to the continuation URI as described in Section 7.2.
      REQUIRED.

   {
       "continue": {
           "access_token": {
               "value": "80UPRY5NM33OMUKMKSKU"
           },
           "uri": "https://server.example.com/continue",
           "wait": 60
       }
   }

   This field is REQUIRED if the grant request is in the _pending_
   state, as the field contains the information needed by the client
   request to continue the request as described in Section 5.  Note that
   the continuation access token is bound to the client instance's key;
   therefore, the client instance MUST sign all continuation requests
   with its key as described in Section 7.3 and MUST present the
   continuation access token in its continuation request.

3.2.  Access Tokens

   If the AS has successfully granted one or more access tokens to the
   client instance, the AS responds with the access_token field.  This
   field contains either a single access token as described in
   Section 3.2.1 or an array of access tokens as described in
   Section 3.2.2.

   The client instance uses any access tokens in this response to call
   the RS as described in Section 7.2.

   The grant request MUST be in the _approved_ state to include this
   field in the response.

3.2.1.  Single Access Token

   If the client instance has requested a single access token and the AS
   has granted that access token, the AS responds with the
   "access_token" field.  The value of this field is an object with the
   following properties.

   value (string):  The value of the access token as a string.  The
      value is opaque to the client instance.  The value MUST be limited
      to the token68 character set defined in Section 11.2 of [HTTP] to
      facilitate transmission over HTTP headers and within other
      protocols without requiring additional encoding.  REQUIRED.

   label (string):  The value of the label the client instance provided
      in the associated token request (Section 2.1), if present.
      REQUIRED for multiple access tokens or if a label was included in
      the single access token request; OPTIONAL for a single access
      token where no label was included in the request.

   manage (object):  Access information for the token management API for
      this access token.  If provided, the client instance MAY manage
      its access token as described in Section 6.  This management API
      is a function of the AS and is separate from the RS the client
      instance is requesting access to.  OPTIONAL.

   access (array of objects/strings):  A description of the rights
      associated with this access token, as defined in Section 8.  If
      included, this MUST reflect the rights associated with the issued
      access token.  These rights MAY vary from what was requested by
      the client instance.  REQUIRED.

   expires_in (integer):  The number of seconds in which the access will
      expire.  The client instance MUST NOT use the access token past
      this time.  Note that the access token MAY be revoked by the AS or
      RS at any point prior to its expiration.  OPTIONAL.

   key (object / string):  The key that the token is bound to, if
      different from the client instance's presented key.  The key MUST
      be an object or string in a format described in Section 7.1.  The
      client instance MUST be able to dereference or process the key
      information in order to be able to sign subsequent requests using
      the access token (Section 7.2).  When the key is provided by value
      from the AS, the token shares some security properties with bearer
      tokens as discussed in Section 11.38.  It is RECOMMENDED that keys
      returned for use with access tokens be key references as described
      in Section 7.1.1 that the client instance can correlate to its
      known keys.  OPTIONAL.

   flags (array of strings):  A set of flags that represent attributes
      or behaviors of the access token issued by the AS.  OPTIONAL.

   The value of the manage field is an object with the following
   properties:

   uri (string):  The URI of the token management API for this access
      token.  This URI MUST be an absolute URI.  This URI MUST NOT
      include the value of the access token being managed or the value
      of the access token used to protect the URI.  This URI SHOULD be
      different for each access token issued in a request.  REQUIRED.

   access_token (object):  A unique access token for continuing the
      request, called the "token management access token".  The value of
      this property MUST be an object in the format specified in
      Section 3.2.1.  This access token MUST be bound to the client
      instance's key used in the request (or its most recent rotation)
      and MUST NOT be a bearer token.  As a consequence, the flags array
      of this access token MUST NOT contain the string bearer, and the
      key field MUST be omitted.  This access token MUST NOT have a
      manage field.  This access token MUST NOT have the same value as
      the token it is managing.  The client instance MUST present the
      continuation access token in all requests to the continuation URI
      as described in Section 7.2.  REQUIRED.

   The values of the flags field defined by this specification are as
   follows:

   "bearer":  Flag indicating whether the token is a bearer token, not
      bound to a key and proofing mechanism.  If the bearer flag is
      present, the access token is a bearer token, and the key field in
      this response MUST be omitted.  See Section 11.9 for additional
      considerations on the use of bearer tokens.

   "durable":  Flag indicating a hint of AS behavior on token rotation.
      If this flag is present, then the client instance can expect a
      previously issued access token to continue to work after it has
      been rotated (Section 6.1) or the underlying grant request has
      been modified (Section 5.3), resulting in the issuance of new
      access tokens.  If this flag is omitted, the client instance can
      anticipate a given access token could stop working after token
      rotation or grant request modification.  Note that a token flagged
      as durable can still expire or be revoked through any normal
      means.

   Flag values MUST NOT be included more than once.

   Additional flags can be defined by extensions using the "GNAP Access
   Token Flags" registry (Section 10.4).

   If the bearer flag and the key field in this response are omitted,
   the token is bound to the key used by the client instance
   (Section 2.3) in its request for access.  If the bearer flag is
   omitted and the key field is present, the token is bound to the key
   and proofing mechanism indicated in the key field.  The means by
   which the AS determines how to bind an access token to a key other
   than that presented by the client instance are out of scope for this
   specification, but common practices include pre-registering specific
   keys in a static fashion.

   The client software MUST reject any access token where the flags
   field contains the bearer flag and the key field is present with any
   value.

   The following non-normative example shows a single access token bound
   to the client instance's key used in the initial request.  The access
   token has a management URI and has access to three described
   resources (one using an object and two described by reference
   strings).

   NOTE: '\' line wrapping per RFC 8792

   "access_token": {
       "value": "OS9M2PMHKUR64TB8N6BW7OZB8CDFONP219RP1LT0",
       "manage": {
           "uri": "https://server.example.com/token/PRY5NM33O",
           "access_token": {
               "value": "B8CDFONP21-4TB8N6.BW7ONM"
           }
       },
       "access": [
           {
               "type": "photo-api",
               "actions": [
                   "read",
                   "write",
                   "dolphin"
               ],
               "locations": [
                   "https://server.example.net/",
                   "https://resource.local/other"
               ],
               "datatypes": [
                   "metadata",
                   "images"
               ]
           },
           "read", "dolphin-metadata"
       ]
   }

   The following non-normative example shows a single bearer access
   token with access to two described resources.

   "access_token": {
       "value": "OS9M2PMHKUR64TB8N6BW7OZB8CDFONP219RP1LT0",
       "flags": ["bearer"],
       "access": [
           "finance", "medical"
       ]
   }

   If the client instance requested a single access token
   (Section 2.1.1), the AS MUST NOT respond with the structure for
   multiple access tokens.

3.2.2.  Multiple Access Tokens

   If the client instance has requested multiple access tokens and the
   AS has granted at least one of them, the AS responds with the
   "access_token" field.  The value of this field is a JSON array, the
   members of which are distinct access tokens as described in
   Section 3.2.1.  Each object MUST have a unique label field,
   corresponding to the token labels chosen by the client instance in
   the request for multiple access tokens (Section 2.1.2).

   In the following non-normative example, two tokens are issued under
   the names token1 and token2, and only the first token has a
   management URI associated with it.

   NOTE: '\' line wrapping per RFC 8792

   "access_token": [
       {
           "label": "token1",
           "value": "OS9M2PMHKUR64TB8N6BW7OZB8CDFONP219RP1LT0",
           "manage": {
               "uri": "https://server.example.com/token/PRY5NM33O",
               "access_token": {
                   "value": "B8CDFONP21-4TB8N6.BW7ONM"
               }
           },
           "access": [ "finance" ]
       },
       {
           "label": "token2",
           "value": "UFGLO2FDAFG7VGZZPJ3IZEMN21EVU71FHCARP4J1",
           "access": [ "medical" ]
       }
   }

   Each access token corresponds to one of the objects in the
   access_token array of the client instance's request (Section 2.1.2).

   The AS MAY refuse to issue one or more of the requested access tokens
   for any reason.  In such cases, the refused token is omitted from the
   response, and all of the other issued access tokens are included in
   the response under their respective requested labels.  If the client
   instance requested multiple access tokens (Section 2.1.2), the AS
   MUST NOT respond with a single access token structure, even if only a
   single access token is granted.  In such cases, the AS MUST respond
   with a structure for multiple access tokens containing one access
   token.

   "access_token": [
       {
           "label": "token2",
           "value": "8N6BW7OZB8CDFONP219-OS9M2PMHKUR64TBRP1LT0",
           "manage": {
               "uri": "https://server.example.com/token/PRY5NM33O",
               "access_token": {
                   "value": "B8CDFONP21-4TB8N6.BW7ONM"
               }
           },
           "access": [ "fruits" ]
       }
   ]

   The parameters of each access token are separate.  For example, each
   access token is expected to have a unique value and (if present)
   label, and each access token likely has different access rights
   associated with it.  Each access token could also be bound to
   different keys with different proofing mechanisms.

3.3.  Interaction Modes

   If the client instance has indicated a capability to interact with
   the RO in its request (Section 2.5) and the AS has determined that
   interaction is both supported and necessary, the AS responds to the
   client instance with any of the following values in the interact
   field of the response.  There is no preference order for interaction
   modes in the response, and it is up to the client instance to
   determine which ones to use.  All supported interaction methods are
   included in the same interact object.

   redirect (string):  Redirect to an arbitrary URI.  REQUIRED if the
      redirect interaction start mode is possible for this request.  See
      Section 3.3.1.

   app (string):  Launch of an application URI.  REQUIRED if the app
      interaction start mode is possible for this request.  See
      Section 3.3.2.

   user_code (string):  Display a short user code.  REQUIRED if the
      user_code interaction start mode is possible for this request.
      See Section 3.3.3.

   user_code_uri (object):  Display a short user code and URI.  REQUIRED
      if the user_code_uri interaction start mode is possible for this
      request.  Section 3.3.4

   finish (string):  A unique ASCII string value provided by the AS as a
      nonce.  This is used by the client instance to verify the callback
      after interaction is completed.  REQUIRED if the interaction
      finish method requested by the client instance is possible for
      this request.  See Section 3.3.5.

   expires_in (integer):  The number of integer seconds after which this
      set of interaction responses will expire and no longer be usable
      by the client instance.  If the interaction methods expire, the
      client MAY restart the interaction process for this grant request
      by sending an update (Section 5.3) with a new interaction request
      field (Section 2.5).  OPTIONAL.  If omitted, the interaction
      response modes returned do not expire but MAY be invalidated by
      the AS at any time.

   Additional interaction mode responses can be defined in the "GNAP
   Interaction Mode Responses" registry (Section 10.13).

   The AS MUST NOT respond with any interaction mode that the client
   instance did not indicate in its request, and the AS MUST NOT respond
   with any interaction mode that the AS does not support.  Since
   interaction responses include secret or unique information, the AS
   SHOULD respond to each interaction mode only once in an ongoing
   request, particularly if the client instance modifies its request
   (Section 5.3).

   The grant request MUST be in the _pending_ state to include this
   field in the response.

3.3.1.  Redirection to an Arbitrary URI

   If the client instance indicates that it can redirect to an arbitrary
   URI (Section 2.5.1.1) and the AS supports this mode for the client
   instance's request, the AS responds with the "redirect" field, which
   is a string containing the URI for the end user to visit.  This URI
   MUST be unique for the request and MUST NOT contain any security-
   sensitive information such as user identifiers or access tokens.

   "interact": {
       "redirect": "https://interact.example.com/4CF492MLVMSW9MKMXKHQ"
   }

   The URI returned is a function of the AS, but the URI itself MAY be
   completely distinct from the grant endpoint URI that the client
   instance uses to request access (Section 2), allowing an AS to
   separate its user-interaction functionality from its backend security
   functionality.  The AS will need to dereference the specific grant
   request and its information from the URI alone.  If the AS does not
   directly host the functionality accessed through the redirect URI,
   then the means for the interaction functionality to communicate with
   the rest of the AS are out of scope for this specification.

   The client instance sends the end user to the URI to interact with
   the AS.  The client instance MUST NOT alter the URI in any way.  The
   means for the client instance to send the end user to this URI are
   out of scope of this specification, but common methods include an
   HTTP redirect, launching the system browser, displaying a scannable
   code, or printing out the URI in an interactive console.  See details
   of the interaction in Section 4.1.1.

3.3.2.  Launch of an Application URI

   If the client instance indicates that it can launch an application
   URI (Section 2.5.1.2) and the AS supports this mode for the client
   instance's request, the AS responds with the "app" field, which is a
   string containing the URI for the client instance to launch.  This
   URI MUST be unique for the request and MUST NOT contain any security-
   sensitive information such as user identifiers or access tokens.

   "interact": {
       "app": "https://app.example.com/launch?tx=4CF492MLV"
   }

   The means for the launched application to communicate with the AS are
   out of scope for this specification.

   The client instance launches the URI as appropriate on its platform;
   the means for the client instance to launch this URI are out of scope
   of this specification.  The client instance MUST NOT alter the URI in
   any way.  The client instance MAY attempt to detect if an installed
   application will service the URI being sent before attempting to
   launch the application URI.  See details of the interaction in
   Section 4.1.4.

3.3.3.  Display of a Short User Code

   If the client instance indicates that it can display a short, user-
   typeable code (Section 2.5.1.3) and the AS supports this mode for the
   client instance's request, the AS responds with a "user_code" field.
   This field is string containing a unique short code that the user can
   type into a web page.  To facilitate usability, this string MUST
   consist only of characters that can be easily typed by the end user
   (such as ASCII letters or numbers) and MUST be processed by the AS in
   a case-insensitive manner (see Section 4.1.2).  The string MUST be
   randomly generated so as to be unguessable by an attacker within the
   time it is accepted.  The time in which this code will be accepted
   SHOULD be short lived, such as several minutes.  It is RECOMMENDED
   that this code be between six and eight characters in length.

   "interact": {
       "user_code": "A1BC3DFF"
   }

   The client instance MUST communicate the "user_code" value to the end
   user in some fashion, such as displaying it on a screen or reading it
   out audibly.  This code is used by the interaction component of the
   AS as a means of identifying the pending grant request and does not
   function as an authentication factor for the RO.

   The URI that the end user is intended to enter the code into MUST be
   stable, since the client instance is expected to have no means of
   communicating a dynamic URI to the end user at runtime.

   As this interaction mode is designed to facilitate interaction via a
   secondary device, it is not expected that the client instance
   redirect the end user to the URI where the code is entered.  If the
   client instance is capable of communicating a short arbitrary URI to
   the end user for use with the user code, the client instance SHOULD
   instead use the "user_code_uri" mode (Section 2.5.1.4).  If the
   client instance is capable of communicating a long arbitrary URI to
   the end user, such as through a scannable code, the client instance
   SHOULD use the "redirect" mode (Section 2.5.1.1) for this purpose,
   instead of or in addition to the user code mode.

   See details of the interaction in Section 4.1.2.

3.3.4.  Display of a Short User Code and URI

   If the client instance indicates that it can display a short, user-
   typeable code (Section 2.5.1.3) and the AS supports this mode for the
   client instance's request, the AS responds with a "user_code_uri"
   object that contains the following members.

   code (string):  A unique short code that the end user can type into a
      provided URI.  To facilitate usability, this string MUST consist
      only of characters that can be easily typed by the end user (such
      as ASCII letters or numbers) and MUST be processed by the AS in a
      case-insensitive manner (see Section 4.1.3).  The string MUST be
      randomly generated so as to be unguessable by an attacker within
      the time it is accepted.  The time in which this code will be
      accepted SHOULD be short lived, such as several minutes.  It is
      RECOMMENDED that this code be between six and eight characters in
      length.  REQUIRED.

   uri (string):  The interaction URI that the client instance will
      direct the RO to.  This URI MUST be short enough to be
      communicated to the end user by the client instance.  It is
      RECOMMENDED that this URI be short enough for an end user to type
      in manually.  The URI MUST NOT contain the code value.  This URI
      MUST be an absolute URI.  REQUIRED.

   "interact": {
       "user_code_uri": {
           "code": "A1BC3DFF",
           "uri": "https://s.example/device"
       }
   }

   The client instance MUST communicate the "code" to the end user in
   some fashion, such as displaying it on a screen or reading it out
   audibly.  This code is used by the interaction component of the AS as
   a means of identifying the pending grant request and does not
   function as an authentication factor for the RO.

   The client instance MUST also communicate the URI to the end user.
   Since it is expected that the end user will continue interaction on a
   secondary device, the URI needs to be short enough to allow the end
   user to type or copy it to a secondary device without mistakes.

   The URI returned is a function of the AS, but the URI itself MAY be
   completely distinct from the grant endpoint URI that the client
   instance uses to request access (Section 2), allowing an AS to
   separate its user-interaction functionality from its backend security
   functionality.  If the AS does not directly host the functionality
   accessed through the given URI, then the means for the interaction
   functionality to communicate with the rest of the AS are out of scope
   for this specification.

   See details of the interaction in Section 4.1.2.

3.3.5.  Interaction Finish

   If the client instance indicates that it can receive a post-
   interaction redirect or push at a URI (Section 2.5.2) and the AS
   supports this mode for the client instance's request, the AS responds
   with a finish field containing a nonce that the client instance will
   use in validating the callback as defined in Section 4.2.

   "interact": {
       "finish": "MBDOFXG4Y5CVJCX821LH"
   }

   When the interaction is completed, the interaction component of the
   AS MUST contact the client instance using the means defined by the
   finish method as described in Section 4.2.

   If the AS returns the finish field, the client instance MUST NOT
   continue a grant request before it receives the associated
   interaction reference on the callback URI.  See details in
   Section 4.2.

3.4.  Returning Subject Information

   If information about the RO is requested and the AS grants the client
   instance access to that data, the AS returns the approved information
   in the "subject" response field.  The AS MUST return the subject
   field only in cases where the AS is sure that the RO and the end user
   are the same party.  This can be accomplished through some forms of
   interaction with the RO (Section 4).

   This field is an object with the following properties.

   sub_ids (array of objects):  An array of Subject Identifiers for the
      RO, as defined by [RFC9493].  REQUIRED if returning Subject
      Identifiers.

   assertions (array of objects):  An array containing assertions as
      objects, each containing the assertion object described below.
      REQUIRED if returning assertions.

   updated_at (string):  Timestamp as a date string as described in
      [RFC3339], indicating when the identified account was last
      updated.  The client instance MAY use this value to determine if
      it needs to request updated profile information through an
      identity API.  The definition of such an identity API is out of
      scope for this specification.  RECOMMENDED.

   Assertion objects contain the following fields:

   format (string):  The assertion format.  Possible formats are listed
      in Section 3.4.1.  Additional assertion formats can be defined in
      the "GNAP Assertion Formats" registry (Section 10.6).  REQUIRED.

   value (string):  The assertion value as the JSON string serialization
      of the assertion.  REQUIRED.

   The following non-normative example contains an opaque identifier and
   an OpenID Connect ID Token:

   "subject": {
     "sub_ids": [ {
       "format": "opaque",
       "id": "XUT2MFM1XBIKJKSDU8QM"
     } ],
     "assertions": [ {
       "format": "id_token",
       "value": "eyj..."
     } ]
   }

   Subject Identifiers returned by the AS SHOULD uniquely identify the
   RO at the AS.  Some forms of Subject Identifiers are opaque to the
   client instance (such as the subject of an issuer and subject pair),
   while other forms (such as email address and phone number) are
   intended to allow the client instance to correlate the identifier
   with other account information at the client instance.  The client
   instance MUST NOT request or use any returned Subject Identifiers for
   communication purposes (see Section 2.2).  That is, a Subject
   Identifier returned in the format of an email address or a phone
   number only identifies the RO to the AS and does not indicate that
   the AS has validated that the represented email address or phone
   number in the identifier is suitable for communication with the
   current user.  To get such information, the client instance MUST use
   an identity protocol to request and receive additional identity
   claims.  The details of an identity protocol and associated schema
   are outside the scope of this specification.

   The AS MUST ensure that the returned subject information represents
   the RO.  In most cases, the AS will also ensure that the returned
   subject information represents the end user authenticated
   interactively at the AS.  The AS SHOULD NOT reuse Subject Identifiers
   for multiple different ROs.

   The "sub_ids" and "assertions" response fields are independent of
   each other.  That is, a returned assertion MAY use a different
   Subject Identifier than other assertions and Subject Identifiers in
   the response.  However, all Subject Identifiers and assertions
   returned MUST refer to the same party.

   The client instance MUST interpret all subject information in the
   context of the AS from which the subject information is received, as
   is discussed in Section 6 of [SP80063C].  For example, one AS could
   return an email identifier of "user@example.com" for one RO, and a
   different AS could return that same email identifier of
   "user@example.com" for a completely different RO.  A client instance
   talking to both ASes needs to differentiate between these two
   accounts by accounting for the AS source of each identifier and not
   assuming that either has a canonical claim on the identifier without
   additional configuration and trust agreements.  Otherwise, a rogue AS
   could exploit this to take over a targeted account asserted by a
   different AS.

   Extensions to this specification MAY define additional response
   properties in the "GNAP Subject Information Response Fields" registry
   (Section 10.14).

   The grant request MUST be in the _approved_ state to return this
   field in the response.

   See Section 11.30 for considerations that the client instance has to
   make when accepting and processing assertions from the AS.

3.4.1.  Assertion Formats

   The following assertion formats are defined in this specification:

   id_token:  An OpenID Connect ID Token [OIDC], in JSON Web Token (JWT)
      compact format as a single string.

   saml2:  A SAML 2.0 assertion [SAML2], encoded as a single base64url
      string with no padding.

3.5.  Returning a Dynamically Bound Client Instance Identifier

   Many parts of the client instance's request can be passed as either a
   value or a reference.  The use of a reference in place of a value
   allows for a client instance to optimize requests to the AS.

   Some references, such as for the client instance's identity
   (Section 2.3.1) or the requested resources (Section 8.1), can be
   managed statically through an admin console or developer portal
   provided by the AS or RS.  The developer of the client software can
   include these values in their code for a more efficient and compact
   request.

   If desired, the AS MAY also generate and return an instance
   identifier dynamically to the client instance in the response to
   facilitate multiple interactions with the same client instance over
   time.  The client instance SHOULD use this instance identifier in
   future requests in lieu of sending the associated data values in the
   client field.

   Dynamically generated client instance identifiers are string values
   that MUST be protected by the client instance as secrets.  Instance
   identifier values MUST be unguessable and MUST NOT contain any
   information that would compromise any party if revealed.  Instance
   identifier values are opaque to the client instance, and their
   content is determined by the AS.  The instance identifier MUST be
   unique per client instance at the AS.

   instance_id (string):  A string value used to represent the
      information in the client object that the client instance can use
      in a future request, as described in Section 2.3.1.  OPTIONAL.

   The following non-normative example shows an instance identifier
   alongside an issued access token.

   {
       "instance_id": "7C7C4AZ9KHRS6X63AJAO",
       "access_token": {
           "value": "OS9M2PMHKUR64TB8N6BW7OZB8CDFONP219RP1LT0"
       }
   }

3.6.  Error Response

   If the AS determines that the request cannot be completed for any
   reason, it responds to the client instance with an error field in the
   response message.  This field is either an object or a string.

   When returned as an object, the object contains the following fields:

   code (string):  A single ASCII error code defining the error.  The
      value MUST be defined in the "GNAP Error Codes" registry
      (Section 10.15).  REQUIRED.

   description (string):  A human-readable string description of the
      error intended for the developer of the client.  The value is
      chosen by the implementation.  OPTIONAL.

   This specification defines the following code values:

   "invalid_request":  The request is missing a required parameter,
      includes an invalid parameter value, or is otherwise malformed.

   "invalid_client":  The request was made from a client that was not
      recognized or allowed by the AS, or the client's signature
      validation failed.

   "invalid_interaction":  The client instance has provided an
      interaction reference that is incorrect for this request, or the
      interaction modes in use have expired.

   "invalid_flag":  The flag configuration is not valid.

   "invalid_rotation":  The token rotation request is not valid.

   "key_rotation_not_supported":  The AS does not allow rotation of this
      access token's key.

   "invalid_continuation":  The continuation of the referenced grant
      could not be processed.

   "user_denied":  The RO denied the request.

   "request_denied":  The request was denied for an unspecified reason.

   "unknown_user":  The user presented in the request is not known to
      the AS or does not match the user present during interaction.

   "unknown_interaction":  The interaction integrity could not be
      established.

   "too_fast":  The client instance did not respect the timeout in the
      wait response before the next call.

   "too_many_attempts":  A limit has been reached in the total number of
      reasonable attempts.  This number is either defined statically or
      adjusted based on runtime conditions by the AS.

   Additional error codes can be defined in the "GNAP Error Codes"
   registry (Section 10.15).

   For example, if the RO denied the request while interacting with the
   AS, the AS would return the following error when the client instance
   tries to continue the grant request:

   {
       "error": {
           "code": "user_denied",
           "description": "The RO denied the request"
       }
   }

   Alternatively, the AS MAY choose to only return the error as codes
   and provide the error as a string.  Since the description field is
   not intended to be machine-readable, the following response is
   considered functionally equivalent to the previous example for the
   purposes of the client software's understanding:

   {
       "error": "user_denied"
   }

   If an error state is reached but the grant is in the _pending_ state
   (and therefore the client instance can continue), the AS MAY include
   the continue field in the response along with the error, as defined
   in Section 3.1.  This allows the client instance to modify its
   request for access, potentially leading to prompting the RO again.
   Other fields MUST NOT be included in the response.

4.  Determining Authorization and Consent

   When the client instance makes its initial request (Section 2) to the
   AS for delegated access, it is capable of asking for several
   different kinds of information in response:

   *  the access being requested, in the access_token request parameter

   *  the subject information being requested, in the subject request
      parameter

   *  any additional requested information defined by extensions of this
      protocol

   When the grant request is in the _processing_ state, the AS
   determines what authorizations and consents are required to fulfill
   this requested delegation.  The details of how the AS makes this
   determination are out of scope for this document.  However, there are
   several common patterns defined and supported by GNAP for fulfilling
   these requirements, including information sent by the client
   instance, information gathered through the interaction process, and
   information supplied by external parties.  An individual AS can
   define its own policies and processes for deciding when and how to
   gather the necessary authorizations and consent and how those are
   applied to the grant request.

   To facilitate the AS fulfilling this request, the client instance
   sends information about the actions the client software can take,
   including:

   *  starting interaction with the end user, in the interact request
      parameter

   *  receiving notification that interaction with the RO has concluded,
      in the interact request parameter

   *  any additional capabilities defined by extensions of this protocol

   The client instance can also supply information directly to the AS in
   its request.  The client instance can send several kinds of things,
   including:

   *  the identity of the client instance, known from the keys or
      identifiers in the client request parameter

   *  the identity of the end user, in the user request parameter

   *  any additional information presented by the client instance in the
      request defined by extensions of this protocol

   The AS will process this presented information in the context of the
   client instance's request and can only trust the information as much
   as it trusts the presentation and context of that request.  If the AS
   determines that the information presented in the initial request is
   sufficient for granting the requested access, the AS MAY move the
   grant request to the _approved_ state and return results immediately
   in its response (Section 3) with access tokens and subject
   information.

   If the AS determines that additional runtime authorization is
   required, the AS can either deny the request outright (if there is no
   possible recovery) or move the grant request to the _pending_ state
   and use a number of means at its disposal to gather that
   authorization from the appropriate ROs, including:

   *  starting interaction with the end user facilitated by the client
      software, such as a redirection or user code

   *  challenging the client instance through a challenge-response
      mechanism

   *  requesting that the client instance present specific additional
      information, such as a user's credential or an assertion

   *  contacting an RO through an out-of-band mechanism, such as a push
      notification

   *  executing an auxiliary software process through an out-of-band
      mechanism, such as querying a digital wallet

   The process of gathering authorization and consent in GNAP is left
   deliberately flexible to allow for a wide variety of different
   deployments, interactions, and methodologies.  In this process, the
   AS can gather consent from the RO or apply the RO's policy as
   necessitated by the access that has been requested.  The AS can
   sometimes determine which RO needs to prompt for consent based on
   what has been requested by the client instance, such as a specific RS
   record, an identified subject, or a request requiring specific access
   such as approval by an administrator.  In other cases, the request is
   applied to whichever RO is present at the time of consent gathering.
   This pattern is especially prevalent when the end user is sent to the
   AS for an interactive session, during which the end user takes on the
   role of the RO.  In these cases, the end user is delegating their own
   access as RO to the client instance.

   The client instance can indicate that it is capable of facilitating
   interaction with the end user, another party, or another piece of
   software through its interaction start request (Section 2.5.1).
   Here, the AS usually needs to interact directly with the end user to
   determine their identity, determine their status as an RO, and
   collect their consent.  If the AS has determined that authorization
   is required and the AS can support one or more of the requested
   interaction start methods, the AS returns the associated interaction
   start responses (Section 3.3).  The client instance SHOULD initiate
   one or more of these interaction methods (Section 4.1) in order to
   facilitate the granting of the request.  If more than one interaction
   start method is available, the means by which the client chooses
   which methods to follow are out of scope of this specification.

   After starting interaction, the client instance can then make a
   continuation request (Section 5) either in response to a signal
   indicating the finish of the interaction (Section 4.2), after a time-
   based polling, or through some other method defined by an extension
   of this specification through the "GNAP Interaction Mode Responses"
   registry (Section 10.13).

   If the grant request is not in the _approved_ state, the client
   instance can repeat the interaction process by sending a grant update
   request (Section 5.3) with new interaction methods (Section 2.5).

   The client instance MUST use each interaction method once at most if
   a response can be detected.  The AS MUST handle any interact request
   as a one-time-use mechanism and SHOULD apply suitable timeouts to any
   interaction start methods provided, including user codes and
   redirection URIs.  The client instance SHOULD apply suitable timeouts
   to any interaction finish method.

   In order to support client software deployed in disadvantaged network
   conditions, the AS MAY allow for processing of the same interaction
   method multiple times if the AS can determine that the request is
   from the same party and the results are idempotent.  For example, if
   a client instance launches a redirect to the AS but does not receive
   a response within a reasonable time, the client software can launch
   the redirect again, assuming that it never reached the AS in the
   first place.  However, if the AS in question receives both requests,
   it could mistakenly process them separately, creating an undefined
   state for the client instance.  If the AS can determine that both
   requests come from the same origin or under the same session, and the
   requests both came before any additional state change to the grant
   occurs, the AS can reasonably conclude that the initial response was
   not received and the same response can be returned to the client
   instance.

   If the AS instead has a means of contacting the RO directly, it could
   do so without involving the client instance in its consent-gathering
   process.  For example, the AS could push a notification to a known RO
   and have the RO approve the pending request asynchronously.  These
   interactions can be through an interface of the AS itself (such as a
   hosted web page), through another application (such as something
   installed on the RO's device), through a messaging fabric, or any
   other means.

   When interacting with an RO, the AS can use various strategies to
   determine the authorization of the requested grant, including:

   *  authenticate the RO through a local account or some other means,
      such as federated login

   *  validate the RO through presentation of claims, attributes, or
      other information

   *  prompt the RO for consent for the requested delegation

   *  describe to the RO what information is being released, to whom,
      and for what purpose

   *  provide warnings to the RO about potential attacks or negative
      effects of allowing the information

   *  allow the RO to modify the client instance's requested access,
      including limiting or expanding that access

   *  provide the RO with artifacts such as receipts to facilitate an
      audit trail of authorizations

   *  allow the RO to deny the requested delegation

   The AS is also allowed to request authorization from more than one
   RO, if the AS deems fit.  For example, a medical record might need to
   be released by both an attending nurse and a physician, or both
   owners of a bank account need to sign off on a transfer request.
   Alternatively, the AS could require N of M possible ROs to approve a
   given request.  In some circumstances, the AS could even determine
   that the end user present during the interaction is not the
   appropriate RO for a given request and reach out to the appropriate
   RO asynchronously.

   The RO is also allowed to define an automated policy at the AS to
   determine which kind of end user can get access to the resource and
   under which conditions.  For instance, such a condition might require
   the end user to log in and accept the RO's legal provisions.
   Alternatively, client software could be acting without an end user,
   and the RO's policy allows issuance of access tokens to specific
   instances of that client software without human interaction.

   While all of these cases are supported by GNAP, the details of their
   implementation and the methods for determining which ROs or related
   policies are required for a given request are out of scope for this
   specification.

4.1.  Starting Interaction with the End User

   When a grant request is in the _pending_ state, the interaction start
   methods sent by the client instance can be used to facilitate
   interaction with the end user.  To initiate an interaction start
   method indicated by the interaction start responses (Section 3.3)
   from the AS, the client instance follows the steps defined by that
   interaction start mode.  The actions of the client instance required
   for the interaction start modes defined in this specification are
   described in the following subsections.  Interaction start modes
   defined in extensions to this specification MUST define the expected
   actions of the client software when that interaction start mode is
   used.

   If the client instance does not start an interaction start mode
   within an AS-determined amount of time, the AS MUST reject attempts
   to use the interaction start modes.  If the client instance has
   already begun one interaction start mode and the interaction has been
   successfully completed, the AS MUST reject attempts to use other
   interaction start modes.  For example, if a user code has been
   successfully entered for a grant request, the AS will need to reject
   requests to an arbitrary redirect URI on the same grant request in
   order to prevent an attacker from capturing and altering an active
   authorization process.

4.1.1.  Interaction at a Redirected URI

   When the end user is directed to an arbitrary URI through the
   "redirect" mode (Section 3.3.1), the client instance facilitates
   opening the URI through the end user's web browser.  The client
   instance could launch the URI through the system browser, provide a
   clickable link, redirect the user through HTTP response codes, or
   display the URI in a form the end user can use to launch, such as a
   multidimensional barcode.  In all cases, the URI is accessed with an
   HTTP GET request, and the resulting page is assumed to allow direct
   interaction with the end user through an HTTP user agent.  With this
   method, it is common (though not required) for the RO to be the same
   party as the end user, since the client instance has to communicate
   the redirection URI to the end user.

   In many cases, the URI indicates a web page hosted at the AS,
   allowing the AS to authenticate the end user as the RO and
   interactively provide consent.  The URI value is used to identify the
   grant request being authorized.  If the URI cannot be associated with
   a currently active request, the AS MUST display an error to the RO
   and MUST NOT attempt to redirect the RO back to any client instance,
   even if a redirect finish method is supplied (Section 2.5.2.1).  If
   the URI is not hosted by the AS directly, the means of communication
   between the AS and the service provided by this URI are out of scope
   for this specification.

   The client instance MUST NOT modify the URI when launching it; in
   particular, the client instance MUST NOT add any parameters to the
   URI.  The URI MUST be reachable from the end user's browser, though
   the URI MAY be opened on a separate device from the client instance
   itself.  The URI MUST be accessible from an HTTP GET request, and it
   MUST be protected by HTTPS, be hosted on a server local to the RO's
   browser ("localhost"), or use an application-specific URI scheme that
   is loaded on the end user's device.

4.1.2.  Interaction at the Static User Code URI

   When the end user is directed to enter a short code through the
   "user_code" mode (Section 3.3.3), the client instance communicates
   the user code to the end user and directs the end user to enter that
   code at an associated URI.  The client instance MAY format the user
   code in such a way as to facilitate memorability and transfer of the
   code, so long as this formatting does not alter the value as accepted
   at the user code URI.  For example, a client instance receiving the
   user code "A1BC3DFF" could choose to display this to the user as
   "A1BC 3DFF", breaking up the long string into two shorter strings.

   When processing input codes, the AS MUST transform the input string
   to remove invalid characters.  In the above example, the space in
   between the two parts would be removed upon its entry into the
   interactive form at the user code URI.  Additionally, the AS MUST
   treat user input as case insensitive.  For example, if the user
   inputs the string "a1bc 3DFF", the AS will treat the input the same
   as "A1BC3DFF".  To facilitate this, it is RECOMMENDED that the AS use
   only ASCII letters and numbers as valid characters for the user code.

   It is RECOMMENDED that the AS choose from character values that are
   easily copied and typed without ambiguity.  For example, some glyphs
   have multiple Unicode code points for the same visual character, and
   the end user could potentially type a different character than what
   the AS has returned.  For additional considerations of
   internationalized character strings, see [RFC8264].

   This mode is designed to be used when the client instance is not able
   to communicate or facilitate launching an arbitrary URI.  The
   associated URI could be statically configured with the client
   instance or in the client software's documentation.  As a
   consequence, these URIs SHOULD be short.  The user code URI MUST be
   reachable from the end user's browser, though the URI is usually
   opened on a separate device from the client instance itself.  The URI
   MUST be accessible from an HTTP GET request, and it MUST be protected
   by HTTPS, be hosted on a server local to the RO's browser
   ("localhost"), or use an application-specific URI scheme that is
   loaded on the end user's device.

   In many cases, the URI indicates a web page hosted at the AS,
   allowing the AS to authenticate the end user as the RO and
   interactively provide consent.  The value of the user code is used to
   identify the grant request being authorized.  If the user code cannot
   be associated with a currently active request, the AS MUST display an
   error to the RO and MUST NOT attempt to redirect the RO back to any
   client instance, even if a redirect finish method is supplied
   (Section 2.5.2.1).  If the interaction component at the user code URI
   is not hosted by the AS directly, the means of communication between
   the AS and this URI, including communication of the user code itself,
   are out of scope for this specification.

   When the RO enters this code at the user code URI, the AS MUST
   uniquely identify the pending request that the code was associated
   with.  If the AS does not recognize the entered code, the interaction
   component MUST display an error to the user.  If the AS detects too
   many unrecognized code enter attempts, the interaction component
   SHOULD display an error to the user indicating too many attempts and
   MAY take additional actions such as slowing down the input
   interactions.  The user should be warned as such an error state is
   approached, if possible.

4.1.3.  Interaction at a Dynamic User Code URI

   When the end user is directed to enter a short code through the
   "user_code_uri" mode (Section 3.3.4), the client instance
   communicates the user code and associated URI to the end user and
   directs the end user to enter that code at the URI.  The client
   instance MAY format the user code in such a way as to facilitate
   memorability and transfer of the code, so long as this formatting
   does not alter the value as accepted at the user code URI.  For
   example, a client instance receiving the user code "A1BC3DFF" could
   choose to display this to the user as "A1BC 3DFF", breaking up the
   long string into two shorter strings.

   When processing input codes, the AS MUST transform the input string
   to remove invalid characters.  In the above example, the space in
   between the two parts would be removed upon its entry into the
   interactive form at the user code URI.  Additionally, the AS MUST
   treat user input as case insensitive.  For example, if the user
   inputs the string "a1bc 3DFF", the AS will treat the input the same
   as "A1BC3DFF".  To facilitate this, it is RECOMMENDED that the AS use
   only ASCII letters and numbers as valid characters for the user code.

   This mode is used when the client instance is not able to facilitate
   launching a complex arbitrary URI but can communicate arbitrary
   values like URIs.  As a consequence, these URIs SHOULD be short
   enough to allow the URI to be typed by the end user, such as a total
   length of 20 characters or fewer.  The client instance MUST NOT
   modify the URI when communicating it to the end user; in particular
   the client instance MUST NOT add any parameters to the URI.  The user
   code URI MUST be reachable from the end user's browser, though the
   URI is usually be opened on a separate device from the client
   instance itself.  The URI MUST be accessible from an HTTP GET
   request, and it MUST be protected by HTTPS, be hosted on a server
   local to the RO's browser ("localhost"), or use an application-
   specific URI scheme that is loaded on the end user's device.

   In many cases, the URI indicates a web page hosted at the AS,
   allowing the AS to authenticate the end user as the RO and
   interactively provide consent.  The value of the user code is used to
   identify the grant request being authorized.  If the user code cannot
   be associated with a currently active request, the AS MUST display an
   error to the RO and MUST NOT attempt to redirect the RO back to any
   client instance, even if a redirect finish method is supplied
   (Section 2.5.2.1).  If the interaction component at the user code URI
   is not hosted by the AS directly, the means of communication between
   the AS and this URI, including communication of the user code itself,
   are out of scope for this specification.

   When the RO enters this code at the given URI, the AS MUST uniquely
   identify the pending request that the code was associated with.  If
   the AS does not recognize the entered code, the interaction component
   MUST display an error to the user.  If the AS detects too many
   unrecognized code enter attempts, the interaction component SHOULD
   display an error to the user indicating too many attempts and MAY
   take additional actions such as slowing down the input interactions.
   The user should be warned as such an error state is approached, if
   possible.

4.1.4.  Interaction through an Application URI

   When the client instance is directed to launch an application through
   the "app" mode (Section 3.3.2), the client launches the URI as
   appropriate to the system, such as through a deep link or custom URI
   scheme registered to a mobile application.  The means by which the AS
   and the launched application communicate with each other and perform
   any of the required actions are out of scope for this specification.

4.2.  Post-Interaction Completion

   If an interaction "finish" method (Section 3.3.5) is associated with
   the current request, the AS MUST follow the appropriate method upon
   completion of interaction in order to signal the client instance to
   continue, except for some limited error cases discussed below.  If a
   finish method is not available, the AS SHOULD instruct the RO to
   return to the client instance upon completion.  In such cases, it is
   expected that the client instance will poll the continuation endpoint
   as described in Section 5.2.

   The AS MUST create an interaction reference and associate that
   reference with the current interaction and the underlying pending
   request.  The interaction reference value is an ASCII string
   consisting of only unreserved characters per Section 2.3 of
   [RFC3986].  The interaction reference value MUST be sufficiently
   random so as not to be guessable by an attacker.  The interaction
   reference MUST be one-time-use to prevent interception and replay
   attacks.

   The AS MUST calculate a hash value based on the client instance, AS
   nonces, and the interaction reference, as described in Section 4.2.3.
   The client instance will use this value to validate the "finish"
   call.

   All interaction finish methods MUST define a way to convey the hash
   and interaction reference back to the client instance.  When an
   interaction finish method is used, the client instance MUST present
   the interaction reference back to the AS as part of its continuation
   request (Section 5.1).

   Note that in many error cases, such as when the RO has denied access,
   the "finish" method is still enacted by the AS.  This pattern allows
   the client instance to potentially recover from the error state by
   modifying its request or providing additional information directly to
   the AS in a continuation request.  The AS MUST NOT follow the
   "finish" method in the following circumstances:

   *  The AS has determined that any URIs involved with the finish
      method are dangerous or blocked.

   *  The AS cannot determine which ongoing grant request is being
      referenced.

   *  The ongoing grant request has been canceled or otherwise blocked.

4.2.1.  Completing Interaction with a Browser Redirect to the Callback
        URI

   When using the redirect interaction finish method defined in Sections
   2.5.2.1 and 3.3.5, the AS signals to the client instance that
   interaction is complete and the request can be continued by directing
   the RO (in their browser) back to the client instance's redirect URI.

   The AS secures this redirect by adding the hash and interaction
   reference as query parameters to the client instance's redirect URI.

   hash:  The interaction hash value as described in Section 4.2.3.
      REQUIRED.

   interact_ref:  The interaction reference generated for this
      interaction.  REQUIRED.

   The means of directing the RO to this URI are outside the scope of
   this specification, but common options include redirecting the RO
   from a web page and launching the system browser with the target URI.
   See Section 11.19 for considerations on which HTTP status code to use
   when redirecting a request that potentially contains credentials.

   NOTE: '\' line wrapping per RFC 8792

   https://client.example.net/return/123455\
     ?hash=x-gguKWTj8rQf7d7i3w3UhzvuJ5bpOlKyAlVpLxBffY\
     &interact_ref=4IFWWIKYBC2PQ6U56NL1

   The client instance MUST be able to process a request on the URI.  If
   the URI is HTTP, the request MUST be an HTTP GET.

   When receiving the request, the client instance MUST parse the query
   parameters to extract the hash and interaction reference values.  The
   client instance MUST calculate and validate the hash value as
   described in Section 4.2.3.  If the hash validates, the client
   instance sends a continuation request to the AS as described in
   Section 5.1, using the interaction reference value received here.  If
   the hash does not validate, the client instance MUST NOT send the
   interaction reference to the AS.

4.2.2.  Completing Interaction with a Direct HTTP Request Callback

   When using the push interaction finish method defined in Sections
   2.5.2.1 and 3.3.5, the AS signals to the client instance that
   interaction is complete and the request can be continued by sending
   an HTTP POST request to the client instance's callback URI.

   The HTTP message content is a JSON object consisting of the following
   two fields:

   hash (string):  The interaction hash value as described in
      Section 4.2.3.  REQUIRED.

   interact_ref (string):  The interaction reference generated for this
      interaction.  REQUIRED.

   POST /push/554321 HTTP/1.1
   Host: client.example.net
   Content-Type: application/json

   {
     "hash": "pjdHcrti02HLCwGU3qhUZ3wZXt8IjrV_BtE3oUyOuKNk",
     "interact_ref": "4IFWWIKYBC2PQ6U56NL1"
   }

   Since the AS is making an outbound connection to a URI supplied by an
   outside party (the client instance), the AS MUST protect itself
   against Server-Side Request Forgery (SSRF) attacks when making this
   call, as discussed in Section 11.34.

   When receiving the request, the client instance MUST parse the JSON
   object and validate the hash value as described in Section 4.2.3.  If
   either fails, the client instance MUST return an unknown_interaction
   error (Section 3.6).  If the hash validates, the client instance
   sends a continuation request to the AS as described in Section 5.1,
   using the interaction reference value received here.

4.2.3.  Calculating the Interaction Hash

   The "hash" parameter in the request to the client instance's callback
   URI ties the front-channel response to an ongoing request by using
   values known only to the parties involved.  This security mechanism
   allows the client instance to protect itself against several kinds of
   session fixation and injection attacks as discussed in Section 11.25.
   The AS MUST always provide this hash, and the client instance MUST
   validate the hash when received.

   To calculate the "hash" value, the party doing the calculation
   creates a hash base string by concatenating the following values in
   the following order using a single newline (0x0A) character to
   separate them:

   *  the "nonce" value sent by the client instance in the interaction
      finish field of the initial request (Section 2.5.2)

   *  the AS's nonce value from the interaction finish response
      (Section 3.3.5)

   *  the "interact_ref" returned from the AS as part of the interaction
      finish method (Section 4.2)

   *  the grant endpoint URI the client instance used to make its
      initial request (Section 2)

   There is no padding or whitespace before or after any of the lines
   and no trailing newline character.  The following non-normative
   example shows a constructed hash base string consisting of these four
   elements.

   VJLO6A4CATR0KRO
   MBDOFXG4Y5CVJCX821LH
   4IFWWIKYB2PQ6U56NL1
   https://server.example.com/tx

   The party then hashes the bytes of the ASCII encoding of this string
   with the appropriate algorithm based on the "hash_method" parameter
   under the "finish" key of the interaction finish request
   (Section 2.5.2).  The resulting byte array from the hash function is
   then encoded using URL-Safe base64 with no padding [RFC4648].  The
   resulting string is the hash value.

   If provided, the "hash_method" value MUST be one of the hash name
   strings defined in the IANA "Named Information Hash Algorithm
   Registry" [HASH-ALG].  If the "hash_method" value is not present in
   the client instance's request, the algorithm defaults to "sha-256".

   For example, the "sha-256" hash method consists of hashing the input
   string with the 256-bit SHA2 algorithm.  The following is the encoded
   "sha-256" hash of the hash base string in the example above.

   x-gguKWTj8rQf7d7i3w3UhzvuJ5bpOlKyAlVpLxBffY

   As another example, the "sha3-512" hash method consists of hashing
   the input string with the 512-bit SHA3 algorithm.  The following is
   the encoded "sha3-512" hash of the hash base string in the example
   above.

   NOTE: '\' line wrapping per RFC 8792

   pyUkVJSmpqSJMaDYsk5G8WCvgY91l-agUPe1wgn-cc5rUtN69gPI2-S_s-Eswed8iB4\
     PJ_a5Hg6DNi7qGgKwSQ

5.  Continuing a Grant Request

   While it is possible for the AS to return an approved grant response
   (Section 3) with all the client instance's requested information
   (including access tokens (Section 3.2) and subject information
   (Section 3.4)) immediately, it's more common that the AS will place
   the grant request into the _pending_ state and require communication
   with the client instance several times over the lifetime of a grant
   request.  This is often part of facilitating interaction (Section 4),
   but it could also be used to allow the AS and client instance to
   continue negotiating the parameters of the original grant request
   (Section 2) through modification of the request.

   The ability to continue an already-started request allows the client
   instance to perform several important functions, including presenting
   additional information from interaction, modifying the initial
   request, and revoking a grant request in progress.

   To enable this ongoing negotiation, the AS provides a continuation
   API to the client software.  The AS returns a continue field in the
   response (Section 3.1) that contains information the client instance
   needs to access this API, including a URI to access as well as a
   special access token to use during the requests, called the
   "continuation access token".

   All requests to the continuation API are protected by a bound
   continuation access token.  The continuation access token is bound to
   the same key and method the client instance used to make the initial
   request (or its most recent rotation).  As a consequence, when the
   client instance makes any calls to the continuation URI, the client
   instance MUST present the continuation access token as described in
   Section 7.2 and present proof of the client instance's key (or its
   most recent rotation) by signing the request as described in
   Section 7.3.  The AS MUST validate the signature and ensure that it
   is bound to the appropriate key for the continuation access token.

   Access tokens other than the continuation access tokens MUST NOT be
   usable for continuation requests.  Conversely, continuation access
   tokens MUST NOT be usable to make authorized requests to RSs, even if
   co-located within the AS.

   In the following non-normative example, the client instance makes a
   POST request to a unique URI and signs the request with HTTP message
   signatures:

   POST /continue/KSKUOMUKM HTTP/1.1
   Authorization: GNAP 80UPRY5NM33OMUKMKSKU
   Host: server.example.com
   Content-Length: 0
   Signature-Input: sig1=...
   Signature: sig1=...

   The AS MUST be able to tell from the client instance's request which
   specific ongoing request is being accessed, using a combination of
   the continuation URI and the continuation access token.  If the AS
   cannot determine a single active grant request to map the
   continuation request to, the AS MUST return an invalid_continuation
   error (Section 3.6).

   In the following non-normative example, the client instance makes a
   POST request to a stable continuation endpoint URI with the
   interaction reference (Section 5.1), includes the access token, and
   signs with HTTP message signatures:

   POST /continue HTTP/1.1
   Host: server.example.com
   Content-Type: application/json
   Authorization: GNAP 80UPRY5NM33OMUKMKSKU
   Signature-Input: sig1=...
   Signature: sig1=...
   Content-Digest: sha-256=...

   {
     "interact_ref": "4IFWWIKYBC2PQ6U56NL1"
   }

   In the following non-normative alternative example, the client
   instance had been provided a continuation URI unique to this ongoing
   grant request:

   POST /tx/rxgIIEVMBV-BQUO7kxbsp HTTP/1.1
   Host: server.example.com
   Content-Type: application/json
   Authorization: GNAP eyJhbGciOiJub25lIiwidHlwIjoiYmFkIn0
   Signature-Input: sig1=...
   Signature: sig1=...
   Content-Digest: sha-256=...

   {
     "interact_ref": "4IFWWIKYBC2PQ6U56NL1"
   }

   In both cases, the AS determines which grant is being asked for based
   on the URI and continuation access token provided.

   If a wait parameter was included in the continuation response
   (Section 3.1), the client instance MUST NOT call the continuation URI
   prior to waiting the number of seconds indicated.  If no wait period
   is indicated, the client instance MUST NOT poll immediately and
   SHOULD wait at least 5 seconds.  If the client instance does not
   respect the given wait period, the AS MUST return the too_fast error
   (Section 3.6).

   The response from the AS is a JSON object of a grant response and MAY
   contain any of the fields described in Section 3, as described in
   more detail in the subsections below.

   If the AS determines that the client instance can make further
   requests to the continuation API, the AS MUST include a new
   continuation response (Section 3.1).  The new continuation response
   MUST include a continuation access token as well, and this token
   SHOULD be a new access token, invalidating the previous access token.
   If the AS does not return a new continuation response, the client
   instance MUST NOT make an additional continuation request.  If a
   client instance does so, the AS MUST return an invalid_continuation
   error (Section 3.6).

   For continuation functions that require the client instance to send
   message content, the content MUST be a JSON object.

   For all requests to the grant continuation API, the AS MAY make use
   of long polling mechanisms such as those discussed in [RFC6202].
   That is to say, instead of returning the current status immediately,
   the long polling technique allows the AS additional time to process
   and fulfill the request before returning the HTTP response to the
   client instance.  For example, when the AS receives a continuation
   request but the grant request is in the _processing_ state, the AS
   could wait until the grant request has moved to the _pending_ or
   _approved_ state before returning the response message.

5.1.  Continuing after a Completed Interaction

   When the AS responds to the client instance's finish method as in
   Section 4.2.1, this response includes an interaction reference.  The
   client instance MUST include that value as the field interact_ref in
   a POST request to the continuation URI.

   POST /continue HTTP/1.1
   Host: server.example.com
   Content-Type: application/json
   Authorization: GNAP 80UPRY5NM33OMUKMKSKU
   Signature-Input: sig1=...
   Signature: sig1=...
   Content-Digest: sha-256=...

   {
     "interact_ref": "4IFWWIKYBC2PQ6U56NL1"
   }

   Since the interaction reference is a one-time-use value as described
   in Section 4.2.1, if the client instance needs to make additional
   continuation calls after this request, the client instance MUST NOT
   include the interaction reference in subsequent calls.  If the AS
   detects a client instance submitting an interaction reference when
   the request is not in the _pending_ state, the AS MUST return a
   too_many_attempts error (Section 3.6) and SHOULD invalidate the
   ongoing request by moving it to the _finalized_ state.

   If the grant request is in the _approved_ state, the grant response
   (Section 3) MAY contain any newly created access tokens (Section 3.2)
   or newly released subject information (Section 3.4).  The response
   MAY contain a new continuation response (Section 3.1) as described
   above.  The response SHOULD NOT contain any interaction responses
   (Section 3.3).

   If the grant request is in the _pending_ state, the grant response
   (Section 3) MUST NOT contain access tokens or subject information and
   MAY contain a new interaction response (Section 3.3) to any
   interaction methods that have not been exhausted at the AS.

   For example, if the request is successful in causing the AS to issue
   access tokens and release opaque subject claims, the response could
   look like this:

   NOTE: '\' line wrapping per RFC 8792

   {
       "access_token": {
           "value": "OS9M2PMHKUR64TB8N6BW7OZB8CDFONP219RP1LT0",
           "manage": {
               "uri": "https://server.example.com/token/PRY5NM33O",
               "access_token": {
                   "value": "B8CDFONP21-4TB8N6.BW7ONM"
               }
           }
       },
       "subject": {
           "sub_ids": [ {
             "format": "opaque",
             "id": "J2G8G8O4AZ"
           } ]
       }
   }

   With the above example, the client instance cannot make an additional
   continuation request because a continue field is not included.

   In the following non-normative example, the RO has denied the client
   instance's request, and the AS responds with the following response:

   {
       "error": "user_denied",
       "continue": {
           "access_token": {
               "value": "33OMUKMKSKU80UPRY5NM"
           },
           "uri": "https://server.example.com/continue",
           "wait": 30
       }
   }

   In the preceding example, the AS includes the continue field in the
   response.  Therefore, the client instance can continue the grant
   negotiation process, perhaps modifying the request as discussed in
   Section 5.3.

5.2.  Continuing during Pending Interaction (Polling)

   When the client instance does not include a finish parameter, the
   client instance will often need to poll the AS until the RO has
   authorized the request.  To do so, the client instance makes a POST
   request to the continuation URI as in Section 5.1 but does not
   include message content.

   POST /continue HTTP/1.1
   Host: server.example.com
   Authorization: GNAP 80UPRY5NM33OMUKMKSKU
   Signature-Input: sig1=...
   Signature: sig1=...

   If the grant request is in the _approved_ state, the grant response
   (Section 3) MAY contain any newly created access tokens (Section 3.2)
   or newly released subject claims (Section 3.4).  The response MAY
   contain a new continuation response (Section 3.1) as described above.
   If a continue field is included, it SHOULD include a wait field to
   facilitate a reasonable polling rate by the client instance.  The
   response SHOULD NOT contain interaction responses (Section 3.3).

   If the grant request is in the _pending_ state, the grant response
   (Section 3) MUST NOT contain access tokens or subject information and
   MAY contain a new interaction response (Section 3.3) to any
   interaction methods that have not been exhausted at the AS.

   For example, if the request has not yet been authorized by the RO,
   the AS could respond by telling the client instance to make another
   continuation request in the future.  In the following non-normative
   example, a new, unique access token has been issued for the call,
   which the client instance will use in its next continuation request.

   {
       "continue": {
           "access_token": {
               "value": "33OMUKMKSKU80UPRY5NM"
           },
           "uri": "https://server.example.com/continue",
           "wait": 30
       }
   }

   If the request is successful in causing the AS to issue access tokens
   and release subject information, the response could look like the
   following non-normative example:

   NOTE: '\' line wrapping per RFC 8792

   {
       "access_token": {
           "value": "OS9M2PMHKUR64TB8N6BW7OZB8CDFONP219RP1LT0",
           "manage": {
               "uri": "https://server.example.com/token/PRY5NM33O",
               "access_token": {
                   "value": "B8CDFONP21-4TB8N6.BW7ONM"
               }
           }
       },
       "subject": {
           "sub_ids": [ {
             "format": "opaque",
             "id": "J2G8G8O4AZ"
           } ]
       }
   }

   See Section 11.23 for considerations on polling for continuation
   without an interaction finish method.

   In error conditions, the AS responds to the client instance with an
   error code as discussed in Section 3.6.  For example, if the client
   instance has polled too many times before the RO has approved the
   request, the AS would respond with a message like the following:

   {
       "error": "too_many_attempts"
   }

   Since this response does not include a continue field, the client
   instance cannot continue to poll the AS for additional updates and
   the grant request is _finalized_. If the client instance still needs
   access to the resource, it will need to start with a new grant
   request.

5.3.  Modifying an Existing Request

   The client instance might need to modify an ongoing request, whether
   or not tokens have already been issued or subject information has
   already been released.  In such cases, the client instance makes an
   HTTP PATCH request to the continuation URI and includes any fields it
   needs to modify.  Fields that aren't included in the request are
   considered unchanged from the original request.

   A grant request associated with a modification request MUST be in the
   _approved_ or _pending_ state.  When the AS receives a valid
   modification request, the AS MUST place the grant request into the
   _processing_ state and re-evaluate the authorization in the new
   context created by the update request, since the extent and context
   of the request could have changed.

   The client instance MAY include the access_token and subject fields
   as described in Sections 2.1 and 2.2.  Inclusion of these fields
   override any values in the initial request, which MAY trigger
   additional requirements and policies by the AS.  For example, if the
   client instance is asking for more access, the AS could require
   additional interaction with the RO to gather additional consent.  If
   the client instance is asking for more limited access, the AS could
   determine that sufficient authorization has been granted to the
   client instance and return the more limited access rights
   immediately.  If the grant request was previously in the _approved_
   state, the AS could decide to remember the larger scale of access
   rights associated with the grant request, allowing the client
   instance to make subsequent requests of different subsets of granted
   access.  The details of this processing are out of scope for this
   specification, but a one possible approach is as follows:

   1.  A client instance requests access to Foo, and this is granted by
       the RO.  This results in an access token: AT1.

   2.  The client instance later modifies the grant request to include
       Foo and Bar together.  Since the client instance was previously
       granted Foo under this grant request, the RO is prompted to allow
       the client instance access to Foo and Bar together.  This results
       in a new access token: AT2.  This access token has access to both
       Foo and Bar. The rights of the original access token AT1 are not
       modified.

   3.  The client instance makes another grant modification to ask only
       for Bar. Since the client instance was previously granted Foo and
       Bar together under this grant request, the RO is not prompted,
       and the access to Bar is granted in a new access token: AT3.
       This new access token does not allow access to Foo.

   4.  The original access token AT1 expires, and the client seeks a new
       access token to replace it.  The client instance makes another
       grant modification to ask only for Foo. Since the client instance
       was previously granted Foo and Bar together under this grant
       request, the RO is not prompted, and the access to Foo is granted
       in a new access token: AT4.  This new access token does not allow
       access to Bar.

   All four access tokens are independent of each other and associated
   with the same underlying grant request.  Each of these access tokens
   could possibly also be rotated using token management, if available.
   For example, instead of asking for a new token to replace AT1, the
   client instance could ask for a refresh of AT1 using the rotation
   method of the token management API.  This would result in a refreshed
   AT1 with a different token value and expiration from the original AT1
   but with the same access rights of allowing only access to Foo.

   The client instance MAY include the interact field as described in
   Section 2.5.  Inclusion of this field indicates that the client
   instance is capable of driving interaction with the end user, and
   this field replaces any values from a previous request.  The AS MAY
   respond to any of the interaction responses as described in
   Section 3.3, just like it would to a new request.

   The client instance MAY include the user field as described in
   Section 2.4 to present new assertions or information about the end
   user.  The AS SHOULD check that this presented user information is
   consistent with any user information previously presented by the
   client instance or otherwise associated with this grant request.

   The client instance MUST NOT include the client field of the request,
   since the client instance is assumed not to have changed.
   Modification of client instance information, including rotation of
   keys associated with the client instance, is outside the scope of
   this specification.

   The client instance MUST NOT include post-interaction responses such
   as those described in Section 5.1.

   Modification requests MUST NOT alter previously issued access tokens.
   Instead, any access tokens issued from a continuation are considered
   new, separate access tokens.  The AS MAY revoke previously issued
   access tokens after a modification has occurred.

   If the modified request can be granted immediately by the AS (the
   grant request is in the _approved_ state), the grant response
   (Section 3) MAY contain any newly created access tokens (Section 3.2)
   or newly released subject claims (Section 3.4).  The response MAY
   contain a new continuation response (Section 3.1) as described above.
   If interaction can occur, the response SHOULD contain interaction
   responses (Section 3.3) as well.

   For example, a client instance initially requests a set of resources
   using references:

   POST /tx HTTP/1.1
   Host: server.example.com
   Content-Type: application/json
   Signature-Input: sig1=...
   Signature: sig1=...
   Content-Digest: sha-256=...

   {
       "access_token": {
           "access": [
               "read", "write"
           ]
       },
       "interact": {
           "start": ["redirect"],
           "finish": {
               "method": "redirect",
               "uri": "https://client.example.net/return/123455",
               "nonce": "LKLTI25DK82FX4T4QFZC"
           }
       },
       "client": "987YHGRT56789IOLK"
   }

   Access is granted by the RO, and a token is issued by the AS.  In its
   final response, the AS includes a continue field, which includes a
   separate access token for accessing the continuation API:

   {
       "continue": {
           "access_token": {
               "value": "80UPRY5NM33OMUKMKSKU"
           },
           "uri": "https://server.example.com/continue",
           "wait": 30
       },
       "access_token": {
           "value": "RP1LT0-OS9M2P_R64TB",
           "access": [
               "read", "write"
           ]
       }
   }

   This continue field allows the client instance to make an eventual
   continuation call.  Some time later, the client instance realizes
   that it no longer needs "write" access and therefore modifies its
   ongoing request, here asking for just "read" access instead of both
   "read" and "write" as before.

   PATCH /continue HTTP/1.1
   Host: server.example.com
   Content-Type: application/json
   Authorization: GNAP 80UPRY5NM33OMUKMKSKU
   Signature-Input: sig1=...
   Signature: sig1=...
   Content-Digest: sha-256=...

   {
       "access_token": {
           "access": [
               "read"
           ]
       }
       ...
   }

   The AS replaces the previous access from the first request, allowing
   the AS to determine if any previously granted consent already
   applies.  In this case, the AS would determine that reducing the
   breadth of the requested access means that new access tokens can be
   issued to the client instance without additional interaction or
   consent.  The AS would likely revoke previously issued access tokens
   that had the greater access rights associated with them, unless they
   had been issued with the durable flag.

   {
       "continue": {
           "access_token": {
               "value": "M33OMUK80UPRY5NMKSKU"
           },
           "uri": "https://server.example.com/continue",
           "wait": 30
       },
       "access_token": {
           "value": "0EVKC7-2ZKwZM_6N760",
           "access": [
               "read"
           ]
       }
   }

   As another example, the client instance initially requests read-only
   access but later needs to step up its access.  The initial request
   could look like the following HTTP message:

   POST /tx HTTP/1.1
   Host: server.example.com
   Content-Type: application/json
   Signature-Input: sig1=...
   Signature: sig1=...
   Content-Digest: sha-256=...

   {
       "access_token": {
           "access": [
               "read"
           ]
       },
       "interact": {
           "start": ["redirect"],
           "finish": {
               "method": "redirect",
               "uri": "https://client.example.net/return/123455",
               "nonce": "LKLTI25DK82FX4T4QFZC"
           }
       },
       "client": "987YHGRT56789IOLK"
   }

   Access is granted by the RO, and a token is issued by the AS.  In its
   final response, the AS includes a continue field:

   {
       "continue": {
           "access_token": {
               "value": "80UPRY5NM33OMUKMKSKU"
           },
           "uri": "https://server.example.com/continue",
           "wait": 30
       },
       "access_token": {
           "value": "RP1LT0-OS9M2P_R64TB",
           "access": [
               "read"
           ]
       }
   }

   This allows the client instance to make an eventual continuation
   call.  The client instance later realizes that it now needs "write"
   access in addition to the "read" access.  Since this is an expansion
   of what it asked for previously, the client instance also includes a
   new interaction field in case the AS needs to interact with the RO
   again to gather additional authorization.  Note that the client
   instance's nonce and callback are different from the initial request.
   Since the original callback was already used in the initial exchange
   and the callback is intended for one-time use, a new one needs to be
   included in order to use the callback again.

   PATCH /continue HTTP/1.1
   Host: server.example.com
   Content-Type: application/json
   Authorization: GNAP 80UPRY5NM33OMUKMKSKU
   Signature-Input: sig1=...
   Signature: sig1=...
   Content-Digest: sha-256=...

   {
       "access_token": {
           "access": [
               "read", "write"
           ]
       },
       "interact": {
           "start": ["redirect"],
           "finish": {
               "method": "redirect",
               "uri": "https://client.example.net/return/654321",
               "nonce": "K82FX4T4LKLTI25DQFZC"
           }
       }
   }

   From here, the AS can determine that the client instance is asking
   for more than it was previously granted, but since the client
   instance has also provided a mechanism to interact with the RO, the
   AS can use that to gather the additional consent.  The protocol
   continues as it would with a new request.  Since the old access
   tokens are good for a subset of the rights requested here, the AS
   might decide to not revoke them.  However, any access tokens granted
   after this update process are new access tokens and do not modify the
   rights of existing access tokens.

5.4.  Revoking a Grant Request

   If the client instance wishes to cancel an ongoing grant request and
   place it into the _finalized_ state, the client instance makes an
   HTTP DELETE request to the continuation URI.

   DELETE /continue HTTP/1.1
   Host: server.example.com
   Content-Type: application/json
   Authorization: GNAP 80UPRY5NM33OMUKMKSKU
   Signature-Input: sig1=...
   Signature: sig1=...

   If the request is successfully revoked, the AS responds with HTTP
   status code 204 (No Content).  The AS SHOULD revoke all associated
   access tokens, if possible.  The AS SHOULD disable all token rotation
   and other token management functions on such access tokens, if
   possible.  Once the grant request is in the _finalized_ state, it
   MUST NOT be moved to any other state.

   If the request is not revoked, the AS responds with an
   invalid_continuation error (Section 3.6).

6.  Token Management

   If an access token response includes the manage field as described in
   Section 3.2.1, the client instance MAY call this URI to manage the
   access token with the rotate and revoke actions defined in the
   following subsections.  Other actions are undefined by this
   specification.

   {
       "access_token": {
           "value": "OS9M2PMHKUR64TB8N6BW7OZB8CDFONP219RP1LT0",
           "flags": ["bearer"],
           "manage": {
               "uri": "https://server.example.com/token/PRY5NM33O",
               "access_token": {
                   "value": "B8CDFONP21-4TB8N6.BW7ONM"
               }
           }
       }
   }

   The token management access token issued under the manage field is
   used to protect all calls to the token management API.  The client
   instance MUST present proof of the key associated with the token
   along with the value of the token management access token.

   The AS MUST validate the proof and ensure that it is associated with
   the token management access token.

   The AS MUST uniquely identify the token being managed from the token
   management URI, the token management access token, or a combination
   of both.

6.1.  Rotating the Access Token Value

   If the client instance has an access token and that access token
   expires, the client instance might want to rotate the access token to
   a new value without expiration.  Rotating an access token consists of
   issuing a new access token in place of an existing access token, with
   the same rights and properties as the original token, apart from an
   updated token value and expiration time.

   To rotate an access token, the client instance makes an HTTP POST to
   the token management URI with no message content, sending the access
   token in the authorization header as described in Section 7.2 and
   signing the request with the appropriate key.

   POST /token/PRY5NM33O HTTP/1.1
   Host: server.example.com
   Authorization: GNAP B8CDFONP21-4TB8N6.BW7ONM
   Signature-Input: sig1=...
   Signature: sig1=...
   Content-Digest: sha-256=...

   The client instance cannot request to alter the access rights
   associated with the access token during a rotation request.  To get
   an access token with different access rights for this grant request,
   the client instance has to call the continuation API's update
   functionality (Section 5.3) to get a new access token.  The client
   instance can also create a new grant request with the required access
   rights.

   The AS validates that the token management access token presented is
   associated with the management URI, that the AS issued the token to
   the given client instance, and that the presented key is the correct
   key for the token management access token.  The AS determines which
   access token is being rotated from the token management URI, the
   token management access token, or both.

   If the token is validated and the key is appropriate for the request,
   the AS MUST invalidate the current access token value associated with
   this URI, if possible.  Note that stateless access tokens can make
   proactive revocation difficult within a system; see Section 11.32.

   For successful rotations, the AS responds with an HTTP status code
   200 (OK) with JSON-formatted message content consisting of the
   rotated access token in the access_token field described in
   Section 3.2.1.  The value of the access token MUST NOT be the same as
   the current value of the access token used to access the management
   API.  The response MUST include an access token management URI, and
   the value of this URI MAY be different from the URI used by the
   client instance to make the rotation call.  The client instance MUST
   use this new URI to manage the rotated access token.

   The access rights in the access array for the rotated access token
   MUST be included in the response and MUST be the same as the token
   before rotation.

   NOTE: '\' line wrapping per RFC 8792

   {
       "access_token": {
           "value": "FP6A8H6HY37MH13CK76LBZ6Y1UADG6VEUPEER5H2",
           "manage": {
               "uri": "https://server.example.com/token/PRY5NM33O",
               "access_token": {
                   "value": "B8CDFONP21-4TB8N6.BW7ONM"
               }
           },
           "expires_in": 3600,
           "access": [
               {
                   "type": "photo-api",
                   "actions": [
                       "read",
                       "write",
                       "dolphin"
                   ],
                   "locations": [
                       "https://server.example.net/",
                       "https://resource.local/other"
                   ],
                   "datatypes": [
                       "metadata",
                       "images"
                   ]
               },
               "read", "dolphin-metadata"
           ]
       }
   }

   If the AS is unable or unwilling to rotate the value of the access
   token, the AS responds with an invalid_rotation error (Section 3.6).
   Upon receiving such an error, the client instance MUST consider the
   access token to not have changed its state.

6.1.1.  Binding a New Key to the Rotated Access Token

   If the client instance wishes to bind a new presentation key to an
   access token, the client instance MUST present both the new key and
   the proof of previous key material in the access token rotation
   request.  The client instance makes an HTTP POST as a JSON object
   with the following field:

   key:  The new key value or reference in the format described in
      Section 7.1.  Note that keys passed by value are always public
      keys.  REQUIRED when doing key rotation.

   The proofing method and parameters for the new key MUST be the same
   as those established for the previous key.

   The client instance MUST prove possession of both the currently bound
   key and the newly requested key simultaneously in the rotation
   request.  Specifically, the signature from the previous key MUST
   cover the value or reference of the new key, and the signature of the
   new key MUST cover the signature value of the old key.  The means of
   doing so vary depending on the proofing method in use.  For example,
   the HTTP message signatures proofing method uses multiple signatures
   in the request as described in Section 7.3.1.1.  This is shown in the
   following example.

   POST /token/PRY5NM33O HTTP/1.1
   Host: server.example.com
   Authorization: GNAP B8CDFONP21-4TB8N6.BW7ONM
   Signature-Input: \
     sig1=("@method" "@target-uri" "content-digest" \
           "authorization"),\
     sig2=("@method" "@target-uri" "content-digest" \
           "authorization" "signature";key="sig1" \
           "signature-input";key="sig1")
   Signature: sig1=..., sig2=...
   Content-Digest: sha-256=...

   {
       "key": {
           "proof": "httpsig",
           "jwk": {
               "kty": "RSA",
               "e": "AQAB",
               "kid": "xyz-2",
               "alg": "RS256",
               "n": "kOB5rR4Jv0GMeLaY6_It_r3ORwdf8ci_JtffXyaSx8xY..."
           }
       }
   }

   Failure to present the appropriate proof of either the new key or the
   previous key for the access token, as defined by the proofing method,
   MUST result in an invalid_rotation error code from the AS
   (Section 3.6).

   An attempt to change the proofing method or parameters, including an
   attempt to rotate the key of a bearer token (which has no key), MUST
   result in an invalid_rotation error code returned from the AS
   (Section 3.6).

   If the AS does not allow rotation of the access token's key for any
   reason, including but not limited to lack of permission for this
   client instance or lack of capability by the AS, the AS MUST return a
   key_rotation_not_supported error code (Section 3.6).

6.2.  Revoking the Access Token

   If the client instance wishes to revoke the access token proactively,
   such as when a user indicates to the client instance that they no
   longer wish for it to have access or the client instance application
   detects that it is being uninstalled, the client instance can use the
   token management URI to indicate to the AS that the AS SHOULD
   invalidate the access token for all purposes.

   The client instance makes an HTTP DELETE request to the token
   management URI, presenting the access token and signing the request
   with the appropriate key.

   DELETE /token/PRY5NM33O HTTP/1.1
   Host: server.example.com
   Authorization: GNAP B8CDFONP21-4TB8N6.BW7ONM
   Signature-Input: sig1=...
   Signature: sig1=...

   If the key presented is associated with the token (or the client
   instance, in the case of a bearer token), the AS MUST invalidate the
   access token, if possible, and return an HTTP response code 204.

   204 No Content

   Though the AS MAY revoke an access token at any time for any reason,
   the token management function is specifically for the client
   instance's use.  If the access token has already expired or has been
   revoked through other means, the AS SHOULD honor the revocation
   request to the token management URI as valid, since the end result is
   that the token is still not usable.

7.  Securing Requests from the Client Instance

   In GNAP, the client instance secures its requests to an AS and RS by
   presenting an access token, proof of a key that it possesses (aka, a
   "key proof"), or both an access token and key proof together.

   *  When an access token is used with a key proof, this is a bound
      token request.  This type of request is used for calls to the RS
      as well as the AS during grant negotiation.

   *  When a key proof is used with no access token, this is a non-
      authorized signed request.  This type of request is used for calls
      to the AS to initiate a grant negotiation.

   *  When an access token is used with no key proof, this is a bearer
      token request.  This type of request is used only for calls to the
      RS and only with access tokens that are not bound to any key as
      described in Section 3.2.1.

   *  When neither an access token nor key proof are used, this is an
      unsecured request.  This type of request is used optionally for
      calls to the RS as part of an RS-first discovery process as
      described in Section 9.1.

7.1.  Key Formats

   Several different places in GNAP require the presentation of key
   material by value or by reference.  Key material sent by value is
   sent using a JSON object with several fields described in this
   section.

   All keys are associated with a specific key proofing method.  The
   proofing method associated with the key is indicated using the proof
   field of the key object.

   proof (string or object):  The form of proof that the client instance
      will use when presenting the key.  The valid values of this field
      and the processing requirements for each are detailed in
      Section 7.3.  REQUIRED.

   A key presented by value MUST be a public key and MUST be presented
   in only one supported format, as discussed in Section 11.35.  Note
   that while most formats present the full value of the public key,
   some formats present a value cryptographically derived from the
   public key.  See additional discussion of the presentation of public
   keys in Section 11.7.

   jwk (object):  The public key and its properties represented as a
      JSON Web Key (JWK) [RFC7517].  A JWK MUST contain the alg
      (Algorithm) and kid (Key ID) parameters.  The alg parameter MUST
      NOT be "none".  The x5c (X.509 Certificate Chain) parameter MAY be
      used to provide the X.509 representation of the provided public
      key.  OPTIONAL.

   cert (string):  The Privacy-Enhanced Mail (PEM) serialized value of
      the certificate used to sign the request, with optional internal
      whitespace per [RFC7468].  The PEM header and footer are
      optionally removed.  OPTIONAL.

   cert#S256 (string):  The certificate thumbprint calculated as per
      MTLS for OAuth [RFC8705] in base64url encoding.  Note that this
      format does not include the full public key.  OPTIONAL.

   Additional key formats can be defined in the "GNAP Key Formats"
   registry (Section 10.17).

   The following non-normative example shows a single key presented in
   two different formats.  The example key is intended to be used with
   the HTTP message signatures proofing mechanism (Section 7.3.1), as
   indicated by the httpsig value of the proof field.

   As a JWK:

   "key": {
       "proof": "httpsig",
       "jwk": {
           "kty": "RSA",
           "e": "AQAB",
           "kid": "xyz-1",
           "alg": "RS256",
           "n": "kOB5rR4Jv0GMeLaY6_It_r3ORwdf8ci_JtffXyaSx8xY..."
       }
   }

   As a certificate in PEM format:

   "key": {
       "proof": "httpsig",
       "cert": "MIIEHDCCAwSgAwIBAgIBATANBgkqhkiG9w0BAQsFA..."
   }

   When the key is presented in GNAP, proof of this key material MUST be
   used to bind the request, the nature of which varies with the
   location in the protocol where the key is used.  For a key used as
   part of a client instance's initial request in Section 2.3, the key
   value represents the client instance's public key, and proof of that
   key MUST be presented in that request.  For a key used as part of an
   access token response in Section 3.2.1, the proof of that key MUST be
   used when the client instance later presents the access token to the
   RS.

7.1.1.  Key References

   Keys in GNAP can also be passed by reference such that the party
   receiving the reference will be able to determine the appropriate
   keying material for use in that part of the protocol.  A key
   reference is a single opaque string.

       "key": "S-P4XJQ_RYJCRTSU1.63N3E"

   Keys referenced in this manner MAY be shared symmetric keys.  See the
   additional considerations for symmetric keys in Section 11.7.  The
   key reference MUST NOT contain any unencrypted private or shared
   symmetric key information.

   Keys referenced in this manner MUST be bound to a single proofing
   mechanism.

   The means of dereferencing this reference to a key value and proofing
   mechanism are out of scope for this specification.  Commonly, key
   references are created by the AS and do not necessarily need to be
   understood by the client.  These types of key references are an
   internal reference to the AS, such as an identifier of a record in a
   database.  In other applications, it can be useful to use key
   references that are resolvable by both clients and the AS, which
   could be accomplished by a client publishing a public key at a URI,
   for example.  For interoperability, this method could later be
   described as an extension, but doing so is out of scope for this
   specification.

7.1.2.  Key Protection

   The security of GNAP relies on the cryptographic security of the keys
   themselves.  When symmetric keys are used in GNAP, a key management
   system or secure key derivation mechanism MUST be used to supply the
   keys.  Symmetric keys MUST NOT be a human-memorable password or a
   value derived from one.  Symmetric keys MUST NOT be passed by value
   from the client instance to the AS.

   Additional security considerations apply when rotating keys (see
   Section 11.22).

7.2.  Presenting Access Tokens

   Access tokens are issued to client instances in GNAP to allow the
   client instance to make an authorized call to an API.  The method the
   client instance uses to send an access token depends on whether the
   token is bound to a key and, if so, which proofing method is
   associated with the key.  This information is conveyed by the key
   parameter and the bearer flag in the access token response structure
   (Section 3.2.1).

   If the flags field does not contain the bearer flag and the key is
   absent, the access token MUST be sent using the same key and proofing
   mechanism that the client instance used in its initial request (or
   its most recent rotation).

   If the flags field does not contain the bearer flag and the key value
   is an object as described in Section 7.1, the access token MUST be
   sent using the key and proofing mechanism defined by the value of the
   proof field within the key object.

   The access token MUST be sent using the HTTP Authorization request
   header field and the "GNAP" authorization scheme along with a key
   proof as described in Section 7.3 for the key bound to the access
   token.  For example, an access token bound using HTTP message
   signatures would be sent as follows:

   NOTE: '\' line wrapping per RFC 8792

   GET /stuff HTTP/1.1
   Host: resource.example.com
   Authorization: GNAP 80UPRY5NM33OMUKMKSKU
   Signature-Input: sig1=("@method" "@target-uri" "authorization")\
     ;created=1618884473;keyid="gnap-rsa";nonce="NAOEJF12ER2";tag="gnap"
   Signature: sig1=:FQ+EjWqc38uLFByKa5y+c4WyYYwCTGUhidWKfr5L1Cha8FiPEw\
     DxG7nWttpBLS/B6VLfkZJogPbclySs9MDIsAIJwHnzlcJjwXWR2lfvm2z3X7EkJHm\
     Zp4SmyKOS34luAiKR1xwf32NYFolHmZf/SbHZJuWvQuS4U33C+BbsXz8MflFH1Dht\
     H/C1E5i244gSbdLCPxzABc/Q0NHVSLo1qaouYIvnxXB8OT3K7mwWjsLh1GC5vFThb\
     3XQ363r6f0OPRa4qWHhubR/d/J/lNOjbBdjq9AJ69oqNJ+A2XT+ZCrVasEJE0OBvD\
     auQoiywhb8BMB7+PEINsPk5/8UvaNxbw==:

   If the flags field contains the bearer flag, the access token is a
   bearer token that MUST be sent using the Authorization request header
   field method defined in [RFC6750].

   Authorization: Bearer OS9M2PMHKUR64TB8N6BW7OZB8CDFONP219RP1LT0

   The Form-Encoded Body Parameter and URI Query Parameter methods of
   [RFC6750] MUST NOT be used for GNAP access tokens.

7.3.  Proving Possession of a Key with a Request

   Any keys presented by the client instance to the AS or RS MUST be
   validated as part of the request in which they are presented.  The
   type of binding used is indicated by the proof parameter of the key
   object in Section 7.1.  Key proofing methods are specified either by
   a string, which consists of the key proofing method name on its own,
   or by a JSON object with the required field method:

   method:  The name of the key proofing method to be used.  REQUIRED.

   Individual methods defined as objects MAY define additional
   parameters as members in this object.

   Values for the method defined by this specification are as follows:

   "httpsig" (string or object):  HTTP message signing.  See
      Section 7.3.1.

   "mtls" (string):  MTLS certificate verification.  See Section 7.3.2.

   "jwsd" (string):  A detached JWS signature header.  See
      Section 7.3.3.

   "jws" (string):  Attached JWS Payload.  See Section 7.3.4.

   Additional proofing methods can be defined in the "GNAP Key Proofing
   Methods" registry (Section 10.16).

   Proofing methods MAY be defined as both an object and a string.  For
   example, the httpsig method can be specified as an object with its
   parameters explicitly declared, such as:

   {
       "proof": {
           "method": "httpsig",
           "alg": "ecdsa-p384-sha384",
           "content-digest-alg": "sha-256"
       }
   }

   The httpsig method also defines default behavior when it is passed as
   a string form, using the signature algorithm specified by the
   associated key material and the content digest is calculated using
   sha-256.  This configuration can be selected using the following
   shortened form:

   {
       "proof": "httpsig"
   }

   All key binding methods used by this specification MUST cover all
   relevant portions of the request, including anything that would
   change the nature of the request, to allow for secure validation of
   the request.  Relevant aspects include the URI being called, the HTTP
   method being used, any relevant HTTP headers and values, and the HTTP
   message content itself.  The verifier of the signed message MUST
   validate all components of the signed message to ensure that nothing
   has been tampered with or substituted in a way that would change the
   nature of the request.  Definitions of key binding methods MUST
   enumerate how these requirements are fulfilled.

   When a key proofing mechanism is bound to an access token, the key
   being presented MUST be the key associated with the access token, and
   the access token MUST be covered by the signature method of the
   proofing mechanism.

   The key binding methods in this section MAY be used by other
   components making calls as part of GNAP, such as the extensions
   allowing the RS to make calls to the AS defined in [GNAP-RS].  To
   facilitate this extended use, "signer" and "verifier" are used as
   generic terms in the subsections below.  In the core functions of
   GNAP specified in this document, the "signer" is the client instance,
   and the "verifier" is the AS (for grant requests) or RS (for resource
   requests), as appropriate.

   When used for delegation in GNAP, these key binding mechanisms allow
   the AS to ensure that the keys presented by the client instance in
   the initial request are in control of the party calling any follow-up
   or continuation requests.  To facilitate this requirement, the
   continuation response (Section 3.1) includes an access token bound to
   the client instance's key (Section 2.3), and that key (or its most
   recent rotation) MUST be proved in all continuation requests
   (Section 5).  Token management requests (Section 6) are similarly
   bound to either the access token's own key or, in the case of bearer
   tokens, the client instance's key.

   In the following subsections, unless otherwise noted, the RS256 JSON
   Object Signing and Encryption (JOSE) signature algorithm (defined in
   Section 3.3 of [RFC7518]) is applied using the following RSA key
   (presented here in JWK format):

   NOTE: '\' line wrapping per RFC 8792

   {
       "kid": "gnap-rsa",
       "p": "xS4-YbQ0SgrsmcA7xDzZKuVNxJe3pCYwdAe6efSy4hdDgF9-vhC5gjaRk\
           i1wWuERSMW4Tv44l5HNrL-Bbj_nCJxr_HAOaesDiPn2PnywwEfg3Nv95Nn-\
           eilhqXRaW-tJKEMjDHu_fmJBeemHNZI412gBnXdGzDVo22dvYoxd6GM",
       "kty": "RSA",
       "q": "rVdcT_uy-CD0GKVLGpEGRR7k4JO6Tktc8MEHkC6NIFXihk_6vAIOCzCD6\
           LMovMinOYttpRndKoGTNdJfWlDFDScAs8C5n2y1STCQPRximBY-bw39-aZq\
           JXMxOLyPjzuVgiTOCBIvLD6-8-mvFjXZk_eefD0at6mQ5qV3U1jZt88",
       "d": "FHlhdTF0ozTliDxMBffT6aJVKZKmbbFJOVNten9c3lXKB3ux3NAb_D2dB\
           7inp9EV23oWrDspFtvCvD9dZrXgRKMHofkEpo_SSvBZfgtH-OTkbY_TqtPF\
           FLPKAw0JX5cFPnn4Q2xE4n-dQ7tpRCKl59vZLHBrHShr90zqzFp0AKXU5fj\
           b1gC9LPwsFA2Fd7KXmI1drQQEVq9R-o18Pnn4BGQNQNjO_VkcJTiBmEIVT_\
           KJRPdpVJAmbgnYWafL_hAfeb_dK8p85yurEVF8nCK5oO3EPrqB7IL4UqaEn\
           5Sl3u0j8x5or-xrrAoNz-gdOv7ONfZY6NFoa-3f8q9wBAHUuQ",
       "e": "AQAB",
       "qi": "ogpNEkDKg22Rj9cDV_-PJBZaXMk66Fp557RT1tafIuqJRHEufSOYnsto\
           bWPJ0gHxv1gVJw3gm-zYvV-wTMNgr2wVsBSezSJjPSjxWZtmT2z68W1DuvK\
           kZy15vz7Jd85hmDlriGcXNCoFEUsGLWkpHH9RwPIzguUHWmTt8y0oXyI",
       "dp": "dvCKGI2G7RLh3WyjoJ_Dr6hZ3LhXweB3YcY3qdD9BnxZ71mrLiMQg4c_\
           EBnwqCETN_5sStn2cRc2JXnvLP3G8t7IFKHTT_i_TSTacJ7uT04MSa053Y3\
           RfwbvLjRNPR0UKAE3ZxROUoIaVNuU_6-QMf8-2ilUv2GIOrCN87gP_Vk",
       "alg": "RS256",
       "dq": "iMZmELaKgT9_W_MRT-UfDWtTLeFjIGRW8aFeVmZk9R7Pnyt8rNzyN-IQ\
           M40ql8u8J6vc2GmQGfokLlPQ6XLSCY68_xkTXrhoU1f-eDntkhP7L6XawSK\
           Onv5F2H7wyBQ75HUmHTg8AK2B_vRlMyFKjXbVlzKf4kvqChSGEz4IjQ",
       "n": "hYOJ-XOKISdMMShn_G4W9m20mT0VWtQBsmBBkI2cmRt4Ai8BfYdHsFzAt\
           YKOjpBR1RpKpJmVKxIGNy0g6Z3ad2XYsh8KowlyVy8IkZ8NMwSrcUIBZGYX\
           jHpwjzvfGvXH_5KJlnR3_uRUp4Z4Ujk2bCaKegDn11V2vxE41hqaPUnhRZx\
           e0jRETddzsE3mu1SK8dTCROjwUl14mUNo8iTrTm4n0qDadz8BkPo-uv4BC0\
           bunS0K3bA_3UgVp7zBlQFoFnLTO2uWp_muLEWGl67gBq9MO3brKXfGhi3kO\
           zywzwPTuq-cVQDyEN7aL0SxCb3Hc4IdqDaMg8qHUyObpPitDQ"
   }

   Key proofing methods SHOULD define a mechanism to allow the rotation
   of keys discussed in Section 6.1.1.  Key rotation mechanisms MUST
   define a way for presenting proof of two keys simultaneously with the
   following attributes:

   *  The value of or reference to the new key material MUST be signed
      by the existing key.  Generally speaking, this amounts to using
      the existing key to sign the content of the message that contains
      the new key.

   *  The signature of the old key MUST be signed by the new key.
      Generally speaking, this means including the signature value of
      the old key under the coverage of the new key.

7.3.1.  HTTP Message Signatures

   This method is indicated by the method value httpsig and can be
   declared in either object form or string form.

   When the proofing method is specified in object form, the following
   parameters are defined:

   alg:  The HTTP signature algorithm, from the "HTTP Signature
      Algorithms" registry.  REQUIRED.

   content-digest-alg:  The algorithm used for the Content-Digest field,
      used to protect the content when present in the message.
      REQUIRED.

   This example uses the Elliptic Curve Digital Signature Algorithm
   (ECDSA) signing algorithm over the P384 curve and the SHA-512 hashing
   algorithm for the content digest.

   {
       "proof": {
           "method": "httpsig",
           "alg": "ecdsa-p384-sha384",
           "content-digest-alg": "sha-512"
       }
   }

   When the proofing method is specified in string form, the signing
   algorithm MUST be derived from the key material (such as using the
   JWS algorithm in a JWK formatted key), and the content digest
   algorithm MUST be sha-256.

   {
       "proof": "httpsig"
   }

   When using this method, the signer creates an HTTP message signature
   as described in [RFC9421].  The covered components of the signature
   MUST include the following:

   "@method":  The method used in the HTTP request.

   "@target-uri":  The full request URI of the HTTP request.

   When the message contains request content, the covered components
   MUST also include the following:

   "content-digest":  The Content-Digest header as defined in [RFC9530].
      When the request message has content, the signer MUST calculate
      this field value and include the field in the request.  The
      verifier MUST validate this field value.  REQUIRED when the
      message request contains message content.

   When the request is bound to an access token, the covered components
   MUST also include the following:

   "authorization":  The Authorization header used to present the access
      token as discussed in Section 7.2.

   Other message components MAY also be included.

   The signer MUST include the tag signature parameter with the value
   gnap, and the verifier MUST verify that the parameter exists with
   this value.  The signer MUST include the created signature parameter
   with a timestamp of when the signature was created, and the verifier
   MUST ensure that the creation timestamp is sufficiently close to the
   current time given expected network delay and clock skew.  The signer
   SHOULD include the nonce parameter with a unique and unguessable
   value.  When included, the verifier MUST determine that the nonce
   value is unique within a reasonably short time period such as several
   minutes.

   If the signer's key presented is a JWK, the keyid parameter of the
   signature MUST be set to the kid value of the JWK, and the signing
   algorithm used MUST be the JWS algorithm denoted by the key's alg
   field of the JWK.

   The explicit alg signature parameter MUST NOT be included in the
   signature, since the algorithm will be derived from either the key
   material or the proof value.

   In the following non-normative example, the message content is a JSON
   object:

   NOTE: '\' line wrapping per RFC 8792

   {
       "access_token": {
           "access": [
               "dolphin-metadata"
           ]
       },
       "interact": {
           "start": ["redirect"],
           "finish": {
               "method": "redirect",
               "uri": "https://client.foo/callback",
               "nonce": "VJLO6A4CAYLBXHTR0KRO"
           }
       },
       "client": {
         "key": {
           "proof": "httpsig",
           "jwk": {
               "kid": "gnap-rsa",
               "kty": "RSA",
               "e": "AQAB",
               "alg": "PS512",
               "n": "hYOJ-XOKISdMMShn_G4W9m20mT0VWtQBsmBBkI2cmRt4Ai8Bf\
     YdHsFzAtYKOjpBR1RpKpJmVKxIGNy0g6Z3ad2XYsh8KowlyVy8IkZ8NMwSrcUIBZG\
     YXjHpwjzvfGvXH_5KJlnR3_uRUp4Z4Ujk2bCaKegDn11V2vxE41hqaPUnhRZxe0jR\
     ETddzsE3mu1SK8dTCROjwUl14mUNo8iTrTm4n0qDadz8BkPo-uv4BC0bunS0K3bA_\
     3UgVp7zBlQFoFnLTO2uWp_muLEWGl67gBq9MO3brKXfGhi3kOzywzwPTuq-cVQDyE\
     N7aL0SxCb3Hc4IdqDaMg8qHUyObpPitDQ"
           }
         }
         "display": {
           "name": "My Client Display Name",
           "uri": "https://client.foo/"
         },
       }
   }

   This content is hashed for the Content-Digest header using sha-256
   into the following encoded value:

   sha-256=:q2XBmzRDCREcS2nWo/6LYwYyjrlN1bRfv+HKLbeGAGg=:

   The HTTP message signature input string is calculated to be the
   following:

   NOTE: '\' line wrapping per RFC 8792

   "@method": POST
   "@target-uri": https://server.example.com/gnap
   "content-digest": \
     sha-256=:q2XBmzRDCREcS2nWo/6LYwYyjrlN1bRfv+HKLbeGAGg=:
   "content-length": 988
   "content-type": application/json
   "@signature-params": ("@method" "@target-uri" "content-digest" \
     "content-length" "content-type");created=1618884473\
     ;keyid="gnap-rsa";nonce="NAOEJF12ER2";tag="gnap"

   This leads to the following full HTTP message request:

   NOTE: '\' line wrapping per RFC 8792

   POST /gnap HTTP/1.1
   Host: server.example.com
   Content-Type: application/json
   Content-Length: 988
   Content-Digest: sha-256=:q2XBmzRDCREcS2nWo/6LYwYyjrlN1bRfv+HKLbeGAG\
     g=:
   Signature-Input: sig1=("@method" "@target-uri" "content-digest" \
     "content-length" "content-type");created=1618884473\
     ;keyid="gnap-rsa";nonce="NAOEJF12ER2";tag="gnap"
   Signature: sig1=:c2uwTa6ok3iHZsaRKl1ediKlgd5cCAYztbym68XgX8gSOgK0Bt\
     +zLJ19oGjSAHDjJxX2gXP2iR6lh9bLMTfPzbFVn4Eh+5UlceP+0Z5mES7v0R1+eHe\
     OqBl0YlYKaSQ11YT7n+cwPnCSdv/6+62m5zwXEEftnBeA1ECorfTuPtau/yrTYEvD\
     9A/JqR2h9VzAE17kSlSSsDHYA6ohsFqcRJavX29duPZDfYgkZa76u7hJ23yVxoUpu\
     2J+7VUdedN/72N3u3/z2dC8vQXbzCPTOiLru12lb6vnBZoDbUGsRR/zHPauxhj9T+\
     218o5+tgwYXw17othJSxIIOZ9PkIgz4g==:

   {
       "access_token": {
           "access": [
               "dolphin-metadata"
           ]
       },
       "interact": {
           "start": ["redirect"],
           "finish": {
               "method": "redirect",
               "uri": "https://client.foo/callback",
               "nonce": "VJLO6A4CAYLBXHTR0KRO"
           }
       },
       "client": {
         "key": {
           "proof": "httpsig",
           "jwk": {
               "kid": "gnap-rsa",
               "kty": "RSA",
               "e": "AQAB",
               "alg": "PS512",
               "n": "hYOJ-XOKISdMMShn_G4W9m20mT0VWtQBsmBBkI2cmRt4Ai8Bf\
     YdHsFzAtYKOjpBR1RpKpJmVKxIGNy0g6Z3ad2XYsh8KowlyVy8IkZ8NMwSrcUIBZG\
     YXjHpwjzvfGvXH_5KJlnR3_uRUp4Z4Ujk2bCaKegDn11V2vxE41hqaPUnhRZxe0jR\
     ETddzsE3mu1SK8dTCROjwUl14mUNo8iTrTm4n0qDadz8BkPo-uv4BC0bunS0K3bA_\
     3UgVp7zBlQFoFnLTO2uWp_muLEWGl67gBq9MO3brKXfGhi3kOzywzwPTuq-cVQDyE\
     N7aL0SxCb3Hc4IdqDaMg8qHUyObpPitDQ"
           }
         }
         "display": {
           "name": "My Client Display Name",
           "uri": "https://client.foo/"
         },
       }
   }

   The verifier MUST ensure that the signature covers all required
   message components.  If the HTTP message includes content, the
   verifier MUST calculate and verify the value of the Content-Digest
   header.  The verifier MUST validate the signature against the
   expected key of the signer.

   A received message MAY include multiple signatures, each with its own
   label.  The verifier MUST examine all included signatures until it
   finds (at least) one that is acceptable according to its policy and
   meets the requirements in this section.

7.3.1.1.  Key Rotation Using HTTP Message Signatures

   When rotating a key using HTTP message signatures, the message, which
   includes the new public key value or reference, is first signed with
   the old key following all of the requirements in Section 7.3.1.  The
   message is then signed again with the new key by following all of the
   requirements in Section 7.3.1 again, with the following additional
   requirements:

   *  The covered components MUST include the Signature and Signature-
      Input values from the signature generated with the old key.

   *  The tag value MUST be gnap-rotate.

   For example, the following request to the token management endpoint
   for rotating a token value contains the new key in the request.  The
   message is first signed using the old key, and the resulting
   signature is placed in "old-key":

   NOTE: '\' line wrapping per RFC 8792

   POST /token/PRY5NM33 HTTP/1.1
   Host: server.example.com
   Authorization: GNAP 4398.34-12-asvDa.a
   Content-Digest: sha-512=:Fb/A5vnawhuuJ5xk2RjGrbbxr6cvinZqd4+JPY85u/\
     JNyTlmRmCOtyVhZ1Oz/cSS4tsYen6fzpCwizy6UQxNBQ==:
   Signature-Input: old-key=("@method" "@target-uri" "content-digest" \
     "authorization");created=1618884475;keyid="test-key-ecc-p256"\
     ;tag="gnap"
   Signature: old-key=:vN4IKYsJl2RLFe+tYEm4dHM4R4BToqx5D2FfH4ge5WOkgxo\
     dI2QRrjB8rysvoSEGvAfiVJOWsGcPD1lU639Amw==:

   {
       "key": {
           "proof": "httpsig",
           "jwk": {
               "kty": "RSA",
               "e": "AQAB",
               "kid": "xyz-2",
               "alg": "RS256",
               "n": "kOB5rR4Jv0GMeLaY6_It_r3ORwdf8ci_JtffXyaSx8xY..."
           }
       }
   }

   The signer then creates a new signature using the new key, adding the
   signature input and value to the signature base.

   NOTE: '\' line wrapping per RFC 8792

   "@method": POST
   "@target-uri": https://server.example.com/token/PRY5NM33
   "content-digest": sha-512=:Fb/A5vnawhuuJ5xk2RjGrbbxr6cvinZqd4+JPY85\
     u/JNyTlmRmCOtyVhZ1Oz/cSS4tsYen6fzpCwizy6UQxNBQ==:
   "authorization": GNAP 4398.34-12-asvDa.a
   "signature";key="old-key": :YdDJjDn2Sq8FR82e5IcOLWmmf6wILoswlnRcz+n\
     M+e8xjFDpWS2YmiMYDqUdri2UiJsZx63T1z7As9Kl6HTGkQ==:
   "signature-input";key="old-key": ("@method" "@target-uri" \
     "content-digest" "authorization");created=1618884475\
     ;keyid="test-key-ecc-p256";tag="gnap"
   "@signature-params": ("@method" "@target-uri" "content-digest" \
     "authorization" "signature";key="old-key" "signature-input"\
     ;key="old-key");created=1618884480;keyid="xyz-2"
     ;tag="gnap-rotate"

   This signature is then added to the message:

   NOTE: '\' line wrapping per RFC 8792

   POST /token/PRY5NM33 HTTP/1.1
   Host: server.example.com
   Authorization: GNAP 4398.34-12-asvDa.a
   Content-Digest: sha-512=:Fb/A5vnawhuuJ5xk2RjGrbbxr6cvinZqd4+JPY85u/\
     JNyTlmRmCOtyVhZ1Oz/cSS4tsYen6fzpCwizy6UQxNBQ==:
   Signature-Input: old-key=("@method" "@target-uri" "content-digest" \
       "authorization");created=1618884475;keyid="test-key-ecc-p256"\
       ;tag="gnap", \
     new-key=("@method" "@target-uri" "content-digest" \
       "authorization" "signature";key="old-key" "signature-input"\
       ;key="old-key");created=1618884480;keyid="xyz-2"
       ;tag="gnap-rotate"
   Signature: old-key=:vN4IKYsJl2RLFe+tYEm4dHM4R4BToqx5D2FfH4ge5WOkgxo\
       dI2QRrjB8rysvoSEGvAfiVJOWsGcPD1lU639Amw==:, \
     new-key=:VWUExXQ0geWeTUKhCfDT7WJyT++OHSVbfPA1ukW0o7mmstdbvIz9iOuH\
       DRFzRBm0MQPFVMpLDFXQdE3vi2SL3ZjzcX2qLwzAtyRB9+RsV2caAA80A5ZGMoo\
       gUsKPk4FFDN7KRUZ0vT9Mo9ycx9Dq/996TOWtAmq5z0YUYEwwn+T6+NcW8rFtms\
       s1ZfXG0EoAfV6ve25p+x40Y1rvDHsfkakTRB4J8jWVDybSe39tjIKQBo3uicDVw\
       twewBMNidIa+66iF3pWj8w9RSb0cncEgvbkHgASqaZeXmxxG4gM8p1HH9v/OqQT\
       Oggm5gTWmCQs4oxEmWsfTOxefunfh3X+Qw==:

   {
       "key": {
           "proof": "httpsig",
           "jwk": {
               "kty": "RSA",
               "e": "AQAB",
               "kid": "xyz-2",
               "alg": "RS256",
               "n": "kOB5rR4Jv0GMeLaY6_It_r3ORwdf8ci_JtffXyaSx8xY..."
           }
       }
   }

   The verifier MUST validate both signatures before processing the
   request for key rotation.

7.3.2.  Mutual TLS

   This method is indicated by the method value mtls in string form.

   {
       "proof": "mtls"
   }

   The signer presents its TLS client certificate during TLS negotiation
   with the verifier.

   In the following non-normative example, the certificate is
   communicated to the application through the Client-Cert header field
   from a TLS reverse proxy as per [RFC9440], leading to the following
   full HTTP request message:

   POST /gnap HTTP/1.1
   Host: server.example.com
   Content-Type: application/jose
   Content-Length: 1567
   Client-Cert: \
     :MIIC6jCCAdKgAwIBAgIGAXjw74xPMA0GCSqGSIb3DQEBCwUAMDYxNDAyBgNVBAMM\
     K05JWU15QmpzRGp5QkM5UDUzN0Q2SVR6a3BEOE50UmppOXlhcEV6QzY2bVEwHhcN\
     MjEwNDIwMjAxODU0WhcNMjIwMjE0MjAxODU0WjA2MTQwMgYDVQQDDCtOSVlNeUJq\
     c0RqeUJDOVA1MzdENklUemtwRDhOdFJqaTl5YXBFekM2Nm1RMIIBIjANBgkqhkiG\
     9w0BAQEFAAOCAQ8AMIIBCgKCAQEAhYOJ+XOKISdMMShn/G4W9m20mT0VWtQBsmBB\
     kI2cmRt4Ai8BfYdHsFzAtYKOjpBR1RpKpJmVKxIGNy0g6Z3ad2XYsh8KowlyVy8I\
     kZ8NMwSrcUIBZGYXjHpwjzvfGvXH/5KJlnR3/uRUp4Z4Ujk2bCaKegDn11V2vxE4\
     1hqaPUnhRZxe0jRETddzsE3mu1SK8dTCROjwUl14mUNo8iTrTm4n0qDadz8BkPo+\
     uv4BC0bunS0K3bA/3UgVp7zBlQFoFnLTO2uWp/muLEWGl67gBq9MO3brKXfGhi3k\
     OzywzwPTuq+cVQDyEN7aL0SxCb3Hc4IdqDaMg8qHUyObpPitDQIDAQABMA0GCSqG\
     SIb3DQEBCwUAA4IBAQBnYFK0eYHy+hVf2D58usj39lhL5znb/q9G35GBd/XsWfCE\
     wHuLOSZSUmG71bZtrOcx0ptle9bp2kKl4HlSTTfbtpuG5onSa3swRNhtKtUy5NH9\
     W/FLViKWfoPS3kwoEpC1XqKY6l7evoTCtS+kTQRSrCe4vbNprCAZRxz6z1nEeCgu\
     NMk38yTRvx8ihZpVOuU+Ih+dOtVe/ex5IAPYxlQsvtfhsUZqc7IyCcy72WHnRHlU\
     fn3pJm0S5270+Yls3Iv6h3oBAP19i906UjiUTNH3g0xMW+V4uLxgyckt4wD4Mlyv\
     jnaQ7Z3sR6EsXMocAbXHIAJhwKdtU/fLgdwL5vtx:


   {
       "access_token": {
           "access": [
               "dolphin-metadata"
           ]
       },
       "interact": {
           "start": ["redirect"],
           "finish": {
               "method": "redirect",
               "uri": "https://client.foo/callback",
               "nonce": "VJLO6A4CAYLBXHTR0KRO"
           }
       },
       "client": {
         "key": {
           "proof": "mtls",
           "cert": "MIIC6jCCAdKgAwIBAgIGAXjw74xPMA0GCSqGSIb3DQEBCwUAMD\
     YxNDAyBgNVBAMMK05JWU15QmpzRGp5QkM5UDUzN0Q2SVR6a3BEOE50UmppOXlhcEV\
     6QzY2bVEwHhcNMjEwNDIwMjAxODU0WhcNMjIwMjE0MjAxODU0WjA2MTQwMgYDVQQD\
     DCtOSVlNeUJqc0RqeUJDOVA1MzdENklUemtwRDhOdFJqaTl5YXBFekM2Nm1RMIIBI\
     jANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAhYOJ+XOKISdMMShn/G4W9m20mT\
     0VWtQBsmBBkI2cmRt4Ai8BfYdHsFzAtYKOjpBR1RpKpJmVKxIGNy0g6Z3ad2XYsh8\
     KowlyVy8IkZ8NMwSrcUIBZGYXjHpwjzvfGvXH/5KJlnR3/uRUp4Z4Ujk2bCaKegDn\
     11V2vxE41hqaPUnhRZxe0jRETddzsE3mu1SK8dTCROjwUl14mUNo8iTrTm4n0qDad\
     z8BkPo+uv4BC0bunS0K3bA/3UgVp7zBlQFoFnLTO2uWp/muLEWGl67gBq9MO3brKX\
     fGhi3kOzywzwPTuq+cVQDyEN7aL0SxCb3Hc4IdqDaMg8qHUyObpPitDQIDAQABMA0\
     GCSqGSIb3DQEBCwUAA4IBAQBnYFK0eYHy+hVf2D58usj39lhL5znb/q9G35GBd/Xs\
     WfCEwHuLOSZSUmG71bZtrOcx0ptle9bp2kKl4HlSTTfbtpuG5onSa3swRNhtKtUy5\
     NH9W/FLViKWfoPS3kwoEpC1XqKY6l7evoTCtS+kTQRSrCe4vbNprCAZRxz6z1nEeC\
     guNMk38yTRvx8ihZpVOuU+Ih+dOtVe/ex5IAPYxlQsvtfhsUZqc7IyCcy72WHnRHl\
     Ufn3pJm0S5270+Yls3Iv6h3oBAP19i906UjiUTNH3g0xMW+V4uLxgyckt4wD4Mlyv\
     jnaQ7Z3sR6EsXMocAbXHIAJhwKdtU/fLgdwL5vtx"
         }
         "display": {
           "name": "My Client Display Name",
           "uri": "https://client.foo/"
         },
       },
       "subject": {
           "formats": ["iss_sub", "opaque"]
       }
   }

   The verifier compares the TLS client certificate presented during
   MTLS negotiation to the expected key of the signer.  Since the TLS
   connection covers the entire message, there are no additional
   requirements to check.

   Note that in many instances, the verifier will not do a full
   certificate chain validation of the presented TLS client certificate,
   as the means of trust for this certificate could be in something
   other than a PKI system, such as a static registration or trust-on-
   first-use.  See Sections 11.3 and 11.4 for some additional
   considerations for this key proofing method.

7.3.2.1.  Key Rotation Using MTLS

   Since it is not possible to present two client authenticated
   certificates to a MTLS connection simultaneously, dynamic key
   rotation for this proofing method is not defined.  Instead, key
   rotation for MTLS-based client instances is expected to be managed
   through deployment practices, as discussed in Section 11.4.

7.3.3.  Detached JWS

   This method is indicated by the method value jwsd in string form.

   {
       "proof": "jwsd"
   }

   The signer creates a JSON Web Signature (JWS) [RFC7515] object as
   follows.

   To protect the request, the JOSE header of the signature contains the
   following claims:

   kid (string):  The key identifier.  REQUIRED if the key is presented
      in JWK format.  This MUST be the value of the kid field of the
      key.

   alg (string):  The algorithm used to sign the request.  The algorithm
      MUST be appropriate to the key presented.  If the key is presented
      as a JWK, this MUST be equal to the alg parameter of the key.  The
      algorithm MUST NOT be none.  REQUIRED.

   typ (string):  The type header, value "gnap-binding-jwsd".  REQUIRED.

   htm (string):  The HTTP method used to make this request, as a case-
      sensitive ASCII string.  Note that most public HTTP methods are in
      uppercase ASCII by convention.  REQUIRED.

   uri (string):  The HTTP URI used for this request.  This value MUST
      be an absolute URI, including all path and query components and no
      fragment components.  REQUIRED.

   created (integer):  A timestamp of when the signature was created, in
      integer seconds since UNIX Epoch.  REQUIRED.

   When the request is bound to an access token, the JOSE header MUST
   also include the following:

   ath (string):  The hash of the access token.  The value MUST be the
      result of base64url encoding (with no padding) the SHA-256 digest
      of the ASCII encoding of the associated access token's value.
      REQUIRED.

   If the HTTP request has content (such as an HTTP POST or PUT method),
   the payload of the JWS object is the base64url encoding (without
   padding) of the SHA-256 digest of the bytes of the content.  If the
   request being made does not have content (such as an HTTP GET,
   OPTIONS, or DELETE method), the JWS signature is calculated over an
   empty payload.

   The signer presents the signed object in compact form [RFC7515] in
   the Detached-JWS header field.

   In the following non-normative example, the JOSE header contains the
   following parameters:

   {
       "alg": "RS256",
       "kid": "gnap-rsa",
       "uri": "https://server.example.com/gnap",
       "htm": "POST",
       "typ": "gnap-binding-jwsd",
       "created": 1618884475
   }

   The request content is the following JSON object:

   NOTE: '\' line wrapping per RFC 8792

   {
       "access_token": {
           "access": [
               "dolphin-metadata"
           ]
       },
       "interact": {
           "start": ["redirect"],
           "finish": {
               "method": "redirect",
               "uri": "https://client.foo/callback",
               "nonce": "VJLO6A4CAYLBXHTR0KRO"
           }
       },
       "client": {
         "key": {
           "proof": "jwsd",
           "jwk": {
               "kid": "gnap-rsa",
               "kty": "RSA",
               "e": "AQAB",
               "alg": "RS256",
               "n": "hYOJ-XOKISdMMShn_G4W9m20mT0VWtQBsmBBkI2cmRt4Ai8Bf\
     YdHsFzAtYKOjpBR1RpKpJmVKxIGNy0g6Z3ad2XYsh8KowlyVy8IkZ8NMwSrcUIBZG\
     YXjHpwjzvfGvXH_5KJlnR3_uRUp4Z4Ujk2bCaKegDn11V2vxE41hqaPUnhRZxe0jR\
     ETddzsE3mu1SK8dTCROjwUl14mUNo8iTrTm4n0qDadz8BkPo-uv4BC0bunS0K3bA_\
     3UgVp7zBlQFoFnLTO2uWp_muLEWGl67gBq9MO3brKXfGhi3kOzywzwPTuq-cVQDyE\
     N7aL0SxCb3Hc4IdqDaMg8qHUyObpPitDQ"
           }
         }
         "display": {
           "name": "My Client Display Name",
           "uri": "https://client.foo/"
         },
       }
   }

   This is hashed to the following base64-encoded value:

   PGiVuOZUcN1tRtUS6tx2b4cBgw9mPgXG3IPB3wY7ctc

   This leads to the following full HTTP request message:

   NOTE: '\' line wrapping per RFC 8792

   POST /gnap HTTP/1.1
   Host: server.example.com
   Content-Type: application/json
   Content-Length: 983
   Detached-JWS: eyJhbGciOiJSUzI1NiIsImNyZWF0ZWQiOjE2MTg4ODQ0NzUsImh0b\
     SI6IlBPU1QiLCJraWQiOiJnbmFwLXJzYSIsInR5cCI6ImduYXAtYmluZGluZytqd3\
     NkIiwidXJpIjoiaHR0cHM6Ly9zZXJ2ZXIuZXhhbXBsZS5jb20vZ25hcCJ9.PGiVuO\
     ZUcN1tRtUS6tx2b4cBgw9mPgXG3IPB3wY7ctc.fUq-SV-A1iFN2MwCRW_yolVtT2_\
     TZA2h5YeXUoi5F2Q2iToC0Tc4drYFOSHIX68knd68RUA7yHqCVP-ZQEd6aL32H69e\
     9zuMiw6O_s4TBKB3vDOvwrhYtDH6fX2hP70cQoO-47OwbqP-ifkrvI3hVgMX9TfjV\
     eKNwnhoNnw3vbu7SNKeqJEbbwZfpESaGepS52xNBlDNMYBQQXxM9OqKJaXffzLFEl\
     -Xe0UnfolVtBraz3aPrPy1C6a4uT7wLda3PaTOVtgysxzii3oJWpuz0WP5kRujzDF\
     wX_EOzW0jsjCSkL-PXaKSpZgEjNjKDMg9irSxUISt1C1T6q3SzRgfuQ


   {
       "access_token": {
           "access": [
               "dolphin-metadata"
           ]
       },
       "interact": {
           "start": ["redirect"],
           "finish": {
               "method": "redirect",
               "uri": "https://client.foo/callback",
               "nonce": "VJLO6A4CAYLBXHTR0KRO"
           }
       },
       "client": {
         "key": {
           "proof": "jwsd",
           "jwk": {
               "kid": "gnap-rsa",
               "kty": "RSA",
               "e": "AQAB",
               "alg": "RS256",
               "n": "hYOJ-XOKISdMMShn_G4W9m20mT0VWtQBsmBBkI2cmRt4Ai8Bf\
     YdHsFzAtYKOjpBR1RpKpJmVKxIGNy0g6Z3ad2XYsh8KowlyVy8IkZ8NMwSrcUIBZG\
     YXjHpwjzvfGvXH_5KJlnR3_uRUp4Z4Ujk2bCaKegDn11V2vxE41hqaPUnhRZxe0jR\
     ETddzsE3mu1SK8dTCROjwUl14mUNo8iTrTm4n0qDadz8BkPo-uv4BC0bunS0K3bA_\
     3UgVp7zBlQFoFnLTO2uWp_muLEWGl67gBq9MO3brKXfGhi3kOzywzwPTuq-cVQDyE\
     N7aL0SxCb3Hc4IdqDaMg8qHUyObpPitDQ"
           }
         }
         "display": {
           "name": "My Client Display Name",
           "uri": "https://client.foo/"
         },
       }
   }

   When the verifier receives the Detached-JWS header, it MUST parse and
   validate the JWS object.  The signature MUST be validated against the
   expected key of the signer.  If the HTTP message request contains
   content, the verifier MUST calculate the hash of the content just as
   the signer does, with no normalization or transformation of the
   request.  All required fields MUST be present, and their values MUST
   be valid.  All fields MUST match the corresponding portions of the
   HTTP message.  For example, the htm field of the JWS header has to be
   the same as the HTTP verb used in the request.

   Note that this proofing method depends on a specific cryptographic
   algorithm, SHA-256, in two ways: 1) the ath hash algorithm is
   hardcoded and 2) the payload of the detached/attached signature is
   computed using a hardcoded hash.  A future version of this document
   may address crypto-agility for both these uses by replacing ath with
   a new header that upgrades the algorithm and possibly defining a new
   JWS header that indicates the HTTP content's hash method.

7.3.3.1.  Key Rotation Using Detached JWS

   When rotating a key using detached JWS, the message, which includes
   the new public key value or reference, is first signed with the old
   key as described above using a JWS object with typ header value
   "gnap-binding-rotation-jwsd".  The value of the JWS object is then
   taken as the payload of a new JWS object, to be signed by the new key
   using the parameters above.

   The value of the new JWS object is sent in the Detached-JWS header.

7.3.4.  Attached JWS

   This method is indicated by the method value jws in string form.

   {
       "proof": "jws"
   }

   The signer creates a JWS [RFC7515] object as follows.

   To protect the request, the JWS header contains the following claims:

   kid (string):  The key identifier.  REQUIRED if the key is presented
      in JWK format.  This MUST be the value of the kid field of the
      key.

   alg (string):  The algorithm used to sign the request.  MUST be
      appropriate to the key presented.  If the key is presented as a
      JWK, this MUST be equal to the alg parameter of the key.  MUST NOT
      be none.  REQUIRED.

   typ (string):  The type header, value "gnap-binding-jws".  REQUIRED.

   htm (string):  The HTTP method used to make this request, as a case-
      sensitive ASCII string.  (Note that most public HTTP methods are
      in uppercase.)  REQUIRED.

   uri (string):  The HTTP URI used for this request, including all path
      and query components and no fragment components.  REQUIRED.

   created (integer):  A timestamp of when the signature was created, in
      integer seconds since UNIX Epoch.  REQUIRED.

   When the request is bound to an access token, the JOSE header MUST
   also include the following:

   ath (string):  The hash of the access token.  The value MUST be the
      result of base64url encoding (with no padding) the SHA-256 digest
      of the ASCII encoding of the associated access token's value.
      REQUIRED.

   If the HTTP request has content (such as an HTTP POST or PUT method),
   the payload of the JWS object is the JSON serialized content of the
   request, and the object is signed according to JWS and serialized
   into compact form [RFC7515].  The signer presents the JWS as the
   content of the request along with a content type of application/jose.
   The verifier MUST extract the payload of the JWS and treat it as the
   request content for further processing.

   If the request being made does not have content (such as an HTTP GET,
   OPTIONS, or DELETE method), the JWS signature is calculated over an
   empty payload and passed in the Detached-JWS header as described in
   Section 7.3.3.

   In the following non-normative example, the JOSE header contains the
   following parameters:

   {
       "alg": "RS256",
       "kid": "gnap-rsa",
       "uri": "https://server.example.com/gnap",
       "htm": "POST",
       "typ": "gnap-binding-jws",
       "created": 1618884475
   }

   The request content, used as the JWS Payload, is the following JSON
   object:

   NOTE: '\' line wrapping per RFC 8792

   {
       "access_token": {
           "access": [
               "dolphin-metadata"
           ]
       },
       "interact": {
           "start": ["redirect"],
           "finish": {
               "method": "redirect",
               "uri": "https://client.foo/callback",
               "nonce": "VJLO6A4CAYLBXHTR0KRO"
           }
       },
       "client": {
         "key": {
           "proof": "jws",
           "jwk": {
               "kid": "gnap-rsa",
               "kty": "RSA",
               "e": "AQAB",
               "alg": "RS256",
               "n": "hYOJ-XOKISdMMShn_G4W9m20mT0VWtQBsmBBkI2cmRt4Ai8Bf\
     YdHsFzAtYKOjpBR1RpKpJmVKxIGNy0g6Z3ad2XYsh8KowlyVy8IkZ8NMwSrcUIBZG\
     YXjHpwjzvfGvXH_5KJlnR3_uRUp4Z4Ujk2bCaKegDn11V2vxE41hqaPUnhRZxe0jR\
     ETddzsE3mu1SK8dTCROjwUl14mUNo8iTrTm4n0qDadz8BkPo-uv4BC0bunS0K3bA_\
     3UgVp7zBlQFoFnLTO2uWp_muLEWGl67gBq9MO3brKXfGhi3kOzywzwPTuq-cVQDyE\
     N7aL0SxCb3Hc4IdqDaMg8qHUyObpPitDQ"
           }
         }
         "display": {
           "name": "My Client Display Name",
           "uri": "https://client.foo/"
         },
       },
       "subject": {
           "formats": ["iss_sub", "opaque"]
       }
   }

   This leads to the following full HTTP request message:

   NOTE: '\' line wrapping per RFC 8792

   POST /gnap HTTP/1.1
   Host: server.example.com
   Content-Type: application/jose
   Content-Length: 1047

   eyJhbGciOiJSUzI1NiIsImNyZWF0ZWQiOjE2MTg4ODQ0NzUsImh0bSI6IlBPU1QiLCJ\
   raWQiOiJnbmFwLXJzYSIsInR5cCI6ImduYXAtYmluZGluZytqd3NkIiwidXJpIjoiaH\
   R0cHM6Ly9zZXJ2ZXIuZXhhbXBsZS5jb20vZ25hcCJ9.CnsKICAgICJhY2Nlc3NfdG9r\
   ZW4iOiB7CiAgICAgICAgImFjY2VzcyI6IFsKICAgICAgICAgICAgImRvbHBoaW4tbWV\
   0YWRhdGEiCiAgICAgICAgXQogICAgfSwKICAgICJpbnRlcmFjdCI6IHsKICAgICAgIC\
   Aic3RhcnQiOiBbInJlZGlyZWN0Il0sCiAgICAgICAgImZpbmlzaCI6IHsKICAgICAgI\
   CAgICAgIm1ldGhvZCI6ICJyZWRpcmVjdCIsCiAgICAgICAgICAgICJ1cmkiOiAiaHR0\
   cHM6Ly9jbGllbnQuZm9vL2NhbGxiYWNrIiwKICAgICAgICAgICAgIm5vbmNlIjogIlZ\
   KTE82QTRDQVlMQlhIVFIwS1JPIgogICAgICAgIH0KICAgIH0sCiAgICAiY2xpZW50Ij\
   ogewogICAgICAicHJvb2YiOiAiandzIiwKICAgICAgImtleSI6IHsKICAgICAgICAia\
   ndrIjogewogICAgICAgICAgICAia2lkIjogImduYXAtcnNhIiwKICAgICAgICAgICAg\
   Imt0eSI6ICJSU0EiLAogICAgICAgICAgICAiZSI6ICJBUUFCIiwKICAgICAgICAgICA\
   gImFsZyI6ICJSUzI1NiIsCiAgICAgICAgICAgICJuIjogImhZT0otWE9LSVNkTU1TaG\
   5fRzRXOW0yMG1UMFZXdFFCc21CQmtJMmNtUnQ0QWk4QmZZZEhzRnpBdFlLT2pwQlIxU\
   nBLcEptVkt4SUdOeTBnNlozYWQyWFlzaDhLb3dseVZ5OElrWjhOTXdTcmNVSUJaR1lY\
   akhwd2p6dmZHdlhIXzVLSmxuUjNfdVJVcDRaNFVqazJiQ2FLZWdEbjExVjJ2eEU0MWh\
   xYVBVbmhSWnhlMGpSRVRkZHpzRTNtdTFTSzhkVENST2p3VWwxNG1VTm84aVRyVG00bj\
   BxRGFkejhCa1BvLXV2NEJDMGJ1blMwSzNiQV8zVWdWcDd6QmxRRm9GbkxUTzJ1V3Bfb\
   XVMRVdHbDY3Z0JxOU1PM2JyS1hmR2hpM2tPenl3endQVHVxLWNWUUR5RU43YUwwU3hD\
   YjNIYzRJZHFEYU1nOHFIVXlPYnBQaXREUSIKICAgICAgICB9CiAgICAgIH0KICAgICA\
   gImRpc3BsYXkiOiB7CiAgICAgICAgIm5hbWUiOiAiTXkgQ2xpZW50IERpc3BsYXkgTm\
   FtZSIsCiAgICAgICAgInVyaSI6ICJodHRwczovL2NsaWVudC5mb28vIgogICAgICB9L\
   AogICAgfSwKICAgICJzdWJqZWN0IjogewogICAgICAgICJmb3JtYXRzIjogWyJpc3Nf\
   c3ViIiwgIm9wYXF1ZSJdCiAgICB9Cn0K.MwNoVMQp5hVxI0mCs9LlOUdFtkDXaA1_eT\
   vOXq7DOGrtDKH7q4vP2xUq3fH2jRAZqnobo0WdPP3eM3NH5QUjW8pa6_QpwdIWkK7r-\
   u_52puE0lPBp7J4U2w4l9gIbg8iknsmWmXeY5F6wiGT8ptfuEYGgmloAJd9LIeNvD3U\
   LW2h2dz1Pn2eDnbyvgB0Ugae0BoZB4f69fKWj8Z9wvTIjk1LZJN1PcL7_zT8Lrlic9a\
   PyzT7Q9ovkd1s-4whE7TrnGUzFc5mgWUn_gsOpsP5mIIljoEEv-FqOW2RyNYulOZl0Q\
   8EnnDHV_vPzrHlUarbGg4YffgtwkQhdK72-JOxYQ

   When the verifier receives an attached JWS request, it MUST parse and
   validate the JWS object.  The signature MUST be validated against the
   expected key of the signer.  All required fields MUST be present, and
   their values MUST be valid.  All fields MUST match the corresponding
   portions of the HTTP message.  For example, the htm field of the JWS
   header has to be the same as the HTTP verb used in the request.

   Note that this proofing method depends on a specific cryptographic
   algorithm, SHA-256, in two ways: the ath hash algorithm is hardcoded,
   and computing the payload of the detached/attached signature also
   uses a hardcoded hash.  A future version of this document may address
   crypto-agility for both these uses by replacing ath with a new header
   that upgrades the algorithm and possibly defining a new header that
   indicates the HTTP content's hash method.

7.3.4.1.  Key Rotation Using Attached JWS

   When rotating a key using attached JWS, the message, which includes
   the new public key value or reference, is first signed with the old
   key using a JWS object with typ header value "gnap-binding-rotation-
   jws".  The value of the JWS object is then taken as the payload of a
   new JWS object, to be signed by the new key.

8.  Resource Access Rights

   GNAP provides a rich structure for describing the protected resources
   hosted by RSs and accessed by client software.  This structure is
   used when the client instance requests an access token (Section 2.1)
   and when an access token is returned (Section 3.2).  GNAP's structure
   is designed to be analogous to the OAuth 2.0 Rich Authorization
   Requests data structure defined in [RFC9396].

   The root of this structure is a JSON array.  The elements of the JSON
   array represent rights of access that are associated with the access
   token.  Individual rights of access can be defined by the RS as
   either an object or a string.  The resulting access is the union of
   all elements within the array.

   The access associated with the access token is described using
   objects that each contain multiple dimensions of access.  Each object
   contains a REQUIRED type property that determines the type of API
   that the token is used for and the structure of the rest of the
   object.  There is no expected interoperability between different type
   definitions.

   type (string):  The type of resource request as a string.  This field
      MAY define which other fields are allowed in the request object.
      REQUIRED.

   The value of the type field is under the control of the AS.  This
   field MUST be compared using an exact byte match of the string value
   against known types by the AS.  The AS MUST ensure that there is no
   collision between different authorization data types that it
   supports.  The AS MUST NOT do any collation or normalization of data
   types during comparison.  It is RECOMMENDED that designers of
   general-purpose APIs use a URI for this field to avoid collisions
   between multiple API types protected by a single AS.

   While it is expected that many APIs will have their own properties,
   this specification defines a set of common data fields that are
   designed to be usable across different types of APIs.  This
   specification does not require the use of these common fields by an
   API definition but, instead, provides them as reusable generic
   components for API designers to make use of.  The allowable values of
   all fields are determined by the API being protected, as defined by a
   particular type value.

   actions (array of strings):  The types of actions the client instance
      will take at the RS as an array of strings (for example, a client
      instance asking for a combination of "read" and "write" access).

   locations (array of strings):  The location of the RS as an array of
      strings.  These strings are typically URIs identifying the
      location of the RS.

   datatypes (array of strings):  The kinds of data available to the
      client instance at the RS's API as an array of strings (for
      example, a client instance asking for access to raw "image" data
      and "metadata" at a photograph API).

   identifier (string):  A string identifier indicating a specific
      resource at the RS (for example, a patient identifier for a
      medical API or a bank account number for a financial API).

   privileges (array of strings):  The types or levels of privilege
      being requested at the resource (for example, a client instance
      asking for administrative-level access or access when the RO is no
      longer online).

   The following non-normative example describes three kinds of access
   (read, write, and delete) to each of two different locations and two
   different data types (metadata and images) for a single access token
   using the fictitious photo-api type definition.

   "access": [
       {
           "type": "photo-api",
           "actions": [
               "read",
               "write",
               "delete"
           ],
           "locations": [
               "https://server.example.net/",
               "https://resource.local/other"
           ],
           "datatypes": [
               "metadata",
               "images"
           ]
       }
   ]

   While the exact semantics of interpreting the fields of an access
   request object are subject to the definition of the type, it is
   expected that the access requested for each object in the array is
   the cross-product of all fields of the object.  That is to say, the
   object represents a request for all actions listed to be used at all
   locations listed for all possible datatypes listed within the object.
   Assuming the request above was granted, the client instance could
   assume that it would be able to do a read action against the images
   on the first server as well as a delete action on the metadata of the
   second server, or any other combination of these fields, using the
   same access token.

   To request a different combination of access, such as requesting one
   of the possible actions against one of the possible locations and a
   different choice of possible actions against a different one of the
   possible locations, the client instance can include multiple separate
   objects in the resources array.  The total access rights for the
   resulting access token are the union of all objects.  The following
   non-normative example uses the same fictitious photo-api type
   definition to request a single access token with more specifically
   targeted access rights by using two discrete objects within the
   request.

   "access": [
       {
           "type": "photo-api",
           "actions": [
               "read"
           ],
           "locations": [
               "https://server.example.net/"
           ],
           "datatypes": [
               "images"
           ]
       },
       {
           "type": "photo-api",
           "actions": [
               "write",
               "delete"
           ],
           "locations": [
               "https://resource.local/other"
           ],
           "datatypes": [
               "metadata"
           ]
       }
   ]

   The access requested here is for read access to images on one server
   as well as write and delete access for metadata on a different server
   (importantly, without requesting write or delete access to images on
   the first server).

   It is anticipated that API designers will use a combination of common
   fields defined in this specification as well as fields specific to
   the API itself.  The following non-normative example shows the use of
   both common and API-specific fields as part of two different
   fictitious API type values.  The first access request includes the
   actions, locations, and datatypes fields specified here as well as
   the API-specific geolocation field.  The second access request
   includes the actions and identifier fields specified here as well as
   the API-specific currency field.

   "access": [
       {
           "type": "photo-api",
           "actions": [
               "read",
               "write"
           ],
           "locations": [
               "https://server.example.net/",
               "https://resource.local/other"
           ],
           "datatypes": [
               "metadata",
               "images"
           ],
           "geolocation": [
               { lat: -32.364, lng: 153.207 },
               { lat: -35.364, lng: 158.207 }
           ]
       },
       {
           "type": "financial-transaction",
           "actions": [
               "withdraw"
           ],
           "identifier": "account-14-32-32-3",
           "currency": "USD"
       }
   ]

   If this request is approved, the resulting access token's access
   rights will be the union of the requested types of access for each of
   the two APIs, just as above.

8.1.  Requesting Resources by Reference

   Instead of sending an object describing the requested resource
   (Section 8), access rights MAY be communicated as a string known to
   the AS representing the access being requested.  Just like access
   rights communicated as an object, access rights communicated as
   reference strings indicate a specific access at a protected resource.
   In the following non-normative example, three distinct resource
   access rights are being requested.

   "access": [
       "read", "dolphin-metadata", "some other thing"
   ]

   This value is opaque to the client instance and MAY be any valid JSON
   string; therefore, it could include spaces, Unicode characters, and
   properly escaped string sequences.  However, in some situations, the
   value is intended to be seen and understood by the client software's
   developer.  In such cases, the API designer choosing any such human-
   readable strings SHOULD take steps to ensure the string values are
   not easily confused by a developer, such as by limiting the strings
   to easily disambiguated characters.

   This functionality is similar in practice to OAuth 2.0's scope
   parameter [RFC6749], where a single string represents the set of
   access rights requested by the client instance.  As such, the
   reference string could contain any valid OAuth 2.0 scope value, as in
   Appendix B.5.  Note that the reference string here is not bound to
   the same character restrictions as OAuth 2.0's scope definition.

   A single access array MAY include both object-type and string-type
   resource items.  In this non-normative example, the client instance
   is requesting access to a photo-api and financial-transaction API
   type as well as the reference values of read, dolphin-metadata, and
   some other thing.

   "access": [
       {
           "type": "photo-api",
           "actions": [
               "read",
               "write",
               "delete"
           ],
           "locations": [
               "https://server.example.net/",
               "https://resource.local/other"
           ],
           "datatypes": [
               "metadata",
               "images"
           ]
       },
       "read",
       "dolphin-metadata",
       {
           "type": "financial-transaction",
           "actions": [
               "withdraw"
           ],
           "identifier": "account-14-32-32-3",
           "currency": "USD"
       },
       "some other thing"
   ]

   The requested access is the union of all elements of the array,
   including both objects and reference strings.

   In order to facilitate the use of both object and reference strings
   to access the same kind of APIs, the API designer can define a clear
   mapping between these forms.  One possible approach for choosing
   reference string values is to use the same value as the type
   parameter from the fully specified object, with the API defining a
   set of default behaviors in this case.  For example, an API
   definition could declare the following string:

   "access": [
       "photo-api"
   ]

   As being equivalent to the following fully defined object:

   "access": [
       {
           "type": "photo-api",
           "actions": [ "read", "write", "delete" ],
           "datatypes": [ "metadata", "image" ]
       }
   ]

   The exact mechanisms for relating reference strings is up to the API
   designer.  These are enforced by the AS, and the details are out of
   scope for this specification.

9.  Discovery

   By design, GNAP minimizes the need for any pre-flight discovery.  To
   begin a request, the client instance only needs to know the grant
   endpoint of the AS (a single URI) and which keys it will use to sign
   the request.  Everything else can be negotiated dynamically in the
   course of the protocol.

   However, the AS can have limits on its allowed functionality.  If the
   client instance wants to optimize its calls to the AS before making a
   request, it MAY send an HTTP OPTIONS request to the grant request
   endpoint to retrieve the server's discovery information.  The AS MUST
   respond with a JSON document with Content-Type application/json
   containing a single object with the following fields:

   grant_request_endpoint (string):  The location of the AS's grant
      request endpoint.  The location MUST be an absolute URL [RFC3986]
      with a scheme component (which MUST be "https"), a host component,
      and optionally port, path, and query components and no fragment
      components.  This URL MUST match the URL the client instance used
      to make the discovery request.  REQUIRED.

   interaction_start_modes_supported (array of strings):  A list of the
      AS's interaction start methods.  The values of this list
      correspond to the possible values for the interaction start field
      of the request (Section 2.5.1) and MUST be values from the "GNAP
      Interaction Start Modes" registry (Section 10.9).  OPTIONAL.

   interaction_finish_methods_supported (array of strings):  A list of
      the AS's interaction finish methods.  The values of this list
      correspond to the possible values for the method element of the
      interaction finish field of the request (Section 2.5.2) and MUST
      be values from the "GNAP Interaction Finish Methods" registry
      (Section 10.10).  OPTIONAL.

   key_proofs_supported (array of strings):  A list of the AS's
      supported key proofing mechanisms.  The values of this list
      correspond to possible values of the proof field of the key
      section of the request (Section 7.1) and MUST be values from the
      "GNAP Key Proofing Methods" registry (Section 10.16).  OPTIONAL.

   sub_id_formats_supported (array of strings):  A list of the AS's
      supported Subject Identifier formats.  The values of this list
      correspond to possible values of the Subject Identifier field of
      the request (Section 2.2) and MUST be values from the "Subject
      Identifier Formats" registry [Subj-ID-Formats].  OPTIONAL.

   assertion_formats_supported (array of strings):  A list of the AS's
      supported assertion formats.  The values of this list correspond
      to possible values of the subject assertion field of the request
      (Section 2.2) and MUST be values from the "GNAP Assertion Formats"
      registry (Section 10.6).  OPTIONAL.

   key_rotation_supported (boolean):  The boolean "true" indicates that
      rotation of access token bound keys by the client (Section 6.1.1)
      is supported by the AS.  The absence of this field or a boolean
      "false" value indicates that this feature is not supported.

   The information returned from this method is for optimization
   purposes only.  The AS MAY deny any request, or any portion of a
   request, even if it lists a capability as supported.  For example, if
   a given client instance can be registered with the mtls key proofing
   mechanism but the AS also returns other proofing methods from the
   discovery document, then the AS will still deny a request from that
   client instance using a different proofing mechanism.  Similarly, an
   AS with key_rotation_supported set to "true" can still deny any
   request for rotating any access token's key for a variety of reasons.

   Additional fields can be defined in the "GNAP Authorization Server
   Discovery Fields" registry (Section 10.18).

9.1.  RS-First Method of AS Discovery

   If the client instance calls an RS without an access token or with an
   invalid access token, the RS SHOULD be explicit about the fact that
   GNAP needs to be used to access the resource by responding with the
   WWW-Authenticate header field and a GNAP challenge.

   In some situations, the client instance might want to know with which
   specific AS it needs to negotiate for access to that RS.  The RS MAY
   additionally return the following OPTIONAL parameters:

   as_uri:  The URI of the grant endpoint of the GNAP AS.  Used by the
      client instance to call the AS to request an access token.

   referrer:  The URI of the GNAP RS.  Sent by the client instance in
      the Referer header as part of the grant request.

   access:  An opaque access reference as defined in Section 8.1.  MUST
      be sufficient for at least the action the client instance was
      attempting to take at the RS and MAY allow additional access
      rights as well.  Sent by the client as an access right in the
      grant request.

   The client instance SHOULD then use both the referrer and access
   parameters in its access token request.  The client instance MUST
   check that the referrer parameter is equal to the URI of the RS using
   the simple string comparison method in Section 6.2.1 of [RFC3986].

   The means for the RS to determine the value for the access reference
   are out of scope of this specification, but some dynamic methods are
   discussed in [GNAP-RS].

   When receiving the following response from the RS:

   NOTE: '\' line wrapping per RFC 8792

   WWW-Authenticate: \
     GNAP as_uri=https://as.example/tx\
     ;access=FWWIKYBQ6U56NL1\
     ;referrer=https://rs.example

   The client instance then makes a request to the as_uri as described
   in Section 2, with the value of referrer passed as an HTTP Referer
   header field and the access reference passed unchanged into the
   access array in the access_token portion of the request.  The client
   instance MAY request additional resources and other information.

   In the following non-normative example, the client instance is
   requesting a single access token using the opaque access reference
   FWWIKYBQ6U56NL1 received from the RS in addition to the dolphin-
   metadata that the client instance has been configured with out of
   band.

   POST /tx HTTP/1.1
   Host: as.example
   Referer: https://rs.example/resource
   Content-Type: application/json
   Signature-Input: sig1=...
   Signature: sig1=...
   Content-Digest: sha-256=...

   {
       "access_token": {
           "access": [
               "FWWIKYBQ6U56NL1",
               "dolphin-metadata"
           ]
       },
       "client": "KHRS6X63AJ7C7C4AZ9AO"
   }

   The client instance includes the Referer header field as a way for
   the AS to know that the process is initiated through a discovery
   process at the RS.

   If issued, the resulting access token would contain sufficient access
   to be used at both referenced resources.

   Security considerations, especially related to the potential of a
   compromised RS (Section 11.37) redirecting the requests of an
   otherwise properly authenticated client, need to be carefully
   considered when allowing such a discovery process.  This risk can be
   mitigated by an alternative pre-registration process so that the
   client knows which AS protects which RS.  There are also privacy
   considerations related to revealing which AS is protecting a given
   resource; these are discussed in Section 12.4.1.

9.2.  Dynamic Grant Endpoint Discovery

   Additional methods of discovering the appropriate grant endpoint for
   a given application are outside the scope of this specification.
   This limitation is intentional, as many applications rely on static
   configuration between the client instance and AS, as is common in
   OAuth 2.0.  However, the dynamic nature of GNAP makes it a prime
   candidate for other extensions defining methods for discovery of the
   appropriate AS grant endpoint at runtime.  Advanced use cases could
   define contextual methods for securely providing this endpoint to the
   client instance.  Furthermore, GNAP's design intentionally requires
   the client instance to only know the grant endpoint and not
   additional parameters, since other functions and values can be
   disclosed and negotiated during the grant process.

10.  IANA Considerations

   IANA has added values to existing registries as well as created 16
   registries for GNAP [GNAP-REG] and populated those registries with
   initial values as described in this section.

   All use of value typing is based on data types in [RFC8259] and MUST
   be one of the following: number, object, string, boolean, or array.
   When the type is array, the contents of the array MUST be specified,
   as in "array of objects" when one subtype is allowed or "array of
   strings/objects" when multiple simultaneous subtypes are allowed.
   When the type is object, the structure of the object MUST be
   specified in the definition.  If a parameter is available in
   different types, each type SHOULD be registered separately.

   General guidance for extension parameters is found in Appendix D.

10.1.  HTTP Authentication Scheme Registration

   IANA has registered of the following scheme in the "HTTP
   Authentication Schemes" registry [Auth-Schemes] defined in
   Section 18.5 of [HTTP]:

   Authentication Scheme Name:  GNAP

   Reference:  Section 7.2 of RFC 9635

10.2.  Media Type Registration

   Per this section, IANA has registered the following media types
   [RFC2046] in the "Media Types" registry [MediaTypes] as described in
   [RFC6838].

10.2.1.  application/gnap-binding-jwsd

   This media type indicates that the content is a GNAP message to be
   bound with a detached JWS mechanism.

   Type name:  application

   Subtype name:  gnap-binding-jwsd

   Required parameters:  N/A

   Optional parameters:  N/A

   Encoding considerations:  binary

   Security considerations:  See Section 11 of RFC 9635.

   Interoperability considerations:  N/A

   Published specification:  RFC 9635

   Applications that use this media type:  GNAP

   Fragment identifier considerations:  N/A

   Additional information:

      Deprecated alias names for this type:  N/A
      Magic number(s):  N/A
      File extension(s):  N/A
      Macintosh file type code(s):  N/A

   Person & email address to contact for further information:  IETF GNAP
      Working Group (txauth@ietf.org)

   Intended usage:  COMMON

   Restrictions on usage:  none

   Author:  IETF GNAP Working Group (txauth@ietf.org)

   Change Controller:  IETF

10.2.2.  application/gnap-binding-jws

   This media type indicates that the content is a GNAP message to be
   bound with an attached JWS mechanism.

   Type name:  application

   Subtype name:  gnap-binding-jws

   Required parameters:  N/A

   Optional parameters:  N/A

   Encoding considerations:  binary

   Security considerations:  See Section 11 of RFC 9635.

   Interoperability considerations:  N/A

   Published specification:  RFC 9635

   Applications that use this media type:  GNAP

   Fragment identifier considerations:  N/A

   Additional information:

      Deprecated alias names for this type:  N/A
      Magic number(s):  N/A
      File extension(s):  N/A
      Macintosh file type code(s):  N/A

   Person & email address to contact for further information:  IETF GNAP
      Working Group (txauth@ietf.org)

   Intended usage:  COMMON

   Restrictions on usage:  none

   Author:  IETF GNAP Working Group (txauth@ietf.org)

   Change Controller:  IETF

10.2.3.  application/gnap-binding-rotation-jwsd

   This media type indicates that the content is a GNAP token rotation
   message to be bound with a detached JWS mechanism.

   Type name:  application

   Subtype name:  gnap-binding-rotation-jwsd

   Required parameters:  N/A

   Optional parameters:  N/A

   Encoding considerations:  binary

   Security considerations:  See Section 11 of RFC 9635.

   Interoperability considerations:  N/A

   Published specification:  RFC 9635

   Applications that use this media type:  GNAP

   Fragment identifier considerations:  N/A

   Additional information:

      Deprecated alias names for this type:  N/A
      Magic number(s):  N/A
      File extension(s):  N/A
      Macintosh file type code(s):  N/A

   Person & email address to contact for further information:  IETF GNAP
      Working Group (txauth@ietf.org)

   Intended usage:  COMMON

   Restrictions on usage:  none

   Author:  IETF GNAP Working Group (txauth@ietf.org)

   Change Controller:  IETF

10.2.4.  application/gnap-binding-rotation-jws

   This media type indicates that the content is a GNAP token rotation
   message to be bound with an attached JWS mechanism.

   Type name:  application

   Subtype name:  gnap-binding-rotation-jws

   Required parameters:  N/A

   Optional parameters:  N/A

   Encoding considerations:  binary

   Security considerations:  See Section 11 of RFC 9635.

   Interoperability considerations:  N/A

   Published specification:  RFC 9635

   Applications that use this media type:  GNAP

   Fragment identifier considerations:  N/A

   Additional information:

      Deprecated alias names for this type:  N/A
      Magic number(s):  N/A
      File extension(s):  N/A
      Macintosh file type code(s):  N/A

   Person & email address to contact for further information:  IETF GNAP
      Working Group (txauth@ietf.org)

   Intended usage:  COMMON

   Restrictions on usage:  none

   Author:  IETF GNAP Working Group (txauth@ietf.org)

   Change Controller:  IETF

10.3.  GNAP Grant Request Parameters

   This document defines a GNAP grant request, for which IANA has
   created and maintains a new registry titled "GNAP Grant Request
   Parameters".  Initial values for this registry are given in
   Section 10.3.2.  Future assignments and modifications to existing
   assignments are to be made through the Specification Required
   registration policy [RFC8126].

   The designated expert (DE) is expected to ensure the following:

   *  All registrations follow the template presented in Section 10.3.1.

   *  The request parameter's definition is sufficiently orthogonal to
      existing functionality provided by existing parameters.

   *  Registrations for the same name with different types are
      sufficiently close in functionality so as not to cause confusion
      for developers.

   *  The request parameter's definition specifies the expected behavior
      of the AS in response to the request parameter for each potential
      state of the grant request.

10.3.1.  Registration Template

   Name:
      An identifier for the parameter.

   Type:
      The JSON type allowed for the value.

   Reference:
      Reference to one or more documents that specify the value,
      preferably including a URI that can be used to retrieve a copy of
      the document(s).  An indication of the relevant sections may also
      be included but is not required.

10.3.2.  Initial Contents

      +==============+==================+===========================+
      | Name         | Type             | Reference                 |
      +==============+==================+===========================+
      | access_token | object           | Section 2.1.1 of RFC 9635 |
      +--------------+------------------+---------------------------+
      | access_token | array of objects | Section 2.1.2 of RFC 9635 |
      +--------------+------------------+---------------------------+
      | subject      | object           | Section 2.2 of RFC 9635   |
      +--------------+------------------+---------------------------+
      | client       | object           | Section 2.3 of RFC 9635   |
      +--------------+------------------+---------------------------+
      | client       | string           | Section 2.3.1 of RFC 9635 |
      +--------------+------------------+---------------------------+
      | user         | object           | Section 2.4 of RFC 9635   |
      +--------------+------------------+---------------------------+
      | user         | string           | Section 2.4.1 of RFC 9635 |
      +--------------+------------------+---------------------------+
      | interact     | object           | Section 2.5 of RFC 9635   |
      +--------------+------------------+---------------------------+
      | interact_ref | string           | Section 5.1 of RFC 9635   |
      +--------------+------------------+---------------------------+

                                  Table 1

10.4.  GNAP Access Token Flags

   This document defines GNAP access token flags, for which IANA has
   created and maintains a new registry titled "GNAP Access Token
   Flags".  Initial values for this registry are given in
   Section 10.4.2.  Future assignments and modifications to existing
   assignments are to be made through the Specification Required
   registration policy [RFC8126].

   The DE is expected to ensure the following:

   *  All registrations follow the template presented in Section 10.4.1.

   *  The flag specifies whether it applies to requests for tokens to
      the AS, responses with tokens from the AS, or both.

10.4.1.  Registration Template

   Name:
      An identifier for the parameter.

   Allowed Use:
      Where the flag is allowed to occur.  Possible values are
      "Request", "Response", and "Request, Response".

   Reference:
      Reference to one or more documents that specify the value,
      preferably including a URI that can be used to retrieve a copy of
      the document(s).  An indication of the relevant sections may also
      be included but is not required.

10.4.2.  Initial Contents

           +=========+===================+====================+
           | Name    | Allowed Use       | Reference          |
           +=========+===================+====================+
           | bearer  | Request, Response | Sections 2.1.1 and |
           |         |                   | 3.2.1 of RFC 9635  |
           +---------+-------------------+--------------------+
           | durable | Response          | Section 3.2.1 of   |
           |         |                   | RFC 9635           |
           +---------+-------------------+--------------------+

                                 Table 2

10.5.  GNAP Subject Information Request Fields

   This document defines a means to request subject information from the
   AS to the client instance, for which IANA has created and maintains a
   new registry titled "GNAP Subject Information Request Fields".
   Initial values for this registry are given in Section 10.5.2.  Future
   assignments and modifications to existing assignments are to be made
   through the Specification Required registration policy [RFC8126].

   The DE is expected to ensure the following:

   *  All registrations follow the template presented in Section 10.5.1.

   *  Registrations for the same name with different types are
      sufficiently close in functionality so as not to cause confusion
      for developers.

10.5.1.  Registration Template

   Name:
      An identifier for the parameter.

   Type:
      The JSON type allowed for the value.

   Reference:
      Reference to one or more documents that specify the value,
      preferably including a URI that can be used to retrieve a copy of
      the document(s).  An indication of the relevant sections may also
      be included but is not required.

10.5.2.  Initial Contents

    +===================+==================+=========================+
    | Name              | Type             | Reference               |
    +===================+==================+=========================+
    | sub_id_formats    | array of strings | Section 2.2 of RFC 9635 |
    +-------------------+------------------+-------------------------+
    | assertion_formats | array of strings | Section 2.2 of RFC 9635 |
    +-------------------+------------------+-------------------------+
    | sub_ids           | array of objects | Section 2.2 of RFC 9635 |
    +-------------------+------------------+-------------------------+

                                 Table 3

10.6.  GNAP Assertion Formats

   This document defines a means to pass identity assertions between the
   AS and client instance, for which IANA has created and maintains a
   new registry titled "GNAP Assertion Formats".  Initial values for
   this registry are given in Section 10.6.2.  Future assignments and
   modifications to existing assignments are to be made through the
   Specification Required registration policy [RFC8126].

   The DE is expected to ensure the following:

   *  All registrations follow the template presented in Section 10.6.1.

   *  The definition specifies the serialization format of the assertion
      value as used within GNAP.

10.6.1.  Registration Template

   Name:
      An identifier for the assertion format.

   Reference:
      Reference to one or more documents that specify the value,
      preferably including a URI that can be used to retrieve a copy of
      the document(s).  An indication of the relevant sections may also
      be included but is not required.

10.6.2.  Initial Contents

                 +==========+===========================+
                 | Name     | Reference                 |
                 +==========+===========================+
                 | id_token | Section 3.4.1 of RFC 9635 |
                 +----------+---------------------------+
                 | saml2    | Section 3.4.1 of RFC 9635 |
                 +----------+---------------------------+

                                 Table 4

10.7.  GNAP Client Instance Fields

   This document defines a means to send information about the client
   instance, for which IANA has created and maintains a new registry
   titled "GNAP Client Instance Fields".  Initial values for this
   registry are given in Section 10.7.2.  Future assignments and
   modifications to existing assignments are to be made through the
   Specification Required registration policy [RFC8126].

   The DE is expected to ensure the following:

   *  All registrations follow the template presented in Section 10.7.1.

   *  Registrations for the same name with different types are
      sufficiently close in functionality so as not to cause confusion
      for developers.

10.7.1.  Registration Template

   Name:
      An identifier for the parameter.

   Type:
      The JSON type allowed for the value.

   Reference:
      Reference to one or more documents that specify the value,
      preferably including a URI that can be used to retrieve a copy of
      the document(s).  An indication of the relevant sections may also
      be included but is not required.

10.7.2.  Initial Contents

             +==========+========+===========================+
             | Name     | Type   | Reference                 |
             +==========+========+===========================+
             | key      | object | Section 7.1 of RFC 9635   |
             +----------+--------+---------------------------+
             | key      | string | Section 7.1.1 of RFC 9635 |
             +----------+--------+---------------------------+
             | class_id | string | Section 2.3 of RFC 9635   |
             +----------+--------+---------------------------+
             | display  | object | Section 2.3.2 of RFC 9635 |
             +----------+--------+---------------------------+

                                  Table 5

10.8.  GNAP Client Instance Display Fields

   This document defines a means to send end-user-facing displayable
   information about the client instance, for which IANA has created and
   maintains a new registry titled "GNAP Client Instance Display
   Fields".  Initial values for this registry are given in
   Section 10.8.2.  Future assignments and modifications to existing
   assignments are to be made through the Specification Required
   registration policy [RFC8126].

   The DE is expected to ensure the following:

   *  All registrations follow the template presented in Section 10.8.1.

   *  Registrations for the same name with different types are
      sufficiently close in functionality so as not to cause confusion
      for developers.

10.8.1.  Registration Template

   Name:
      An identifier for the parameter.

   Type:
      The JSON type allowed for the value.

   Reference:
      Reference to one or more documents that specify the value,
      preferably including a URI that can be used to retrieve a copy of
      the document(s).  An indication of the relevant sections may also
      be included but is not required.

10.8.2.  Initial Contents

             +==========+========+===========================+
             | Name     | Type   | Reference                 |
             +==========+========+===========================+
             | name     | string | Section 2.3.2 of RFC 9635 |
             +----------+--------+---------------------------+
             | uri      | string | Section 2.3.2 of RFC 9635 |
             +----------+--------+---------------------------+
             | logo_uri | string | Section 2.3.2 of RFC 9635 |
             +----------+--------+---------------------------+

                                  Table 6

10.9.  GNAP Interaction Start Modes

   This document defines a means for the client instance to begin
   interaction between the end user and the AS, for which IANA has
   created and maintains a new registry titled "GNAP Interaction Start
   Modes".  Initial values for this registry are given in
   Section 10.9.2.  Future assignments and modifications to existing
   assignments are to be made through the Specification Required
   registration policy [RFC8126].

   The DE is expected to ensure the following:

   *  All registrations follow the template presented in Section 10.9.1.

   *  Registrations for the same name with different types are
      sufficiently close in functionality so as not to cause confusion
      for developers.

   *  Any registration using an "object" type declares all additional
      parameters, their optionality, and their purpose.

   *  The start mode clearly defines what actions the client is expected
      to take to begin interaction, what the expected user experience
      is, and any security considerations for this communication from
      either party.

   *  The start mode documents incompatibilities with other start modes
      or finish methods, if applicable.

   *  The start mode provides enough information to uniquely identify
      the grant request during the interaction.  For example, in the
      redirect and app modes, this is done using a unique URI (including
      its parameters).  In the user_code and user_code_uri modes, this
      is done using the value of the user code.

10.9.1.  Registration Template

   Mode:
      An identifier for the interaction start mode.

   Type:
      The JSON type for the value, either "string" or "object", as
      described in Section 2.5.1.

   Reference:
      Reference to one or more documents that specify the value,
      preferably including a URI that can be used to retrieve a copy of
      the document(s).  An indication of the relevant sections may also
      be included but is not required.

10.9.2.  Initial Contents

         +===============+========+=============================+
         | Mode          | Type   | Reference                   |
         +===============+========+=============================+
         | redirect      | string | Section 2.5.1.1 of RFC 9635 |
         +---------------+--------+-----------------------------+
         | app           | string | Section 2.5.1.2 of RFC 9635 |
         +---------------+--------+-----------------------------+
         | user_code     | string | Section 2.5.1.3 of RFC 9635 |
         +---------------+--------+-----------------------------+
         | user_code_uri | string | Section 2.5.1.4 of RFC 9635 |
         +---------------+--------+-----------------------------+

                                 Table 7

10.10.  GNAP Interaction Finish Methods

   This document defines a means for the client instance to be notified
   of the end of interaction between the end user and the AS, for which
   IANA has created and maintains a new registry titled "GNAP
   Interaction Finish Methods".  Initial values for this registry are
   given in Section 10.10.2.  Future assignments and modifications to
   existing assignments are to be made through the Specification
   Required registration policy [RFC8126].

   The DE is expected to ensure the following:

   *  All registrations follow the template presented in
      Section 10.10.1.

   *  All finish methods clearly define what actions the AS is expected
      to take, what listening methods the client instance needs to
      enable, and any security considerations for this communication
      from either party.

   *  All finish methods document incompatibilities with any start
      modes, if applicable.

10.10.1.  Registration Template

   Method:
      An identifier for the interaction finish method.

   Reference:
      Reference to one or more documents that specify the value,
      preferably including a URI that can be used to retrieve a copy of
      the document(s).  An indication of the relevant sections may also
      be included but is not required.

10.10.2.  Initial Contents

                +==========+=============================+
                | Method   | Reference                   |
                +==========+=============================+
                | redirect | Section 2.5.2.1 of RFC 9635 |
                +----------+-----------------------------+
                | push     | Section 2.5.2.2 of RFC 9635 |
                +----------+-----------------------------+

                                 Table 8

10.11.  GNAP Interaction Hints

   This document defines a set of hints that a client instance can
   provide to the AS to facilitate interaction with the end user, for
   which IANA has created and maintains a new registry titled "GNAP
   Interaction Hints".  Initial values for this registry are given in
   Section 10.11.2.  Future assignments and modifications to existing
   assignments are to be made through the Specification Required
   registration policy [RFC8126].

   The DE is expected to ensure the following:

   *  All registrations follow the template presented in
      Section 10.11.1.

   *  All interaction hints clearly document the expected behaviors of
      the AS in response to the hint, and an AS not processing the hint
      does not impede the operation of the AS or client instance.

10.11.1.  Registration Template

   Name:
      An identifier for the parameter.

   Reference:
      Reference to one or more documents that specify the value,
      preferably including a URI that can be used to retrieve a copy of
      the document(s).  An indication of the relevant sections may also
      be included but is not required.

10.11.2.  Initial Contents

                +============+===========================+
                | Name       | Reference                 |
                +============+===========================+
                | ui_locales | Section 2.5.3 of RFC 9635 |
                +------------+---------------------------+

                                 Table 9

10.12.  GNAP Grant Response Parameters

   This document defines a GNAP grant response, for which IANA has
   created and maintains a new registry titled "GNAP Grant Response
   Parameters".  Initial values for this registry are given in
   Section 10.12.2.  Future assignments and modifications to existing
   assignments are to be made through the Specification Required
   registration policy [RFC8126].

   The DE is expected to ensure the following:

   *  All registrations follow the template presented in
      Section 10.12.1.

   *  The response parameter's definition is sufficiently orthogonal to
      existing functionality provided by existing parameters.

   *  Registrations for the same name with different types are
      sufficiently close in functionality so as not to cause confusion
      for developers.

   *  The response parameter's definition specifies grant states for
      which the client instance can expect this parameter to appear in a
      response message.

10.12.1.  Registration Template

   Name:
      An identifier for the parameter.

   Type:
      The JSON type allowed for the value.

   Reference:
      Reference to one or more documents that specify the value,
      preferably including a URI that can be used to retrieve a copy of
      the document(s).  An indication of the relevant sections may also
      be included but is not required.

10.12.2.  Initial Contents

      +==============+==================+===========================+
      | Name         | Type             | Reference                 |
      +==============+==================+===========================+
      | continue     | object           | Section 3.1 of RFC 9635   |
      +--------------+------------------+---------------------------+
      | access_token | object           | Section 3.2.1 of RFC 9635 |
      +--------------+------------------+---------------------------+
      | access_token | array of objects | Section 3.2.2 of RFC 9635 |
      +--------------+------------------+---------------------------+
      | interact     | object           | Section 3.3 of RFC 9635   |
      +--------------+------------------+---------------------------+
      | subject      | object           | Section 3.4 of RFC 9635   |
      +--------------+------------------+---------------------------+
      | instance_id  | string           | Section 3.5 of RFC 9635   |
      +--------------+------------------+---------------------------+
      | error        | object           | Section 3.6 of RFC 9635   |
      +--------------+------------------+---------------------------+

                                  Table 10

10.13.  GNAP Interaction Mode Responses

   This document defines a means for the AS to provide the client
   instance with information that is required to complete a particular
   interaction mode, for which IANA has created and maintains a new
   registry titled "GNAP Interaction Mode Responses".  Initial values
   for this registry are given in Section 10.13.2.  Future assignments
   and modifications to existing assignments are to be made through the
   Specification Required registration policy [RFC8126].

   The DE is expected to ensure the following:

   *  All registrations follow the template presented in
      Section 10.13.1.

   *  If the name of the registration matches the name of an interaction
      start mode, the response parameter is unambiguously associated
      with the interaction start mode of the same name.

10.13.1.  Registration Template

   Name:
      An identifier for the parameter.

   Reference:
      Reference to one or more documents that specify the value,
      preferably including a URI that can be used to retrieve a copy of
      the document(s).  An indication of the relevant sections may also
      be included but is not required.

10.13.2.  Initial Contents

                +===============+=========================+
                | Name          | Reference               |
                +===============+=========================+
                | redirect      | Section 3.3 of RFC 9635 |
                +---------------+-------------------------+
                | app           | Section 3.3 of RFC 9635 |
                +---------------+-------------------------+
                | user_code     | Section 3.3 of RFC 9635 |
                +---------------+-------------------------+
                | user_code_uri | Section 3.3 of RFC 9635 |
                +---------------+-------------------------+
                | finish        | Section 3.3 of RFC 9635 |
                +---------------+-------------------------+
                | expires_in    | Section 3.3 of RFC 9635 |
                +---------------+-------------------------+

                                  Table 11

10.14.  GNAP Subject Information Response Fields

   This document defines a means to return subject information from the
   AS to the client instance, for which IANA has created and maintains a
   new registry titled "GNAP Subject Information Response Fields".
   Initial values for this registry are given in Section 10.14.2.
   Future assignments and modifications to existing assignments are to
   be made through the Specification Required registration policy
   [RFC8126].

   The DE is expected to ensure the following:

   *  All registrations follow the template presented in
      Section 10.14.1.

   *  Registrations for the same name with different types are
      sufficiently close in functionality so as not to cause confusion
      for developers.

10.14.1.  Registration Template

   Name:
      An identifier for the parameter.

   Type:
      The JSON type allowed for the value.

   Reference:
      Reference to one or more documents that specify the value,
      preferably including a URI that can be used to retrieve a copy of
      the document(s).  An indication of the relevant sections may also
      be included but is not required.

10.14.2.  Initial Contents

        +============+==================+=========================+
        | Name       | Type             | Reference               |
        +============+==================+=========================+
        | sub_ids    | array of objects | Section 3.4 of RFC 9635 |
        +------------+------------------+-------------------------+
        | assertions | array of objects | Section 3.4 of RFC 9635 |
        +------------+------------------+-------------------------+
        | updated_at | string           | Section 3.4 of RFC 9635 |
        +------------+------------------+-------------------------+

                                  Table 12

10.15.  GNAP Error Codes

   This document defines a set of errors that the AS can return to the
   client instance, for which IANA has created and maintains a new
   registry titled "GNAP Error Codes".  Initial values for this registry
   are given in Section 10.15.2.  Future assignments and modifications
   to existing assignments are to be made through the Specification
   Required registration policy [RFC8126].

   The DE is expected to ensure the following:

   *  All registrations follow the template presented in
      Section 10.15.1.

   *  The error response is sufficiently unique from other errors to
      provide actionable information to the client instance.

   *  The definition of the error response specifies all conditions in
      which the error response is returned and the client instance's
      expected action.

10.15.1.  Registration Template

   Error:
      A unique string code for the error.

   Reference:
      Reference to one or more documents that specify the value,
      preferably including a URI that can be used to retrieve a copy of
      the document(s).  An indication of the relevant sections may also
      be included but is not required.

10.15.2.  Initial Contents

         +============================+=========================+
         | Error                      | Reference               |
         +============================+=========================+
         | invalid_request            | Section 3.6 of RFC 9635 |
         +----------------------------+-------------------------+
         | invalid_client             | Section 3.6 of RFC 9635 |
         +----------------------------+-------------------------+
         | invalid_interaction        | Section 3.6 of RFC 9635 |
         +----------------------------+-------------------------+
         | invalid_flag               | Section 3.6 of RFC 9635 |
         +----------------------------+-------------------------+
         | invalid_rotation           | Section 3.6 of RFC 9635 |
         +----------------------------+-------------------------+
         | key_rotation_not_supported | Section 3.6 of RFC 9635 |
         +----------------------------+-------------------------+
         | invalid_continuation       | Section 3.6 of RFC 9635 |
         +----------------------------+-------------------------+
         | user_denied                | Section 3.6 of RFC 9635 |
         +----------------------------+-------------------------+
         | request_denied             | Section 3.6 of RFC 9635 |
         +----------------------------+-------------------------+
         | unknown_user               | Section 3.6 of RFC 9635 |
         +----------------------------+-------------------------+
         | unknown_interaction        | Section 3.6 of RFC 9635 |
         +----------------------------+-------------------------+
         | too_fast                   | Section 3.6 of RFC 9635 |
         +----------------------------+-------------------------+
         | too_many_attempts          | Section 3.6 of RFC 9635 |
         +----------------------------+-------------------------+

                                 Table 13

10.16.  GNAP Key Proofing Methods

   This document defines methods that the client instance can use to
   prove possession of a key, for which IANA has created and maintains a
   new registry titled "GNAP Key Proofing Methods".  Initial values for
   this registry are given in Section 10.16.2.  Future assignments and
   modifications to existing assignments are to be made through the
   Specification Required registration policy [RFC8126].

   The DE is expected to ensure the following:

   *  All registrations follow the template presented in
      Section 10.16.1.

   *  Registrations for the same name with different types are
      sufficiently close in functionality so as not to cause confusion
      for developers.

   *  The proofing method provides sufficient coverage of and binding to
      the protocol messages to which it is applied.

   *  The proofing method definition clearly enumerates how all
      requirements in Section 7.3 are fulfilled by the definition.

10.16.1.  Registration Template

   Method:
      A unique string code for the key proofing method.

   Type:
      The JSON type allowed for the value.

   Reference:
      Reference to one or more documents that specify the value,
      preferably including a URI that can be used to retrieve a copy of
      the document(s).  An indication of the relevant sections may also
      be included but is not required.

10.16.2.  Initial Contents

             +=========+========+===========================+
             | Method  | Type   | Reference                 |
             +=========+========+===========================+
             | httpsig | string | Section 7.3.1 of RFC 9635 |
             +---------+--------+---------------------------+
             | httpsig | object | Section 7.3.1 of RFC 9635 |
             +---------+--------+---------------------------+
             | mtls    | string | Section 7.3.2 of RFC 9635 |
             +---------+--------+---------------------------+
             | jwsd    | string | Section 7.3.3 of RFC 9635 |
             +---------+--------+---------------------------+
             | jws     | string | Section 7.3.4 of RFC 9635 |
             +---------+--------+---------------------------+

                                 Table 14

10.17.  GNAP Key Formats

   This document defines formats for a public key value, for which IANA
   has created and maintains a new registry titled "GNAP Key Formats".
   Initial values for this registry are given in Section 10.17.2.
   Future assignments and modifications to existing assignments are to
   be made through the Specification Required registration policy
   [RFC8126].

   The DE is expected to ensure the following:

   *  All registrations follow the template presented in
      Section 10.17.1.

   *  The key format specifies the structure and serialization of the
      key material.

10.17.1.  Registration Template

   Format:
      A unique string code for the key format.

   Reference:
      Reference to one or more documents that specify the value,
      preferably including a URI that can be used to retrieve a copy of
      the document(s).  An indication of the relevant sections may also
      be included but is not required.

10.17.2.  Initial Contents

                  +===========+=========================+
                  | Format    | Reference               |
                  +===========+=========================+
                  | jwk       | Section 7.1 of RFC 9635 |
                  +-----------+-------------------------+
                  | cert      | Section 7.1 of RFC 9635 |
                  +-----------+-------------------------+
                  | cert#S256 | Section 7.1 of RFC 9635 |
                  +-----------+-------------------------+

                                  Table 15

10.18.  GNAP Authorization Server Discovery Fields

   This document defines a discovery document for an AS, for which IANA
   has created and maintains a new registry titled "GNAP Authorization
   Server Discovery Fields".  Initial values for this registry are given
   in Section 10.18.2.  Future assignments and modifications to existing
   assignments are to be made through the Specification Required
   registration policy [RFC8126].

   The DE is expected to ensure the following:

   *  All registrations follow the template presented in
      Section 10.18.1.

   *  Registrations for the same name with different types are
      sufficiently close in functionality so as not to cause confusion
      for developers.

   *  The values in the discovery document are sufficient to provide
      optimization and hints to the client instance, but knowledge of
      the discovered value is not required for starting a transaction
      with the AS.

10.18.1.  Registration Template

   Name:
      An identifier for the parameter.

   Type:
      The JSON type allowed for the value.

   Reference:
      Reference to one or more documents that specify the value,
      preferably including a URI that can be used to retrieve a copy of
      the document(s).  An indication of the relevant sections may also
      be included but is not required.

10.18.2.  Initial Contents

     +======================================+==========+=============+
     | Name                                 | Type     | Reference   |
     +======================================+==========+=============+
     | grant_request_endpoint               | string   | Section 9   |
     |                                      |          | of RFC 9635 |
     +--------------------------------------+----------+-------------+
     | interaction_start_modes_supported    | array of | Section 9   |
     |                                      | strings  | of RFC 9635 |
     +--------------------------------------+----------+-------------+
     | interaction_finish_methods_supported | array of | Section 9   |
     |                                      | strings  | of RFC 9635 |
     +--------------------------------------+----------+-------------+
     | key_proofs_supported                 | array of | Section 9   |
     |                                      | strings  | of RFC 9635 |
     +--------------------------------------+----------+-------------+
     | sub_id_formats_supported             | array of | Section 9   |
     |                                      | strings  | of RFC 9635 |
     +--------------------------------------+----------+-------------+
     | assertion_formats_supported          | array of | Section 9   |
     |                                      | strings  | of RFC 9635 |
     +--------------------------------------+----------+-------------+
     | key_rotation_supported               | boolean  | Section 9   |
     |                                      |          | of RFC 9635 |
     +--------------------------------------+----------+-------------+

                                  Table 16

11.  Security Considerations

   In addition to the normative requirements in this document,
   implementors are strongly encouraged to consider these additional
   security considerations in implementations and deployments of GNAP.

11.1.  TLS Protection in Transit

   All requests in GNAP made over untrusted network connections have to
   be made over TLS as outlined in [BCP195] to protect the contents of
   the request and response from manipulation and interception by an
   attacker.  This includes all requests from a client instance to the
   AS, all requests from the client instance to an RS, and any requests
   back to a client instance such as the push-based interaction finish
   method.  Additionally, all requests between a browser and other
   components, such as during redirect-based interaction, need to be
   made over TLS or use equivalent protection such as a network
   connection local to the browser ("localhost").

   Even though requests from the client instance to the AS are signed,
   the signature method alone does not protect the request from
   interception by an attacker.  TLS protects the response as well as
   the request, preventing an attacker from intercepting requested
   information as it is returned.  This is particularly important in
   this specification for security artifacts such as nonces and for
   personal information such as subject information.

   The use of key-bound access tokens does not negate the requirement
   for protecting calls to the RS with TLS.  The keys and signatures
   associated with a bound access token will prevent an attacker from
   using a stolen token; however, without TLS, an attacker would be able
   to watch the data being sent to the RS and returned from the RS
   during legitimate use of the client instance under attack.
   Additionally, without TLS, an attacker would be able to profile the
   calls made between the client instance and RS, possibly gaining
   information about the functioning of the API between the client
   software and RS software that would otherwise be unknown to the
   attacker.

   Note that connections from the end user and RO's browser also need to
   be protected with TLS.  This applies during initial redirects to an
   AS's components during interaction, during any interaction with the
   RO, and during any redirect back to the client instance.  Without TLS
   protection on these portions of the process, an attacker could wait
   for a valid request to start and then take over the RO's interaction
   session.

11.2.  Signing Requests from the Client Software

   Even though all requests in GNAP need to be transmitted over TLS or
   its equivalent, the use of TLS alone is not sufficient to protect all
   parts of a multi-party and multi-stage protocol like GNAP, and TLS is
   not targeted at tying multiple requests to each other over time.  To
   account for this, GNAP makes use of message-level protection and key
   presentation mechanisms that strongly associate a request with a key
   held by the client instance (see Section 7).

   During the initial request from a client instance to the AS, the
   client instance has to identify and prove possession of a
   cryptographic key.  If the key is known to the AS, e.g., previously
   registered or dereferenceable to a trusted source, the AS can
   associate a set of policies to the client instance identified by the
   key.  Without the requirement that the client instance prove that it
   holds that key, the AS could not trust that the connection came from
   any particular client and could not apply any associated policies.

   Even more importantly, the client instance proving possession of a
   key on the first request allows the AS to associate future requests
   with each other by binding all future requests in that transaction to
   the same key.  The access token used for grant continuation is bound
   to the same key and proofing mechanism used by the client instance in
   its initial request; this means that the client instance needs to
   prove possession of that same key in future requests, which allows
   the AS to be sure that the same client instance is executing the
   follow-ups for a given ongoing grant request.  Therefore, the AS has
   to ensure that all subsequent requests for a grant are associated
   with the same key that started the grant or with the most recent
   rotation of that key.  This need holds true even if the initial key
   is previously unknown to the AS, such as would be the case when a
   client instance creates an ephemeral key for its request.  Without
   this ongoing association, an attacker would be able to impersonate a
   client instance in the midst of a grant request, potentially stealing
   access tokens and subject information with impunity.

   Additionally, all access tokens in GNAP default to be associated with
   the key that was presented during the grant request that created the
   access token.  This association allows an RS to know that the
   presenter of the access token is the same party that the token was
   issued to, as identified by their keys.  While non-bound bearer
   tokens are an option in GNAP, these types of tokens have their own
   trade-offs, which are discussed in Section 11.9.

   TLS functions at the transport layer, ensuring that only the parties
   on either end of that connection can read the information passed
   along that connection.  Each time a new connection is made, such as
   for a new HTTP request, a new trust that is mostly unrelated to
   previous connections is re-established.  While modern TLS does make
   use of session resumption, this still needs to be augmented with
   authentication methods to determine the identity of parties on the
   connections.  In other words, it is not possible with TLS alone to
   know that the same party is making a set of calls over time, since
   each time a new TLS connection is established, both the client and
   the server (or the server only when using MTLS (Section 7.3.2)) have
   to validate the other party's identity.  Such a verification can be
   achieved via methods described in [RFC9525], but these are not enough
   to establish the identity of the client instance in many cases.

   To counter this, GNAP defines a set of key binding methods in
   Section 7.3 that allows authentication and proof of possession by the
   caller, which is usually the client instance.  These methods are
   intended to be used in addition to TLS on all connections.

11.3.  MTLS Message Integrity

   The MTLS key proofing mechanism (Section 7.3.2) provides a means for
   a client instance to present a key using a certificate at the TLS
   layer.  Since TLS protects the entire HTTP message in transit,
   verification of the TLS client certificate presented with the message
   provides a sufficient binding between the two.  However, since TLS is
   functioning at a separate layer from HTTP, there is no direct
   connection between the TLS key presentation and the message itself,
   other than the fact that the message was presented over the TLS
   channel.  That is to say, any HTTP message can be presented over the
   TLS channel in question with the same level of trust.  The verifier
   is responsible for ensuring the key in the TLS client certificate is
   the one expected for a particular request.  For example, if the
   request is a grant request (Section 2), the AS needs to compare the
   TLS client certificate presented at the TLS layer to the key
   identified in the request content itself (either by value or through
   a referenced identifier).

   Furthermore, the prevalence of the TLS terminating reverse proxy
   (TTRP) pattern in deployments adds a wrinkle to the situation.  In
   this common pattern, the TTRP validates the TLS connection and then
   forwards the HTTP message contents onward to an internal system for
   processing.  The system processing the HTTP message no longer has
   access to the original TLS connection's information and context.  To
   compensate for this, the TTRP could inject the TLS client certificate
   into the forwarded request using the HTTP Client-Cert header field
   [RFC9111], giving the downstream system access to the certificate
   information.  The TTRP has to be trusted to provide accurate
   certificate information, and the connection between the TTRP and the
   downstream system also has to be protected.  The TTRP could provide
   some additional assurance, for example, by adding its own signature
   to the Client-Cert header field using HTTP message signatures
   [RFC9421].  This signature would be effectively ignored by GNAP
   (since it would not use GNAP's tag parameter value) but would be
   understood by the downstream service as part of its deployment.

   Additional considerations for different types of deployment patterns
   and key distribution mechanisms for MTLS are found in Section 11.4.

11.4.  MTLS Deployment Patterns

   GNAP does not specify how a client instance's keys could be made
   known to the AS ahead of time.  The Public Key Infrastructure (PKI)
   can be used to manage the keys used by client instances when calling
   the AS, allowing the AS to trust a root key from a trusted authority.
   This method is particularly relevant to the MTLS key proofing method,
   where the client instance presents its certificate to the AS as part
   of the TLS connection.  An AS using PKI to validate the MTLS
   connection would need to ensure that the presented certificate was
   issued by a trusted certificate authority before allowing the
   connection to continue.  PKI-based certificates would allow a key to
   be revoked and rotated through management at the certificate
   authority without requiring additional registration or management at
   the AS.  The PKI required to manage mutually authenticated TLS has
   historically been difficult to deploy, especially at scale, but it
   remains an appropriate solution for systems where the required
   management overhead is not an impediment.

   MTLS in GNAP need not use a PKI backing, as self-signed certificates
   and certificates from untrusted authorities can still be presented as
   part of a TLS connection.  In this case, the verifier would validate
   the connection but accept whatever certificate was presented by the
   client software.  This specific certificate can then be bound to all
   future connections from that client software by being bound to the
   resulting access tokens, in a trust-on-first-use pattern.  See
   Section 11.3 for more considerations on MTLS as a key proofing
   mechanism.

11.5.  Protection of Client Instance Key Material

   Client instances are identified by their unique keys, and anyone with
   access to a client instance's key material will be able to
   impersonate that client instance to all parties.  This is true for
   both calls to the AS as well as calls to an RS using an access token
   bound to the client instance's unique key.  As a consequence, it is
   of utmost importance for a client instance to protect its private key
   material.

   Different types of client software have different methods for
   creating, managing, and registering keys.  GNAP explicitly allows for
   ephemeral clients such as single-page applications (SPAs) and single-
   user clients (such as mobile applications) to create and present
   their own keys during the initial grant request without any explicit
   pre-registration step.  The client software can securely generate a
   key pair on the device and present the public key, along with proof
   of holding the associated private key, to the AS as part of the
   initial request.  To facilitate trust in these ephemeral keys, GNAP
   further allows for an extensible set of client information to be
   passed with the request.  This information can include device posture
   and third-party attestations of the client software's provenance and
   authenticity, depending on the needs and capabilities of the client
   software and its deployment.

   From GNAP's perspective, each distinct key is a different client
   instance.  However, multiple client instances can be grouped together
   by an AS policy and treated similarly to each other.  For instance,
   if an AS knows of several different keys for different servers within
   a cluster, the AS can decide that authorization of one of these
   servers applies to all other servers within the cluster.  An AS that
   chooses to do this needs to be careful with how it groups different
   client keys together in its policy, since the breach of one instance
   would have direct effects on the others in the cluster.

   Additionally, if an end user controls multiple instances of a single
   type of client software, such as having an application installed on
   multiple devices, each of these instances is expected to have a
   separate key and be issued separate access tokens.  However, if the
   AS is able to group these separate instances together as described
   above, it can streamline the authorization process for new instances
   of the same client software.  For example, if two client instances
   can present proof of a valid installation of a piece of client
   software, the AS would be able to associate the approval of the first
   instance of this software to all related instances.  The AS could
   then choose to bypass an explicit prompt of the RO for approval
   during authorization, since such approval has already been given.  An
   AS doing such a process would need to take assurance measures that
   the different instances are in fact correlated and authentic, as well
   as ensure that the expected RO is in control of the client instance.

   Finally, if multiple instances of client software each have the same
   key, then from GNAP's perspective, these are functionally the same
   client instance as GNAP has no reasonable way to differentiate
   between them.  This situation could happen if multiple instances
   within a cluster can securely share secret information among
   themselves.  Even though there are multiple copies of the software,
   the shared key makes these copies all present as a single instance.
   It is considered bad practice to share keys between copies of
   software unless they are very tightly integrated with each other and
   can be closely managed.  It is particularly bad practice to allow an
   end user to copy keys between client instances and to willingly use
   the same key in multiple instances.

11.6.  Protection of Authorization Server

   The AS performs critical functions in GNAP, including authenticating
   client software, managing interactions with end users to gather
   consent and provide notice, and issuing access tokens for client
   instances to present to RSs.  As such, protecting the AS is central
   to any GNAP deployment.

   If an attacker is able to gain control over an AS, they would be able
   to create fraudulent tokens and manipulate registration information
   to allow for malicious clients.  These tokens and clients would be
   trusted by other components in the ecosystem under the protection of
   the AS.

   If the AS uses signed access tokens, an attacker in control of the
   AS's signing keys would be able to manufacture fraudulent tokens for
   use at RSs under the protection of the AS.

   If an attacker is able to impersonate an AS, they would be able to
   trick legitimate client instances into making signed requests for
   information that could potentially be proxied to a real AS.  To
   combat this, all communications to the AS need to be made over TLS or
   its equivalent, and the software making the connection has to
   validate the certificate chain of the host it is connecting to.

   Consequently, protecting, monitoring, and auditing the AS is
   paramount to preserving the security of a GNAP-protected ecosystem.
   The AS presents attackers with a valuable target for attack.
   Fortunately, the core focus and function of the AS is to provide
   security for the ecosystem, unlike the RS whose focus is to provide
   an API or the client software whose focus is to access the API.

11.7.  Symmetric and Asymmetric Client Instance Keys

   Many of the cryptographic methods used by GNAP for key proofing can
   support both asymmetric and symmetric cryptography, and they can be
   extended to use a wide variety of mechanisms.  Implementors will find
   the available guidelines on cryptographic key management provided in
   [RFC4107] useful.  While symmetric cryptographic systems have some
   benefits in speed and simplicity, they have a distinct drawback --
   both parties need access to the same key in order to do both signing
   and verification of the message.  When more than two parties share
   the same symmetric key, data origin authentication is not provided.
   Any party that knows the symmetric key can compute a valid MAC;
   therefore, the contents could originate from any one of the parties.

   Use of symmetric cryptography means that when the client instance
   calls the AS to request a token, the AS needs to know the exact value
   of the client instance's key (or be able to derive it) in order to
   validate the key proof signature.  With asymmetric keys, the client
   needs to only send its public key to the AS to allow for verification
   that the client holds the associated private key, regardless of
   whether or not that key was pre-registered with the AS.

   Symmetric keys also have the expected advantage of providing better
   protection against quantum threats in the future.  Also, these types
   of keys (and their secure derivations) are widely supported among
   many cloud-based key management systems.

   When used to bind to an access token, a key value must be known by
   the RS in order to validate the proof signature on the request.
   Common methods for communicating these proofing keys include putting
   information in a structured access token and allowing the RS to look
   up the associated key material against the value of the access token.
   With symmetric cryptography, both of these methods would expose the
   signing key to the RS and, in the case of a structured access token,
   potentially to any party that can see the access token itself unless
   the token's payload has been encrypted.  Any of these parties would
   then be able to make calls using the access token by creating a valid
   signature using the shared key.  With asymmetric cryptography, the RS
   needs to only know the public key associated with the token in order
   to validate the request; therefore, the RS cannot create any new
   signed calls.

   While both signing approaches are allowed, GNAP treats these two
   classes of keys somewhat differently.  Only the public portion of
   asymmetric keys are allowed to be sent by value in requests to the AS
   when establishing a connection.  Since sending a symmetric key (or
   the private portion of an asymmetric key) would expose the signing
   material to any parties on the request path, including any attackers,
   sending these kinds of keys by value is prohibited.  Symmetric keys
   can still be used by client instances, but only if the client
   instance can send a reference to the key and not its value.  This
   approach allows the AS to use pre-registered symmetric keys as well
   as key derivation schemes to take advantage of symmetric cryptography
   without requiring key distribution at runtime, which would expose the
   keys in transit.

   Both the AS and client software can use systems such as hardware
   security modules to strengthen their key security storage and
   generation for both asymmetric and symmetric keys (see also
   Section 7.1.2).

11.8.  Generation of Access Tokens

   The contents of access tokens need to be such that only the
   generating AS would be able to create them, and the contents cannot
   be manipulated by an attacker to gain different or additional access
   rights.

   One method for accomplishing this is to use a cryptographically
   random value for the access token, generated by the AS using a secure
   randomization function with sufficiently high entropy.  The odds of
   an attacker guessing the output of the randomization function to
   collide with a valid access token are exceedingly small, and even
   then, the attacker would not have any control over what the access
   token would represent since that information would be held close by
   the AS.

   Another method for accomplishing this is to use a structured token
   that is cryptographically signed.  In this case, the payload of the
   access token declares to the RS what the token is good for, but the
   signature applied by the AS during token generation covers this
   payload.  Only the AS can create such a signature; therefore, only
   the AS can create such a signed token.  The odds of an attacker being
   able to guess a signature value with a useful payload are exceedingly
   small.  This technique only works if all targeted RSs check the
   signature of the access token.  Any RS that does not validate the
   signature of all presented tokens would be susceptible to injection
   of a modified or falsified token.  Furthermore, an AS has to
   carefully protect the keys used to sign access tokens, since anyone
   with access to these signing keys would be able to create seemingly
   valid access tokens using them.

11.9.  Bearer Access Tokens

   Bearer access tokens can be used by any party that has access to the
   token itself, without any additional information.  As a natural
   consequence, any RS that a bearer token is presented to has the
   technical capability of presenting that bearer token to another RS,
   as long as the token is valid.  It also means that any party that is
   able to capture the token value in storage or in transit is able to
   use the access token.  While bearer tokens are inherently simpler,
   this simplicity has been misapplied and abused in making needlessly
   insecure systems.  The downsides of bearer tokens have become more
   pertinent lately as stronger authentication systems have caused some
   attacks to shift to target tokens and APIs.

   In GNAP, key-bound access tokens are the default due to their higher
   security properties.  While bearer tokens can be used in GNAP, their
   use should be limited to cases where the simplicity benefits outweigh
   the significant security downsides.  One common deployment pattern is
   to use a gateway that takes in key-bound tokens on the outside and
   verifies the signatures on the incoming requests but translates the
   requests to a bearer token for use by trusted internal systems.  The
   bearer tokens are never issued or available outside of the internal
   systems, greatly limiting the exposure of the less-secure tokens but
   allowing the internal deployment to benefit from the advantages of
   bearer tokens.

11.10.  Key-Bound Access Tokens

   Key-bound access tokens, as the name suggests, are bound to a
   specific key and must be presented along with proof of that key
   during use.  The key itself is not presented at the same time as the
   token, so even if a token value is captured, it cannot be used to
   make a new request.  This is particularly true for an RS, which will
   see the token value but will not see the keys used to make the
   request (assuming asymmetric cryptography is in use, see
   Section 11.7).

   Key-bound access tokens provide this additional layer of protection
   only when the RS checks the signature of the message presented with
   the token.  Acceptance of an invalid presentation signature, or
   failure to check the signature entirely, would allow an attacker to
   make calls with a captured access token without having access to the
   related signing key material.

   In addition to validating the signature of the presentation message
   itself, the RS also needs to ensure that the signing key used is
   appropriate for the presented token.  If an RS does not ensure that
   the right keys were used to sign a message with a specific token, an
   attacker would be able to capture an access token and sign the
   request with their own keys, thereby negating the benefits of using
   key-bound access tokens.

   The RS also needs to ensure that sufficient portions of the message
   are covered by the signature.  Any items outside the signature could
   still affect the API's processing decisions, but these items would
   not be strongly bound to the token presentation.  As such, an
   attacker could capture a valid request and then manipulate portions
   of the request outside of the signature envelope in order to cause
   unwanted actions at the protected API.

   Some key-bound tokens are susceptible to replay attacks, depending on
   the details of the signing method used.  Therefore, key proofing
   mechanisms used with access tokens need to use replay-protection
   mechanisms covered under the signature such as a per-message nonce, a
   reasonably short time validity window, or other uniqueness
   constraints.  The details of using these will vary depending on the
   key proofing mechanism in use.  For example, HTTP message signatures
   have both a created and nonce signature parameter as well as the
   ability to cover significant portions of the HTTP message.  All of
   these can be used to limit the attack surface.

11.11.  Exposure of End-User Credentials to Client Instance

   As a delegation protocol, one of the main goals of GNAP is to prevent
   the client software from being exposed to any credentials or
   information about the end user or RO as a requirement of the
   delegation process.  By using the variety of interaction mechanisms,
   the RO can interact with the AS without ever authenticating to the
   client software and without the client software having to impersonate
   the RO through replay of their credentials.

   Consequently, no interaction methods defined in this specification
   require the end user to enter their credentials, but it is
   technologically possible for an extension to be defined to carry such
   values.  Such an extension would be dangerous as it would allow rogue
   client software to directly collect, store, and replay the end user's
   credentials outside of any legitimate use within a GNAP request.

   The concerns of such an extension could be mitigated through use of a
   challenge and response unlocked by the end user's credentials.  For
   example, the AS presents a challenge as part of an interaction start
   method, and the client instance signs that challenge using a key
   derived from a password presented by the end user.  It would be
   possible for the client software to collect this password in a secure
   software enclave without exposing the password to the rest of the
   client software or putting it across the wire to the AS.  The AS can
   validate this challenge response against a known password for the
   identified end user.  While an approach such as this does not remove
   all of the concerns surrounding such a password-based scheme, it is
   at least possible to implement in a more secure fashion than simply
   collecting and replaying the password.  Even so, such schemes should
   only ever be used by trusted clients due to the ease of abusing them.

11.12.  Mixing Up Authorization Servers

   If a client instance is able to work with multiple ASes
   simultaneously, it is possible for an attacker to add a compromised
   AS to the client instance's configuration and cause the client
   software to start a request at the compromised AS.  This AS could
   then proxy the client's request to a valid AS in order to attempt to
   get the RO to approve access for the legitimate client instance.

   A client instance needs to always be aware of which AS it is talking
   to throughout a grant process and ensure that any callback for one AS
   does not get conflated with the callback to different AS.  The
   interaction finish hash calculation in Section 4.2.3 allows a client
   instance to protect against this kind of substitution, but only if
   the client instance validates the hash.  If the client instance does
   not use an interaction finish method or does not check the
   interaction finish hash value, the compromised AS can be granted a
   valid access token on behalf of the RO.  See Sections 4.5.5 and 5.5
   of [AXELAND2021] for details of one such attack, which has been
   addressed in this document by including the grant endpoint in the
   interaction hash calculation.  Note that the client instance still
   needs to validate the hash for the attack to be prevented.

11.13.  Processing of Client-Presented User Information

   GNAP allows the client instance to present assertions and identifiers
   of the current user to the AS as part of the initial request.  This
   information should only ever be taken by the AS as a hint, since the
   AS has no way to tell if the represented person is present at the
   client software without using an interaction mechanism.  This
   information does not guarantee the given user is there, but it does
   constitute a statement by the client software that the AS can take
   into account.

   For example, if a specific user is claimed to be present prior to
   interaction, but a different user is shown to be present during
   interaction, the AS can either determine this to be an error or
   signal to the client instance through returned subject information
   that the current user has changed from what the client instance
   thought.  This user information can also be used by the AS to
   streamline the interaction process when the user is present.  For
   example, instead of having the user type in their account identifier
   during interaction at a redirected URI, the AS can immediately
   challenge the user for their account credentials.  Alternatively, if
   an existing session is detected, the AS can determine that it matches
   the identifier provided by the client and subsequently skip an
   explicit authentication event by the RO.

   In cases where the AS trusts the client software more completely, due
   to policy or previous approval of a given client instance, the AS can
   take this user information as a statement that the user is present
   and could issue access tokens and release subject information without
   interaction.  The AS should only take such action in very limited
   circumstances, as a client instance could assert whatever it likes
   for the user's identifiers in its request.  The AS can limit the
   possibility of this by issuing randomized opaque identifiers to
   client instances to represent different end-user accounts after an
   initial login.

   When a client instance presents an assertion to the AS, the AS needs
   to evaluate that assertion.  Since the AS is unlikely to be the
   intended audience of an assertion held by the client software, the AS
   will need to evaluate the assertion in a different context.  Even in
   this case, the AS can still evaluate that the assertion was generated
   by a trusted party, was appropriately signed, and is within any time
   validity windows stated by the assertion.  If the client instance's
   audience identifier is known to the AS and can be associated with the
   client instance's presented key, the AS can also evaluate that the
   appropriate client instance is presenting the claimed assertion.  All
   of this will prevent an attacker from presenting a manufactured
   assertion or one captured from an untrusted system.  However, without
   validating the audience of the assertion, a captured assertion could
   be presented by the client instance to impersonate a given end user.
   In such cases, the assertion offers little more protection than a
   simple identifier would.

   A special case exists where the AS is the generator of the assertion
   being presented by the client instance.  In these cases, the AS can
   validate that it did issue the assertion and it is associated with
   the client instance presenting the assertion.

11.14.  Client Instance Pre-registration

   Each client instance is identified by its own unique key, and for
   some kinds of client software such as a web server or backend system,
   this identification can be facilitated by registering a single key
   for a piece of client software ahead of time.  This registration can
   be associated with a set of display attributes to be used during the
   authorization process to identify the client software to the user.
   In these cases, it can be assumed that only one instance of client
   software will exist, likely to serve many different users.

   A client's registration record needs to include its identifying key.
   Furthermore, it is the case that any clients using symmetric
   cryptography for key proofing mechanisms need to have their keys pre-
   registered.  The registration should also include any information
   that would aid in the authorization process, such as a display name
   and logo.  The registration record can also limit a given client to
   ask for certain kinds of information or use specific interaction
   mechanisms at runtime.

   It also is sensible to pre-register client instances when the
   software is acting autonomously, without the need for a runtime
   approval by an RO or any interaction with an end user.  In these
   cases, an AS needs to rely on the trust decisions that have been
   determined prior to runtime to determine what rights and tokens to
   grant to a given client instance.

   However, it does not make sense to pre-register many types of
   clients.  Single-page applications (SPAs) and mobile/desktop
   applications in particular present problems with pre-registration.
   For SPAs, the instances are ephemeral in nature, and long-term
   registration of a single instance leads to significant storage and
   management overhead at the AS.  For mobile applications, each
   installation of the client software is a separate instance, and
   sharing a key among all instances would be detrimental to security as
   the compromise of any single installation would compromise all copies
   for all users.

   An AS can treat these classes of client software differently from
   each other, perhaps by allowing access to certain high-value APIs
   only to pre-registered known clients or by requiring an active end-
   user delegation of authority to any client software not pre-
   registered.

   An AS can also provide warnings and caveats to ROs during the
   authorization process, allowing the user to make an informed decision
   regarding the software they are authorizing.  For example, if the AS
   has vetted the client software and this specific instance, it can
   present a different authorization screen compared to a client
   instance that is presenting all of its information at runtime.

   Finally, an AS can use platform attestations and other signals from
   the client instance at runtime to determine whether or not the
   software making the request is legitimate.  The details of such
   attestations are outside the scope of this specification, but the
   client portion of a grant request provides a natural extension point
   to such information through the "GNAP Client Instance Fields"
   registry (Section 10.7).

11.15.  Client Instance Impersonation

   If client instances are allowed to set their own user-facing display
   information, such as a display name and website URL, a malicious
   client instance could impersonate legitimate client software for the
   purposes of tricking users into authorizing the malicious client.

   Requiring clients to pre-register does not fully mitigate this
   problem since many pre-registration systems have self-service portals
   for management of client registration, allowing authenticated
   developers to enter self-asserted information into the management
   portal.

   An AS can mitigate this by actively filtering all self-asserted
   values presented by client software, both dynamically as part of GNAP
   and through a registration portal, to limit the kinds of
   impersonation that could be done.

   An AS can also warn the RO about the provenance of the information it
   is displaying, allowing the RO to make a more informed delegation
   decision.  For example, an AS can visually differentiate between a
   client instance that can be traced back to a specific developer's
   registration and an instance that has self-asserted its own display
   information.

11.16.  Client-Hosted Logo URI

   The logo_uri client display field defined in Section 2.3.2 allows the
   client instance to specify a URI from which an image can be fetched
   for display during authorization decisions.  When the URI points to
   an externally hosted resource (as opposed to a data: URI), the
   logo_uri field presents challenges in addition to the considerations
   in Section 11.15.

   When a logo_uri is externally hosted, the client software (or the
   host of the asset) can change the contents of the logo without
   informing the AS.  Since the logo is considered an aspect of the
   client software's identity, this flexibility allows for a more
   dynamically managed client instance that makes use of the distributed
   systems.

   However, this same flexibility allows the host of the asset to change
   the hosted file in a malicious way, such as replacing the image
   content with malicious software for download or imitating a different
   piece of client software.  Additionally, the act of fetching the URI
   could accidentally leak information to the image host in the HTTP
   Referer header field, if one is sent.  Even though GNAP intentionally
   does not include security parameters in front-channel URIs wherever
   possible, the AS still should take steps to ensure that this
   information does not leak accidentally, such as setting a referrer
   policy on image links or displaying images only from pages served
   from a URI with no sensitive security or identity information.

   To avoid these issues, the AS can insist on the use of data: URIs,
   though that might not be practical for all types of client software.
   Alternatively, the AS could pre-fetch the content of the URI and
   present its own copy to the RO instead.  This practice opens the AS
   to potential SSRF attacks, as discussed in Section 11.34.

11.17.  Interception of Information in the Browser

   Most information passed through the web browser is susceptible to
   interception and possible manipulation by elements within the browser
   such as scripts loaded within pages.  Information in the URI is
   exposed through browser and server logs, and it can also leak to
   other parties through HTTP Referer headers.

   GNAP's design limits the information passed directly through the
   browser, allowing for opaque URIs in most circumstances.  For the
   redirect-based interaction finish mechanism, named query parameters
   are used to carry unguessable opaque values.  For these, GNAP
   requires creation and validation of a cryptographic hash to protect
   the query parameters added to the URI and associate them with an
   ongoing grant process and values not passed in the URI.  The client
   instance has to properly validate this hash to prevent an attacker
   from injecting an interaction reference intended for a different AS
   or client instance.

   Several interaction start mechanisms use URIs created by the AS and
   passed to the client instance.  While these URIs are opaque to the
   client instance, it's possible for the AS to include parameters,
   paths, and other pieces of information that could leak security data
   or be manipulated by a party in the middle of the transaction.  An AS
   implementation can avoid this problem by creating URIs using
   unguessable values that are randomized for each new grant request.

11.18.  Callback URI Manipulation

   The callback URI used in interaction finish mechanisms is defined by
   the client instance.  This URI is opaque to the AS but can contain
   information relevant to the client instance's operations.  In
   particular, the client instance can include state information to
   allow the callback request to be associated with an ongoing grant
   request.

   Since this URI is exposed to the end user's browser, it is
   susceptible to both logging and manipulation in transit before the
   request is made to the client software.  As such, a client instance
   should never put security-critical or private information into the
   callback URI in a cleartext form.  For example, if the client
   software includes a post-redirect target URI in its callback URI to
   the AS, this target URI could be manipulated by an attacker, creating
   an open redirector at the client.  Instead, a client instance can use
   an unguessable identifier in the URI that can then be used by the
   client software to look up the details of the pending request.  Since
   this approach requires some form of statefulness by the client
   software during the redirection process, clients that are not capable
   of holding state through a redirect should not use redirect-based
   interaction mechanisms.

11.19.  Redirection Status Codes

   As described in [OAUTH-SEC-TOPICS], a server should never use HTTP
   status code 307 (Temporary Redirect) to redirect a request that
   potentially contains user credentials.  If an HTTP redirect is used
   for such a request, HTTP status code 303 (See Other) should be used
   instead.

   Status code 307 (Temporary Redirect), as defined in the HTTP standard
   [HTTP], requires the user agent to preserve the method and content of
   a request, thus submitting the content of the POST request to the
   redirect target.  In the HTTP standard [HTTP], only status code 303
   (See Other) unambiguously enforces rewriting the HTTP POST request to
   an HTTP GET request, which eliminates the POST content from the
   redirected request.  For all other status codes, including status
   code 302 (Found), user agents are allowed to keep a redirected POST
   request as a POST and thus can resubmit the content.

   The use of status code 307 (Temporary Redirect) results in a
   vulnerability when using the redirect interaction finish method
   (Section 3.3.5).  With this method, the AS potentially prompts the RO
   to enter their credentials in a form that is then submitted back to
   the AS (using an HTTP POST request).  The AS checks the credentials
   and, if successful, may immediately redirect the RO to the client
   instance's redirect URI.  Due to the use of status code 307
   (Temporary Redirect), the RO's user agent now transmits the RO's
   credentials to the client instance.  A malicious client instance can
   then use the obtained credentials to impersonate the RO at the AS.

   Redirection away from the initial URI in an interaction session could
   also leak information found in that initial URI through the HTTP
   Referer header field, which would be sent by the user agent to the
   redirect target.  To avoid such leakage, a server can first redirect
   to an internal interstitial page without any identifying or sensitive
   information on the URI before processing the request.  When the user
   agent is ultimately redirected from this page, no part of the
   original interaction URI will be found in the Referer header.

11.20.  Interception of Responses from the AS

   Responses from the AS contain information vital to both the security
   and privacy operations of GNAP.  This information includes nonces
   used in cryptographic calculations, Subject Identifiers, assertions,
   public keys, and information about what client software is requesting
   and was granted.

   In addition, if bearer tokens are used or keys are issued alongside a
   bound access token, the response from the AS contains all information
   necessary for use of the contained access token.  Any party that is
   capable of viewing such a response, such as an intermediary proxy,
   would be able to exfiltrate and use this token.  If the access token
   is instead bound to the client instance's presented key,
   intermediaries no longer have sufficient information to use the
   token.  They can still, however, gain information about the end user
   as well as the actions of the client software.

11.21.  Key Distribution

   GNAP does not define ways for the client instances keys to be
   provided to the client instances, particularly in light of how those
   keys are made known to the AS.  These keys could be generated
   dynamically on the client software or pre-registered at the AS in a
   static developer portal.  The keys for client instances could also be
   distributed as part of the deployment process of instances of the
   client software.  For example, an application installation framework
   could generate a key pair for each copy of client software and then
   both install it into the client software upon installation and
   register that instance with the AS.

   Alternatively, it's possible for the AS to generate keys to be used
   with access tokens that are separate from the keys used by the client
   instance to request tokens.  In this method, the AS would generate
   the asymmetric key pair or symmetric key and return the public key or
   key reference to the client instance alongside the access token
   itself.  The means for the AS to return generated key values to the
   client instance are out of scope, since GNAP does not allow the
   transmission of private or shared key information within the protocol
   itself.

   Additionally, if the token is bound to a key other than the client
   instance's presented key, this opens a possible attack surface for an
   attacker's AS to request an access token and then substitute their
   own key material in the response to the client instance.  The
   attacker's AS would need to be able to use the same key as the client
   instance, but this setup would allow an attacker's AS to make use of
   a compromised key within a system.  This attack can be prevented by
   only binding access tokens to the client instance's presented keys
   and by having client instances have a strong association between
   which keys they expect to use and the AS they expect to use them on.
   This attack is also only able to be propagated on client instances
   that talk to more than one AS at runtime, which can be limited by the
   client software.

11.22.  Key Rotation Policy

   When keys are rotated, there could be a delay in the propagation of
   that rotation to various components in the AS's ecosystem.  The AS
   can define its own policy regarding the timeout of the previously
   bound key, either making it immediately obsolete or allowing for a
   limited grace period during which both the previously bound key and
   the current key can be used for signing requests.  Such a grace
   period can be useful when there are multiple running copies of the
   client that are coordinated with each other.  For example, the client
   software could be deployed as a cloud service with multiple
   orchestrated nodes.  Each of these copies is deployed using the same
   key; therefore, all the nodes represent the same client instance to
   the AS.  In such cases, it can be difficult, or even impossible, to
   update the keys on all these copies in the same instant.

   The need to accommodate such known delays in the system needs to be
   balanced with the risk of allowing an old key to still be used.
   Narrowly restricting the exposure opportunities for exploit at the AS
   in terms of time, place, and method makes exploit significantly more
   difficult, especially if the exception happens only once.  For
   example, the AS can reject requests from the previously bound key (or
   any previous one before it) to cause rotation to a new key or at
   least ensure that the rotation happens in an idempotent way to the
   same new key.

   See also the related considerations for token values in
   Section 11.33.

11.23.  Interaction Finish Modes and Polling

   During the interaction process, the client instance usually hands
   control of the user experience over to another component, be it the
   system browser, another application, or some action the RO is
   instructed to take on another device.  By using an interaction finish
   method, the client instance can be securely notified by the AS when
   the interaction is completed and the next phase of the protocol
   should occur.  This process includes information that the client
   instance can use to validate the finish call from the AS and prevent
   some injection, session hijacking, and phishing attacks.

   Some types of client deployment are unable to receive an interaction
   finish message.  Without an interaction finish method to notify it,
   the client instance will need to poll the grant continuation API
   while waiting for the RO to approve or deny the request.  An attacker
   could take advantage of this situation by capturing the interaction
   start parameters and phishing a legitimate user into authorizing the
   attacker's waiting client instance, which would in turn have no way
   of associating the completed interaction from the targeted user with
   the start of the request from the attacker.

   However, it is important to note that this pattern is practically
   indistinguishable from some legitimate use cases.  For example, a
   smart device emits a code for the RO to enter on a separate device.
   The smart device has to poll because the expected behavior is that
   the interaction will take place on the separate device, without a way
   to return information to the original device's context.

   As such, developers need to weigh the risks of forgoing an
   interaction finish method against the deployment capabilities of the
   client software and its environment.  Due to the increased security,
   an interaction finish method should be employed whenever possible.

11.24.  Session Management for Interaction Finish Methods

   When using an interaction finish method such as redirect or push, the
   client instance receives an unsolicited inbound request from an
   unknown party over HTTPS.  The client instance needs to be able to
   successfully associate this incoming request with a specific pending
   grant request being managed by the client instance.  If the client
   instance is not careful and precise about this, an attacker could
   associate their own session at the client instance with a stolen
   interaction response.  The means of preventing this vary by the type
   of client software and interaction methods in use.  Some common
   patterns are enumerated here.

   If the end user interacts with the client instance through a web
   browser and the redirect interaction finish method is used, the
   client instance can ensure that the incoming HTTP request from the
   finish method is presented in the same browser session that the grant
   request was started in.  This technique is particularly useful when
   the redirect interaction start mode is used as well, since in many
   cases, the end user will follow the redirection with the same browser
   that they are using to interact with the client instance.  The client
   instance can then store the relevant pending grant information in the
   session, either in the browser storage directly (such as with a
   single-page application) or in an associated session store on a
   backend server.  In both cases, when the incoming request reaches the
   client instance, the session information can be used to ensure that
   the same party that started the request is present as the request
   finishes.

   Ensuring that the same party that started a request is present when
   that request finishes can prevent phishing attacks, where an attacker
   starts a request at an honest client instance and tricks an honest RO
   into authorizing it.  For example, if an honest end user (that also
   acts as the RO) wants to start a request through a client instance
   controlled by the attacker, the attacker can start a request at an
   honest client instance and then redirect the honest end user to the
   interaction URI from the attackers session with the honest client
   instance.  If the honest end user then fails to realize that they are
   not authorizing the attacker-controlled client instance (with which
   it started its request) but instead the honest client instance when
   interacting with the AS, the attacker's session with the honest
   client instance would be authorized.  This would give the attacker
   access to the honest end user's resources that the honest client
   instance is authorized to access.  However, if after the interaction,
   the AS redirects the honest end user back to the client instance
   whose grant request the end user just authorized, the honest end user
   is redirected to the honest client instance.  The honest client
   instance can then detect that the end user is not the party that
   started the request, since the request at the honest client instance
   was started by the attacker.  This detection can prevent the attack.
   This is related to the discussion in Section 11.15, because again the
   attack can be prevented by the AS informing the user as much as
   possible about the client instance that is to be authorized.

   If the end user does not interact with the client instance through a
   web browser or the interaction start method does not use the same
   browser or device that the end user is interacting through (such as
   the launch of a second device through a scannable code or
   presentation of a user code), the client instance will not be able to
   strongly associate an incoming HTTP request with an established
   session with the end user.  This is also true when the push
   interaction finish method is used, since the HTTP request comes
   directly from the interaction component of the AS.  In these
   circumstances, the client instance can at least ensure that the
   incoming HTTP request can be uniquely associated with an ongoing
   grant request by making the interaction finish callback URI unique
   for the grant when making the interaction request (Section 2.5.2).
   Mobile applications and other client instances that generally serve
   only a single end user at a time can use this unique incoming URL to
   differentiate between a legitimate incoming request and an attacker's
   stolen request.

11.25.  Calculating Interaction Hash

   While the use of GNAP's signing mechanisms and token-protected grant
   API provides significant security protections to the protocol, the
   interaction reference mechanism is susceptible to monitoring,
   capture, and injection by an attacker.  To combat this, GNAP requires
   the calculation and verification of an interaction hash.  A client
   instance might be tempted to skip this step, but doing so leaves the
   client instance open to injection and manipulation by an attacker
   that could lead to additional issues.

   The calculation of the interaction hash value provides defense in
   depth, allowing a client instance to protect itself from spurious
   injection of interaction references when using an interaction finish
   method.  The AS is protected during this attack through the
   continuation access token being bound to the expected interaction
   reference, but without hash calculation, the attacker could cause the
   client to make an HTTP request on command, which could itself be
   manipulated -- for example, by including a malicious value in the
   interaction reference designed to attack the AS.  With both of these
   in place, an attacker attempting to substitute the interaction
   reference is stopped in several places.

    .----.        .------.       +--------+      +--------+
   | User |      |Attacker|      | Client |      |   AS   |
   |      |      |        |      |Instance|      |        |
   |      |      |        |      |        |      |        |
   |      |      |        +=(1)=>|        |      |        |
   |      |      |        |      |        +-(2)->|        |
   |      |      |        |      |        |<-(3)-+        |
   |      |      |        |<=(4)=+        |      |        |
   |      |      |        |      |        |      |        |
   |      |      |        +==(5)================>|        |
   |      |      |        |      |        |      |        |
   |      |      |        |<================(6)==+        |
   |      |      |        |      |        |      |        |
   |      +==(A)================>|        |      |        |
   |      |      |        |      |        +-(B)->|        |
   |      |      |        |      |        |<-(C)-+        |
   |      |<=================(D)=+        |      |        |
   |      |      |        |      |        |      |        |
   |      +==(E)================================>|        |
   |      |      |        |      |        |      |        |
   |      |<=(7)=+        |      |        |      |        |
   |      |      |        |      |        |      |        |
   |      +==(F)================>|        |      |        |
   |      |      |        |      |        +-(G)->|        |
   |      |      |        |      |        |      |        |
    `----`        `------`       +--------+      +--------+

                     Figure 11: Interaction Hash Attack

   Prerequisites: The client instance can allow multiple end users to
   access the same AS.  The attacker is attempting to associate their
   rights with the target user's session.

   *  (1) The attacker starts a session at the client instance.

   *  (2) The client instance creates a grant request with nonce CN1.

   *  (3) The AS responds to the grant request with a need to interact,
      nonce SN1, and a continuation token, CT1.

   *  (4) The client instructs the attacker to interact at the AS.

   *  (5) The attacker interacts at the AS.

   *  (6) The AS completes the interact finish with interact reference
      IR1 and interact hash IH1 calculated from (CN1 + SN1 + IR1 + AS).
      The attacker prevents IR1 from returning to the client instance.

   *  (A) The target user starts a session at the client instance.

   *  (B) The client instance creates a grant request with nonce CN2.

   *  (C) The AS responds to the grant request with a need to interact,
      nonce SN2, and a continuation token, CT2.

   *  (D) The client instance instructs the user to interact at the AS.

   *  (E) The target user interacts at the AS.

   *  (7) Before the target user can complete their interaction, the
      attacker delivers their own interact reference IR1 into the user's
      session.  The attacker cannot calculate the appropriate hash
      because the attacker does not have access to CN2 and SN2.

   *  (F) The target user triggers the interaction finish in their own
      session with the attacker's IR1.

   *  (G) If the client instance is checking the interaction hash, the
      attack stops here because the hash calculation of (CN2 + SN2 + IR1
      + AS) will fail.  If the client instance does not check the
      interaction hash, the client instance will be tricked into
      submitting the interaction reference to the AS.  Here, the AS will
      reject the interaction request because it is presented against CT2
      and not CT1 as expected.  However, an attacker who has potentially
      injected CT1 as the value of CT2 would be able to continue the
      attack.

   Even with additional checks in place, client instances using
   interaction finish mechanisms are responsible for checking the
   interaction hash to provide security to the overall system.

11.26.  Storage of Information during Interaction and Continuation

   When starting an interactive grant request, a client application has
   a number of protocol elements that it needs to manage, including
   nonces, references, keys, access tokens, and other elements.  During
   the interaction process, the client instance usually hands control of
   the user experience over to another component, be it the system
   browser, another application, or some action the RO is instructed to
   take on another device.  In order for the client instance to make its
   continuation call, it will need to recall all of these protocol
   elements at a future time.  Usually, this means the client instance
   will need to store these protocol elements in some retrievable
   fashion.

   If the security protocol elements are stored on the end user's
   device, such as in browser storage or in local application data
   stores, capture and exfiltration of this information could allow an
   attacker to continue a pending transaction instead of the client
   instance.  Client software can make use of secure storage mechanisms,
   including hardware-based key and data storage, to prevent such
   exfiltration.

   Note that in GNAP, the client instance has to choose its interaction
   finish URI prior to making the first call to the AS.  As such, the
   interaction finish URI will often have a unique identifier for the
   ongoing request, allowing the client instance to access the correct
   portion of its storage.  Since this URI is passed to other parties
   and often used through a browser, this URI should not contain any
   security-sensitive information that would be valuable to an attacker,
   such as any token identifier, nonce, or user information.  Instead, a
   cryptographically random value is suggested, and that value should be
   used to index into a secure session or storage mechanism.

11.27.  Denial of Service (DoS) through Grant Continuation

   When a client instance starts off an interactive process, it will
   eventually need to continue the grant request in a subsequent message
   to the AS.  It's possible for a naive client implementation to
   continuously send continuation requests to the AS while waiting for
   approval, especially if no interaction finish method is used.  Such
   constant requests could overwhelm the AS's ability to respond to both
   these and other requests.

   To mitigate this for well-behaved client software, the continuation
   response contains a wait parameter that is intended to tell the
   client instance how long it should wait until making its next
   request.  This value can be used to back off client software that is
   checking too quickly by returning increasing wait times for a single
   client instance.

   If client software ignores the wait value and makes its continuation
   calls too quickly or if the client software assumes the absence of
   the wait values means it should poll immediately, the AS can choose
   to return errors to the offending client instance, including possibly
   canceling the ongoing grant request.  With well-meaning client
   software, these errors can indicate a need to change the client
   software's programmed behavior.

11.28.  Exhaustion of Random Value Space

   Several parts of the GNAP process make use of unguessable randomized
   values, such as nonces, tokens, user codes, and randomized URIs.
   Since these values are intended to be unique, a sufficiently powerful
   attacker could make a large number of requests to trigger generation
   of randomized values in an attempt to exhaust the random number
   generation space.  While this attack is particularly applicable to
   the AS, client software could likewise be targeted by an attacker
   triggering new grant requests against an AS.

   To mitigate this, software can ensure that its random values are
   chosen from a significantly large pool so that exhaustion of that
   pool is prohibitive for an attacker.  Additionally, the random values
   can be time-boxed in such a way that their validity windows are
   reasonably short.  Since many of the random values used within GNAP
   are used within limited portions of the protocol, it is reasonable
   for a particular random value to be valid for only a small amount of
   time.  For example, the nonces used for interaction finish hash
   calculation need only to be valid while the client instance is
   waiting for the finish callback and can be functionally expired when
   the interaction has completed.  Similarly, artifacts like access
   tokens and the interaction reference can be limited to have lifetimes
   tied to their functional utility.  Finally, each different category
   of artifact (nonce, token, reference, identifier, etc.) can be
   generated from a separate random pool of values instead of a single
   global value space.

11.29.  Front-Channel URIs

   Some interaction methods in GNAP make use of URIs accessed through
   the end user's browser, known collectively as front-channel
   communication.  These URIs are most notably present in the redirect
   interaction start method and the redirect interaction finish mode.
   Since these URIs are intended to be given to the end user, the end
   user and their browser will be subjected to anything hosted at that
   URI including viruses, malware, and phishing scams.  This kind of
   risk is inherent to all redirection-based protocols, including GNAP,
   when used in this way.

   When talking to a new or unknown AS, a client instance might want to
   check the URI from the interaction start against a blocklist and warn
   the end user before redirecting them.  Many client instances will
   provide an interstitial message prior to redirection in order to
   prepare the user for control of the user experience being handed to
   the domain of the AS, and such a method could be used to warn the
   user of potential threats (for instance, a rogue AS impersonating a
   well-known service provider).  Client software can also prevent this
   by managing an allowlist of known and trusted ASes.

   Alternatively, an attacker could start a GNAP request with a known
   and trusted AS but include their own attack site URI as the callback
   for the redirect finish method.  The attacker would then send the
   interaction start URI to the victim and get them to click on it.
   Since the URI is at the known AS, the victim is inclined to do so.
   The victim will then be prompted to approve the attacker's
   application, and in most circumstances, the victim will then be
   redirected to the attacker's site whether or not the user approved
   the request.  The AS could mitigate this partially by using a
   blocklist and allowlist of interaction finish URIs during the client
   instance's initial request, but this approach can be especially
   difficult if the URI has any dynamic portion chosen by the client
   software.  The AS can couple these checks with policies associated
   with the client instance that has been authenticated in the request.
   If the AS has any doubt about the interaction finish URI, the AS can
   provide an interstitial warning to the end user before processing the
   redirect.

   Ultimately, all protocols that use redirect-based communication
   through the user's browser are susceptible to having an attacker try
   to co-opt one or more of those URIs in order to harm the user.  It is
   the responsibility of the AS and the client software to provide
   appropriate warnings, education, and mitigation to protect end users.

11.30.  Processing Assertions

   Identity assertions can be used in GNAP to convey subject
   information, both from the AS to the client instance in a response
   (Section 3.4) and from the client instance to the AS in a request
   (Section 2.2).  In both of these circumstances, when an assertion is
   passed in GNAP, the receiver of the assertion needs to parse and
   process the assertion.  As assertions are complex artifacts with
   their own syntax and security, special care needs to be taken to
   prevent the assertion values from being used as an attack vector.

   All assertion processing needs to account for the security aspects of
   the assertion format in use.  In particular, the processor needs to
   parse the assertion from a JSON string object and apply the
   appropriate cryptographic processes to ensure the integrity of the
   assertion.

   For example, when SAML 2.0 assertions are used, the receiver has to
   parse an XML document.  There are many well-known security
   vulnerabilities in XML parsers, and the XML standard itself can be
   attacked through the use of processing instructions and entity
   expansions to cause problems with the processor.  Therefore, any
   system capable of processing SAML 2.0 assertions also needs to have a
   secure and correct XML parser.  In addition to this, the SAML 2.0
   specification uses XML Signatures, which have their own
   implementation problems that need to be accounted for.  Similar
   requirements exist for OpenID Connect ID Token, which is based on the
   JWT format and the related JOSE cryptography suite.

11.31.  Stolen Token Replay

   If a client instance can request tokens at multiple ASes and the
   client instance uses the same keys to make its requests across those
   different ASes, then it is possible for an attacker to replay a
   stolen token issued by an honest AS from a compromised AS, thereby
   binding the stolen token to the client instance's key in a different
   context.  The attacker can manipulate the client instance into using
   the stolen token at an RS, particularly at an RS that is expecting a
   token from the honest AS.  Since the honest AS issued the token and
   the client instance presents the token with its expected bound key,
   the attack succeeds.

   This attack has several preconditions.  In this attack, the attacker
   does not need access to the client instance's key and cannot use the
   stolen token directly at the RS, but the attacker is able to get the
   access token value in some fashion.  The client instance also needs
   to be configured to talk to multiple ASes, including the attacker's
   controlled AS.  Finally, the client instance needs to be able to be
   manipulated by the attacker to call the RS while using a token issued
   from the stolen AS.  The RS does not need to be compromised or made
   to trust the attacker's AS.

   To protect against this attack, the client instance can use a
   different key for each AS that it talks to.  Since the replayed token
   will be bound to the key used at the honest AS, the uncompromised RS
   will reject the call since the client instance will be using the key
   used at the attacker's AS instead with the same token.  When the MTLS
   key proofing method is used, a client instance can use self-signed
   certificates to use a different key for each AS that it talks to, as
   discussed in Section 11.4.

   Additionally, the client instance can keep a strong association
   between the RS and a specific AS that it trusts to issue tokens for
   that RS.  This strong binding also helps against some forms of AS
   mix-up attacks (Section 11.12).  Managing this binding is outside the
   scope of this specification, but it can be managed either as a
   configuration element for the client instance or dynamically through
   discovering the AS from the RS (Section 9.1).

   The details of this attack, with additional discussion and
   considerations, are available in Section 3.2 of [HELMSCHMIDT2022].

11.32.  Self-Contained Stateless Access Tokens

   The contents and format of the access token are at the discretion of
   the AS and are opaque to the client instance within GNAP.  As
   discussed in [GNAP-RS], the AS and RS can make use of stateless
   access tokens with an internal structure and format.  These access
   tokens allow an RS to validate the token without having to make any
   external calls at runtime, allowing for benefits in some deployments,
   the discussion of which is outside the scope of this specification.

   However, the use of such self-contained access tokens has an effect
   on the ability of the AS to provide certain functionality defined
   within this specification.  Specifically, since the access token is
   self-contained, it is difficult or impossible for an AS to signal to
   all RSs within an ecosystem when a specific access token has been
   revoked.  Therefore, an AS in such an ecosystem should probably not
   offer token revocation functionality to client instances, since the
   client instance's calls to such an endpoint are effectively
   meaningless.  However, a client instance calling the token revocation
   function will also throw out its copy of the token, so such a placebo
   endpoint might not be completely meaningless.  Token rotation is
   similarly difficult because the AS has to revoke the old access token
   after a rotation call has been made.  If the access tokens are
   completely self-contained and non-revocable, this means that there
   will be a period of time during which both the old and new access
   tokens are valid and usable, which is an increased security risk for
   the environment.

   These problems can be mitigated by keeping the validity time windows
   of self-contained access tokens reasonably short, limiting the time
   after a revocation event that a revoked token could be used.
   Additionally, the AS could proactively signal to RSs under its
   control identifiers for revoked tokens that have yet to expire.  This
   type of information push would be expected to be relatively small and
   infrequent, and its implementation is outside the scope of this
   specification.

11.33.  Network Problems and Token and Grant Management

   If a client instance makes a call to rotate an access token but the
   network connection is dropped before the client instance receives the
   response with the new access token, the system as a whole can end up
   in an inconsistent state, where the AS has already rotated the old
   access token and invalidated it, but the client instance only has
   access to the invalidated access token and not the newly rotated
   token value.  If the client instance retries the rotation request, it
   would fail because the client is no longer presenting a valid and
   current access token.  A similar situation can occur during grant
   continuation, where the same client instance calls to continue or
   update a grant request without successfully receiving the results of
   the update.

   To combat this, both grant management (Section 5) and token
   management (Section 6) can be designed to be idempotent, where
   subsequent calls to the same function with the same credentials are
   meant to produce the same results.  For example, multiple calls to
   rotate the same access token need to result in the same rotated token
   value, within a reasonable time window.

   In practice, an AS can hold onto an old token value for such limited
   purposes.  For example, to support rotating access tokens over
   unreliable networks, the AS receives the initial request to rotate an
   access token and creates a new token value and returns it.  The AS
   also marks the old token value as having been used to create the
   newly rotated token value.  If the AS sees the old token value within
   a small enough time window, such as a few seconds since the first
   rotation attempt, the AS can return the same rotated access token
   value.  Furthermore, once the system has seen the newly rotated token
   in use, the original token can be discarded because the client
   instance has proved that it did receive the token.  The result of
   this is a system that is eventually self-consistent without placing
   an undue complexity burden on the client instance to manage
   problematic networks.

11.34.  Server-Side Request Forgery (SSRF)

   There are several places within GNAP where a URI can be given to a
   party, causing it to fetch that URI during normal operation of the
   protocol.  If an attacker is able to control the value of one of
   these URIs within the protocol, the attacker could cause the target
   system to execute a request on a URI that is within reach of the
   target system but normally unavailable to the attacker.  Examples
   include an attacker sending a URL of http://localhost/admin to cause
   the server to access an internal function on itself or
   https://192.168.0.14/ to call a service behind a firewall.  Even if
   the attacker does not gain access to the results of the call, the
   side effects of such requests coming from a trusted host can be
   problematic to the security and sanctity of such otherwise unexposed
   endpoints.  This can be particularly problematic if such a URI is
   used to call non-HTTP endpoints, such as remote code execution
   services local to the AS.

   The most vulnerable place in this specification is the push-based
   post-interaction finish method (Section 4.2.2), as the client
   instance is less trusted than the AS and can use this method to make
   the AS call an arbitrary URI.  While it is not required by the
   protocol, the AS can fetch other URIs provided by the client
   instance, such as the logo image or home page, for verification or
   privacy-preserving purposes before displaying them to the RO as part
   of a consent screen.  Even if the AS does not fetch these URIs, their
   use in GNAP's normal operation could cause an attack against the end
   user's browser as it fetches these same attack URIs.  Furthermore,
   extensions to GNAP that allow or require URI fetch could also be
   similarly susceptible, such as a system for having the AS fetch a
   client instance's keys from a presented URI instead of the client
   instance presenting the key by value.  Such extensions are outside
   the scope of this specification, but any system deploying such an
   extension would need to be aware of this issue.

   To help mitigate this problem, similar approaches that protect
   parties against malicious redirects (Section 11.29) can be used.  For
   example, all URIs that can result in a direct request being made by a
   party in the protocol can be filtered through an allowlist or
   blocklist.  For example, an AS that supports the push-based
   interaction finish method can compare the callback URI in the
   interaction request to a known URI for a pre-registered client
   instance, or it can ensure that the URI is not on a blocklist of
   sensitive URLs such as internal network addresses.  However, note
   that because these types of calls happen outside of the view of human
   interaction, it is not usually feasible to provide notification and
   warning to someone before the request needs to be executed, as is the
   case with redirection URLs.  As such, SSRF is somewhat more difficult
   to manage at runtime, and systems should generally refuse to fetch a
   URI if unsure.

11.35.  Multiple Key Formats

   All keys presented by value are only allowed to be in a single
   format.  While it would seem beneficial to allow keys to be sent in
   multiple formats in case the receiver doesn't understand one or more
   of the formats used, there are security issues with such a feature.
   If multiple keys formats are allowed, receivers of these key
   definitions would need to be able to make sure that it's the same key
   represented in each field and not simply use one of the key formats
   without checking for equivalence.  If equivalence is not carefully
   checked, it is possible for an attacker to insert their own key into
   one of the formats without needing to have control over the other
   formats.  This could potentially lead to a situation where one key is
   used by part of the system (such as identifying the client instance)
   and a different key in a different format in the same message is used
   for other things (such as calculating signature validity).  However,
   in such cases, it is impossible for the receiver to ensure that all
   formats contain the same key information since it is assumed that the
   receiver cannot understand all of the formats.

   To combat this, all keys presented by value have to be in exactly one
   supported format known by the receiver as discussed in Section 7.1.
   In most cases, a client instance is going to be configured with its
   keys in a single format, and it will simply present that format as is
   to the AS in its request.  A client instance capable of multiple
   formats can use AS discovery (Section 9) to determine which formats
   are supported, if desired.  An AS should be generous in supporting
   many different key formats to allow different types of client
   software and client instance deployments.  An AS implementation
   should try to support multiple formats to allow a variety of client
   software to connect.

11.36.  Asynchronous Interactions

   GNAP allows the RO to be contacted by the AS asynchronously, outside
   the regular flow of the protocol.  This allows for some advanced use
   cases, such as cross-user authentication or information release, but
   such advanced use cases have some distinct issues that implementors
   need to be fully aware of before using these features.

   First, in many applications, the return of subject information to the
   client instance could indicate to the client instance that the end
   user is the party represented by that information, functionally
   allowing the end user to authenticate to the client application.
   While the details of a fully functional authentication protocol are
   outside the scope of GNAP, it is a common exercise for a client
   instance to request information about the end user.  This is
   facilitated by several interaction methods (Section 4.1) defined in
   GNAP that allow the end user to begin interaction directly with the
   AS.  However, when the subject of the information is intentionally
   not the end user, the client application will need some way to
   differentiate between requests for authentication of the end user and
   requests for information about a different user.  Confusing these
   states could lead to an attacker having their account associated with
   a privileged user.  Client instances can mitigate this by having
   distinct code paths for primary end-user authentication and for
   requesting subject information about secondary users, such as in a
   call center.  In such use cases, the client software used by the RO
   (the caller) and the end user (the agent) are generally distinct,
   allowing the AS to differentiate between the agent's corporate device
   making the request and the caller's personal device approving the
   request.

   Second, ROs that interact asynchronously do not usually have the same
   context as an end user in an application attempting to perform the
   task needing authorization.  As such, the asynchronous requests for
   authorization coming to the RO from the AS might have very little to
   do with what the RO is doing at the time.  This situation can
   consequently lead to authorization fatigue on the part of the RO,
   where any incoming authorization request is quickly approved and
   dispatched without the RO making a proper verification of the
   request.  An attacker can exploit this fatigue and get the RO to
   authorize the attacker's system for access.  To mitigate this, AS
   systems deploying asynchronous authorization should only prompt the
   RO when the RO is expecting such a request, and significant user
   experience engineering efforts need to be employed to ensure that the
   RO can clearly make the appropriate security decision.  Furthermore,
   audit capability and the ability to undo access decisions that may be
   ongoing are particularly important in the asynchronous case.

11.37.  Compromised RS

   An attacker may aim to gain access to confidential or sensitive
   resources.  The measures for hardening and monitoring RS systems
   (beyond protection with access tokens) are out of the scope of this
   document, but the use of GNAP to protect a system does not absolve
   the RS of following best practices.  GNAP generally considers that a
   breach can occur and therefore advises to prefer key-bound tokens
   whenever possible, which at least limits the impact of access token
   leakage by a compromised or malicious RS.

11.38.  AS-Provided Token Keys

   While the most common token-issuance pattern is to bind the access
   token to the client instance's presented key, it is possible for the
   AS to provide a binding key along with an access token, as shown by
   the key field of the token response in Section 3.2.1.  This practice
   allows for an AS to generate and manage the keys associated with
   tokens independently of the keys known to client instances.

   If the key material is returned by value from the AS, then the client
   instance will simply use this key value when presenting the token.
   This can be exploited by an attacker to issue a compromised token to
   an unsuspecting client, assuming that the client instance trusts the
   attacker's AS to issue tokens for the target RS.  In this attack, the
   attacker first gets a token bound to a key under the attacker's
   control.  This token is likely bound to an authorization or account
   controlled by the attacker.  The attacker then reissues that same
   token to the client instance, this time acting as an AS.  The
   attacker can return their own key to the client instance, tricking
   the client instance into using the attacker's token.  Such an attack
   is also possible when the key is returned by reference, if the
   attacker is able to provide a reference meaningful to the client
   instance that references a key under the attacker's control.  This
   substitution attack is similar to some of the main issues found with
   bearer tokens as discussed in Section 11.9.

   Returning a key with an access token should be limited to
   circumstances where both the client and AS can be verified to be
   honest and when the trade-off of not using a client instance's own
   keys is worth the additional risk.

12.  Privacy Considerations

   The privacy considerations in this section are modeled after the list
   of privacy threats in "Privacy Considerations for Internet Protocols"
   [RFC6973] and either explain how these threats are mitigated or
   advise how the threats relate to GNAP.

12.1.  Surveillance

   Surveillance is the observation or monitoring of an individual's
   communications or activities.  Surveillance can be conducted by
   observers or eavesdroppers at any point along the communications
   path.

   GNAP assumes the TLS protection used throughout the spec is intact.
   Without the protection of TLS, there are many points throughout the
   use of GNAP that could lead to possible surveillance.  Even with the
   proper use of TLS, surveillance could occur by several parties
   outside of the TLS-protected channels, as discussed in the
   subsections below.

12.1.1.  Surveillance by the Client

   The purpose of GNAP is to authorize clients to be able to access
   information on behalf of a user.  So while it is expected that the
   client may be aware of the user's identity as well as data being
   fetched for that user, in some cases, the extent of the client may be
   beyond what the user is aware of.  For example, a client may be
   implemented as multiple distinct pieces of software, such as a
   logging service or a mobile application that reports usage data to an
   external backend service.  Each of these pieces could gain
   information about the user without the user being aware of this
   action.

   When the client software uses a hosted asset for its components, such
   as its logo image, the fetch of these assets can reveal user actions
   to the host.  If the AS presents the logo URI to the RO in a browser
   page, the browser will fetch the logo URL from the authorization
   screen.  This fetch will tell the host of the logo image that someone
   is accessing an instance of the client software and requesting access
   for it.  This is particularly problematic when the host of the asset
   is not the client software itself, such as when a content delivery
   network is used.

12.1.2.  Surveillance by the Authorization Server

   The role of the AS is to manage the authorization of client instances
   to protect access to the user's data.  In this role, the AS is by
   definition aware of each authorization of a client instance by a
   user.  When the AS shares user information with the client instance,
   it needs to make sure that it has the permission from that user to do
   so.

   Additionally, as part of the authorization grant process, the AS may
   be aware of which RSs the client intends to use an access token at.
   However, it is possible to design a system using GNAP in which this
   knowledge is not made available to the AS, such as by avoiding the
   use of the locations object in the authorization request.

   If the AS's implementation of access tokens is such that it requires
   an RS callback to the AS to validate them, then the AS will be aware
   of which RSs are actively in use and by which users and clients.  To
   avoid this possibility, the AS would need to structure access tokens
   in such a way that they can be validated by the RS without notifying
   the AS that the token is being validated.

12.2.  Stored Data

   Several parties in the GNAP process are expected to persist data at
   least temporarily, if not semi-permanently, for the normal
   functioning of the system.  If compromised, this could lead to
   exposure of sensitive information.  This section documents the
   potentially sensitive information each party in GNAP is expected to
   store for normal operation.  Naturally, it is possible for any party
   to store information related to protocol mechanics (such as audit
   logs, etc.) for longer than is technically necessary.

   The AS is expected to store Subject Identifiers for users
   indefinitely, in order to be able to include them in the responses to
   clients.  The AS is also expected to store client key identifiers
   associated with display information about the client, such as its
   name and logo.

   The client is expected to store its client instance key indefinitely,
   in order to authenticate to the AS for the normal functioning of the
   GNAP flows.  Additionally, the client will be temporarily storing
   artifacts issued by the AS during a flow, and these artifacts ought
   to be discarded by the client when the transaction is complete.

   The RS is not required to store any state for its normal operation,
   as far as its part in implementing GNAP.  Depending on the
   implementation of access tokens, the RS may need to cache public keys
   from the AS in order to validate access tokens.

12.3.  Intrusion

   Intrusion refers to the ability of various parties to send
   unsolicited messages or cause denial of service for unrelated
   parties.

   If the RO is different from the end user, there is an opportunity for
   the end user to cause unsolicited messages to be sent to the RO if
   the system prompts the RO for consent when an end user attempts to
   access their data.

   The format and contents of Subject Identifiers are intentionally not
   defined by GNAP.  If the AS uses values for Subject Identifiers that
   are also identifiers for communication channels (e.g., an email
   address or phone number), this opens up the possibility for a client
   to learn this information when it was not otherwise authorized to
   access this kind of data about the user.

12.4.  Correlation

   The threat of correlation is the combination of various pieces of
   information related to an individual in a way that defies their
   expectations of what others know about them.

12.4.1.  Correlation by Clients

   The biggest risk of correlation in GNAP is when an AS returns stable,
   consistent user identifiers to multiple different applications.  In
   this case, applications created by different parties would be able to
   correlate these user identifiers out of band in order to know which
   users they have in common.

   The most common example of this in practice is tracking for
   advertising purposes, such that a client shares their list of user
   IDs with an ad platform that is then able to retarget ads to
   applications created by other parties.  In contrast, a positive
   example of correlation is a corporate acquisition where two
   previously unrelated clients now do need to be able to identify the
   same user between the two clients, such as when software systems are
   intentionally connected by the end user.

   Another means of correlation comes from the use of RS-first discovery
   (Section 9.1).  A client instance that knows nothing other than an
   RS's URL could make an unauthenticated call to the RS and learn which
   AS protects the resources there.  If the client instance knows
   something about the AS, such as it being a single-user AS or
   belonging to a specific organization, the client instance could,
   through association, learn things about the resource without ever
   gaining access to the resource itself.

12.4.2.  Correlation by Resource Servers

   Unrelated RSs also have an opportunity to correlate users if the AS
   includes stable user identifiers in access tokens or in access token
   introspection responses.

   In some cases, an RS may not actually need to be able to identify
   users (such as an RS providing access to a company cafeteria menu,
   which only needs to validate whether the user is a current employee),
   so ASes should be thoughtful of when user identifiers are actually
   necessary to communicate to RSs for the functioning of the system.

   However, note that the lack of inclusion of a user identifier in an
   access token may be a risk if there is a concern that two users may
   voluntarily share access tokens between them in order to access
   protected resources.  For example, if a website wants to limit access
   to only people over 18, and such does not need to know any user
   identifiers, an access token may be issued by an AS contains only the
   claim "over 18".  If the user is aware that this access token doesn't
   reference them individually, they may be willing to share the access
   token with a user who is under 18 in order to let them get access to
   the website.  (Note that the binding of an access token to a non-
   extractable client instance key also prevents the access token from
   being voluntarily shared.)

12.4.3.  Correlation by Authorization Servers

   Clients are expected to be identified by their client instance key.
   If a particular client instance key is used at more than one AS, this
   could open up the possibility for multiple unrelated ASes to
   correlate client instances.  This is especially a problem in the
   common case where a client instance is used by a single individual,
   as it would allow the ASes to correlate that individual between them.
   If this is a concern of a client, the client should use distinct keys
   with each AS.

12.5.  Disclosure in Shared References

   Throughout many parts of GNAP, the parties pass shared references
   between each other, sometimes in place of the values themselves (for
   example, the interact_ref value used throughout the flow).  These
   references are intended to be random strings and should not contain
   any private or sensitive data that could potentially leak information
   between parties.

13.  References

13.1.  Normative References

   [BCP195]   Best Current Practice 195,
              <https://www.rfc-editor.org/info/bcp195>.
              At the time of writing, this BCP comprises the following:

              Moriarty, K. and S. Farrell, "Deprecating TLS 1.0 and TLS
              1.1", BCP 195, RFC 8996, DOI 10.17487/RFC8996, March 2021,
              <https://www.rfc-editor.org/info/rfc8996>.

              Sheffer, Y., Saint-Andre, P., and T. Fossati,
              "Recommendations for Secure Use of Transport Layer
              Security (TLS) and Datagram Transport Layer Security
              (DTLS)", BCP 195, RFC 9325, DOI 10.17487/RFC9325, November
              2022, <https://www.rfc-editor.org/info/rfc9325>.

   [HASH-ALG] IANA, "Named Information Hash Algorithm Registry",
              <https://www.iana.org/assignments/named-information/>.

   [HTTP]     Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "HTTP Semantics", STD 97, RFC 9110,
              DOI 10.17487/RFC9110, June 2022,
              <https://www.rfc-editor.org/info/rfc9110>.

   [OIDC]     Sakimura, N., Bradley, J., Jones, M., de Medeiros, B., and
              C. Mortimore, "OpenID Connect Core 1.0 incorporating
              errata set 2", December 2023,
              <https://openid.net/specs/openid-connect-core-1_0.html>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC2397]  Masinter, L., "The "data" URL scheme", RFC 2397,
              DOI 10.17487/RFC2397, August 1998,
              <https://www.rfc-editor.org/info/rfc2397>.

   [RFC3339]  Klyne, G. and C. Newman, "Date and Time on the Internet:
              Timestamps", RFC 3339, DOI 10.17487/RFC3339, July 2002,
              <https://www.rfc-editor.org/info/rfc3339>.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <https://www.rfc-editor.org/info/rfc3986>.

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
              <https://www.rfc-editor.org/info/rfc4648>.

   [RFC5646]  Phillips, A., Ed. and M. Davis, Ed., "Tags for Identifying
              Languages", BCP 47, RFC 5646, DOI 10.17487/RFC5646,
              September 2009, <https://www.rfc-editor.org/info/rfc5646>.

   [RFC6749]  Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
              RFC 6749, DOI 10.17487/RFC6749, October 2012,
              <https://www.rfc-editor.org/info/rfc6749>.

   [RFC6750]  Jones, M. and D. Hardt, "The OAuth 2.0 Authorization
              Framework: Bearer Token Usage", RFC 6750,
              DOI 10.17487/RFC6750, October 2012,
              <https://www.rfc-editor.org/info/rfc6750>.

   [RFC7468]  Josefsson, S. and S. Leonard, "Textual Encodings of PKIX,
              PKCS, and CMS Structures", RFC 7468, DOI 10.17487/RFC7468,
              April 2015, <https://www.rfc-editor.org/info/rfc7468>.

   [RFC7515]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web
              Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May
              2015, <https://www.rfc-editor.org/info/rfc7515>.

   [RFC7517]  Jones, M., "JSON Web Key (JWK)", RFC 7517,
              DOI 10.17487/RFC7517, May 2015,
              <https://www.rfc-editor.org/info/rfc7517>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8259]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", STD 90, RFC 8259,
              DOI 10.17487/RFC8259, December 2017,
              <https://www.rfc-editor.org/info/rfc8259>.

   [RFC8705]  Campbell, B., Bradley, J., Sakimura, N., and T.
              Lodderstedt, "OAuth 2.0 Mutual-TLS Client Authentication
              and Certificate-Bound Access Tokens", RFC 8705,
              DOI 10.17487/RFC8705, February 2020,
              <https://www.rfc-editor.org/info/rfc8705>.

   [RFC9111]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "HTTP Caching", STD 98, RFC 9111,
              DOI 10.17487/RFC9111, June 2022,
              <https://www.rfc-editor.org/info/rfc9111>.

   [RFC9421]  Backman, A., Ed., Richer, J., Ed., and M. Sporny, "HTTP
              Message Signatures", RFC 9421, DOI 10.17487/RFC9421,
              February 2024, <https://www.rfc-editor.org/info/rfc9421>.

   [RFC9493]  Backman, A., Ed., Scurtescu, M., and P. Jain, "Subject
              Identifiers for Security Event Tokens", RFC 9493,
              DOI 10.17487/RFC9493, December 2023,
              <https://www.rfc-editor.org/info/rfc9493>.

   [RFC9530]  Polli, R. and L. Pardue, "Digest Fields", RFC 9530,
              DOI 10.17487/RFC9530, February 2024,
              <https://www.rfc-editor.org/info/rfc9530>.

   [SAML2]    Cantor, S., Ed., Kemp, J., Ed., Philpott, R., Ed., and E.
              Maler, Ed., "Assertions and Protocol for the OASIS
              Security Assertion Markup Language (SAML) V2.0", OASIS
              Standard, March 2005, <https://docs.oasis-
              open.org/security/saml/v2.0/saml-core-2.0-os.pdf>.

13.2.  Informative References

   [Auth-Schemes]
              IANA, "HTTP Authentication Schemes",
              <https://www.iana.org/assignments/http-authschemes>.

   [AXELAND2021]
              Axeland, Å. and O. Oueidat, "Security Analysis of Attack
              Surfaces on the Grant Negotiation and Authorization
              Protocol", Master's thesis, Department of Computer Science
              and Engineering, Chalmers University of Technology and
              University of Gothenburg, 2021,
              <https://hdl.handle.net/20.500.12380/304105>.

   [GNAP-REG] IANA, "Grant Negotiation and Authorization Protocol
              (GNAP)", <https://www.iana.org/assignments/gnap>.

   [GNAP-RS]  Richer, J., Ed. and F. Imbault, "Grant Negotiation and
              Authorization Protocol Resource Server Connections", Work
              in Progress, Internet-Draft, draft-ietf-gnap-resource-
              servers-09, 23 September 2024,
              <https://datatracker.ietf.org/doc/html/draft-ietf-gnap-
              resource-servers-09>.

   [HELMSCHMIDT2022]
              Helmschmidt, F., "Security Analysis of the Grant
              Negotiation and Authorization Protocol", Master's thesis,
              Institute of Information Security, University of Stuggart,
              DOI 10.18419/opus-12203, 2022,
              <http://dx.doi.org/10.18419/opus-12203>.

   [MediaTypes]
              IANA, "Media Types",
              <https://www.iana.org/assignments/media-types>.

   [OAUTH-SEC-TOPICS]
              Lodderstedt, T., Bradley, J., Labunets, A., and D. Fett,
              "OAuth 2.0 Security Best Current Practice", Work in
              Progress, Internet-Draft, draft-ietf-oauth-security-
              topics-29, 3 June 2024,
              <https://datatracker.ietf.org/doc/html/draft-ietf-oauth-
              security-topics-29>.

   [promise-theory]
              Bergstra, J. and M. Burgess, "Promise Theory: Principles
              and Applications", Second Edition, XtAxis Press, 2019,
              <http://markburgess.org/promises.html>.

   [RFC2046]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part Two: Media Types", RFC 2046,
              DOI 10.17487/RFC2046, November 1996,
              <https://www.rfc-editor.org/info/rfc2046>.

   [RFC4107]  Bellovin, S. and R. Housley, "Guidelines for Cryptographic
              Key Management", BCP 107, RFC 4107, DOI 10.17487/RFC4107,
              June 2005, <https://www.rfc-editor.org/info/rfc4107>.

   [RFC6202]  Loreto, S., Saint-Andre, P., Salsano, S., and G. Wilkins,
              "Known Issues and Best Practices for the Use of Long
              Polling and Streaming in Bidirectional HTTP", RFC 6202,
              DOI 10.17487/RFC6202, April 2011,
              <https://www.rfc-editor.org/info/rfc6202>.

   [RFC6838]  Freed, N., Klensin, J., and T. Hansen, "Media Type
              Specifications and Registration Procedures", BCP 13,
              RFC 6838, DOI 10.17487/RFC6838, January 2013,
              <https://www.rfc-editor.org/info/rfc6838>.

   [RFC6973]  Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
              Morris, J., Hansen, M., and R. Smith, "Privacy
              Considerations for Internet Protocols", RFC 6973,
              DOI 10.17487/RFC6973, July 2013,
              <https://www.rfc-editor.org/info/rfc6973>.

   [RFC7518]  Jones, M., "JSON Web Algorithms (JWA)", RFC 7518,
              DOI 10.17487/RFC7518, May 2015,
              <https://www.rfc-editor.org/info/rfc7518>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

   [RFC8264]  Saint-Andre, P. and M. Blanchet, "PRECIS Framework:
              Preparation, Enforcement, and Comparison of
              Internationalized Strings in Application Protocols",
              RFC 8264, DOI 10.17487/RFC8264, October 2017,
              <https://www.rfc-editor.org/info/rfc8264>.

   [RFC8707]  Campbell, B., Bradley, J., and H. Tschofenig, "Resource
              Indicators for OAuth 2.0", RFC 8707, DOI 10.17487/RFC8707,
              February 2020, <https://www.rfc-editor.org/info/rfc8707>.

   [RFC8792]  Watsen, K., Auerswald, E., Farrel, A., and Q. Wu,
              "Handling Long Lines in Content of Internet-Drafts and
              RFCs", RFC 8792, DOI 10.17487/RFC8792, June 2020,
              <https://www.rfc-editor.org/info/rfc8792>.

   [RFC9396]  Lodderstedt, T., Richer, J., and B. Campbell, "OAuth 2.0
              Rich Authorization Requests", RFC 9396,
              DOI 10.17487/RFC9396, May 2023,
              <https://www.rfc-editor.org/info/rfc9396>.

   [RFC9440]  Campbell, B. and M. Bishop, "Client-Cert HTTP Header
              Field", RFC 9440, DOI 10.17487/RFC9440, July 2023,
              <https://www.rfc-editor.org/info/rfc9440>.

   [RFC9525]  Saint-Andre, P. and R. Salz, "Service Identity in TLS",
              RFC 9525, DOI 10.17487/RFC9525, November 2023,
              <https://www.rfc-editor.org/info/rfc9525>.

   [SP80063C] Grassi, P., Richer, J., Squire, S., Fenton, J., Nadeau,
              E., Lefkovitz, N., Danker, J., Choong, Y., Greene, K., and
              M. Theofanos, "Digital Identity Guidelines: Federation and
              Assertions", NIST SP 800-63C, DOI 10.6028/NIST.SP.800-63c,
              June 2017,
              <https://nvlpubs.nist.gov/nistpubs/SpecialPublications/
              NIST.SP.800-63c.pdf>.

   [Subj-ID-Formats]
              IANA, "Subject Identifier Formats",
              <https://www.iana.org/assignments/secevent>.

Appendix A.  Comparison with OAuth 2.0

   GNAP's protocol design differs from OAuth 2.0's in several
   fundamental ways:

   1.  *Consent and authorization flexibility:*

       OAuth 2.0 generally assumes the user has access to a web browser.
       The type of interaction available is fixed by the grant type, and
       the most common interactive grant types start in the browser.
       OAuth 2.0 assumes that the user using the client software is the
       same user that will interact with the AS to approve access.

       GNAP allows various patterns to manage authorizations and
       consents required to fulfill this requested delegation, including
       information sent by the client instance, information supplied by
       external parties, and information gathered through the
       interaction process.  GNAP allows a client instance to list
       different ways that it can start and finish an interaction, and
       these can be mixed together as needed for different use cases.
       GNAP interactions can use a browser, but they don't have to.
       Methods can use inter-application messaging protocols, out-of-
       band data transfer, or anything else.  GNAP allows extensions to
       define new ways to start and finish an interaction, as new
       methods and platforms are expected to become available over time.
       GNAP is designed to allow the end user and the RO to be two
       different people, but it still works in the optimized case of
       them being the same party.

   2.  *Intent registration and inline negotiation:*

       OAuth 2.0 uses different "grant types" that start at different
       endpoints for different purposes.  Many of these require
       discovery of several interrelated parameters.

       GNAP requests all start with the same type of request to the same
       endpoint at the AS.  Next steps are negotiated between the client
       instance and AS based on software capabilities, policies
       surrounding requested access, and the overall context of the
       ongoing request.  GNAP defines a continuation API that allows the
       client instance and AS to request and send additional information
       from each other over multiple steps.  This continuation API uses
       the same access token protection that other GNAP-protected APIs
       use.  GNAP allows discovery to optimize the requests, but it
       isn't required thanks to the negotiation capabilities.

       GNAP is able to handle the life cycle of an authorization request
       and therefore simplifies the mental model surrounding OAuth2.
       For instance, there's no need for refresh tokens when the API
       enables proper rotation of access tokens.

   3.  *Client instances:*

       OAuth 2.0 requires all clients to be registered at the AS and to
       use a client_id known to the AS as part of the protocol.  This
       client_id is generally assumed to be assigned by a trusted
       authority during a registration process, and OAuth places a lot
       of trust on the client_id as a result.  Dynamic registration
       allows different classes of clients to get a client_id at
       runtime, even if they only ever use it for one request.

       GNAP allows the client instance to present an unknown key to the
       AS and use that key to protect the ongoing request.  GNAP's
       client instance identifier mechanism allows for pre-registered
       clients and dynamically registered clients to exist as an
       optimized case without requiring the identifier as part of the
       protocol at all times.

   4.  *Expanded delegation:*

       OAuth 2.0 defines the "scope" parameter for controlling access to
       APIs.  This parameter has been coopted to mean a number of
       different things in different protocols, including flags for
       turning special behavior on and off and the return of data apart
       from the access token.  The "resource" indicator (defined in
       [RFC8707]) and Rich Authorization Request (RAR) extensions (as
       defined in [RFC9396]) expand on the "scope" concept in similar
       but different ways.

       GNAP defines a rich structure for requesting access (analogous to
       RAR), with string references as an optimization (analogous to
       scopes).  GNAP defines methods for requesting directly returned
       user information, separate from API access.  This information
       includes identifiers for the current user and structured
       assertions.  GNAP makes no assumptions or demands on the format
       or contents of the access token, but the RS extension allows a
       negotiation of token formats between the AS and RS.

   5.  *Cryptography-based security:*

       OAuth 2.0 uses shared bearer secrets, including the client_secret
       and access token, and advanced authentication and sender
       constraints have been built on after the fact in inconsistent
       ways.

       In GNAP, all communication between the client instance and AS is
       bound to a key held by the client instance.  GNAP uses the same
       cryptographic mechanisms for both authenticating the client (to
       the AS) and binding the access token (to the RS and the AS).
       GNAP allows extensions to define new cryptographic protection
       mechanisms, as new methods are expected to become available over
       time.  GNAP does not have the notion of "public clients" because
       key information can always be sent and used dynamically.

   6.  *Privacy and usable security:*

       OAuth 2.0's deployment model assumes a strong binding between the
       AS and the RS.

       GNAP is designed to be interoperable with decentralized identity
       standards and to provide a human-centric authorization layer.  In
       addition to this specification, GNAP supports various patterns of
       communication between RSs and ASes through extensions.  GNAP
       tries to limit the odds of a consolidation to just a handful of
       popular AS services.

Appendix B.  Example Protocol Flows

   The protocol defined in this specification provides a number of
   features that can be combined to solve many different kinds of
   authentication scenarios.  This section seeks to show examples of how
   the protocol could be applied for different situations.

   Some longer fields, particularly cryptographic information, have been
   truncated for display purposes in these examples.

B.1.  Redirect-Based User Interaction

   In this scenario, the user is the RO and has access to a web browser,
   and the client instance can take front-channel callbacks on the same
   device as the user.  This combination is analogous to the OAuth 2.0
   Authorization Code grant type.

   The client instance initiates the request to the AS.  Here, the
   client instance identifies itself using its public key.

   POST /tx HTTP/1.1
   Host: server.example.com
   Content-Type: application/json
   Signature-Input: sig1=...
   Signature: sig1=...
   Content-Digest: sha-256=...

   {
       "access_token": {
           "access": [
               {
                   "actions": [
                       "read",
                       "write",
                       "dolphin"
                   ],
                   "locations": [
                       "https://server.example.net/",
                       "https://resource.local/other"
                   ],
                   "datatypes": [
                       "metadata",
                       "images"
                   ]
               }
           ],
       },
       "client": {
         "key": {
           "proof": "httpsig",
           "jwk": {
               "kty": "RSA",
               "e": "AQAB",
               "kid": "xyz-1",
               "alg": "RS256",
               "n": "kOB5rR4Jv0GMeLaY6_It_r3ORwdf8ci_JtffXyaSx8..."
           }
         }
       },
       "interact": {
           "start": ["redirect"],
           "finish": {
               "method": "redirect",
               "uri": "https://client.example.net/return/123455",
               "nonce": "LKLTI25DK82FX4T4QFZC"
           }
       }
   }

   The AS processes the request and determines that the RO needs to
   interact.  The AS returns the following response that gives the
   client instance the information it needs to connect.  The AS has also
   indicated to the client instance that it can use the given instance
   identifier to identify itself in future requests (Section 2.3.1).

   HTTP/1.1 200 OK
   Content-Type: application/json
   Cache-Control: no-store

   {
       "interact": {
         "redirect":
           "https://server.example.com/interact/4CF492MLVMSW9MKM",
         "finish": "MBDOFXG4Y5CVJCX821LH"
       }
       "continue": {
           "access_token": {
               "value": "80UPRY5NM33OMUKMKSKU"
           },
           "uri": "https://server.example.com/continue"
       },
       "instance_id": "7C7C4AZ9KHRS6X63AJAO"
   }

   The client instance saves the response and redirects the user to the
   interaction start mode's "redirect" URI by sending the following HTTP
   message to the user's browser.

   HTTP 303 Found
   Location: https://server.example.com/interact/4CF492MLVMSW9MKM

   The user's browser fetches the AS's interaction URI.  The user logs
   in, is identified as the RO for the resource being requested, and
   approves the request.  Since the AS has a callback parameter that was
   sent in the initial request's interaction finish method, the AS
   generates the interaction reference, calculates the hash, and
   redirects the user back to the client instance with these additional
   values added as query parameters.

   NOTE: '\' line wrapping per RFC 8792

   HTTP 302 Found
   Location: https://client.example.net/return/123455\
     ?hash=x-gguKWTj8rQf7d7i3w3UhzvuJ5bpOlKyAlVpLxBffY\
     &interact_ref=4IFWWIKYBC2PQ6U56NL1

   The client instance receives this request from the user's browser.
   The client instance ensures that this is the same user that was sent
   out by validating session information and retrieves the stored
   pending request.  The client instance uses the values in this to
   validate the hash parameter.  The client instance then calls the
   continuation URI using the associated continuation access token and
   presents the interaction reference in the request content.  The
   client instance signs the request as above.

   POST /continue HTTP/1.1
   Host: server.example.com
   Content-Type: application/json
   Authorization: GNAP 80UPRY5NM33OMUKMKSKU
   Signature-Input: sig1=...
   Signature: sig1=...
   Content-Digest: sha-256=...

   {
       "interact_ref": "4IFWWIKYBC2PQ6U56NL1"
   }

   The AS retrieves the pending request by looking up the pending grant
   request associated with the presented continuation access token.
   Seeing that the grant is approved, the AS issues an access token and
   returns this to the client instance.

   NOTE: '\' line wrapping per RFC 8792

   HTTP/1.1 200 OK
   Content-Type: application/json
   Cache-Control: no-store

   {
       "access_token": {
           "value": "OS9M2PMHKUR64TB8N6BW7OZB8CDFONP219RP1LT0",
           "manage": "https://server.example.com/token/PRY5NM33O\
               M4TB8N6BW7OZB8CDFONP219RP1L",
           "access": [{
               "actions": [
                   "read",
                   "write",
                   "dolphin"
               ],
               "locations": [
                   "https://server.example.net/",
                   "https://resource.local/other"
               ],
               "datatypes": [
                   "metadata",
                   "images"
               ]
           }]
       },
       "continue": {
           "access_token": {
               "value": "80UPRY5NM33OMUKMKSKU"
           },
           "uri": "https://server.example.com/continue"
       }
   }

B.2.  Secondary Device Interaction

   In this scenario, the user does not have access to a web browser on
   the device and must use a secondary device to interact with the AS.
   The client instance can display a user code or a printable QR code.
   The client instance is not able to accept callbacks from the AS and
   needs to poll for updates while waiting for the user to authorize the
   request.

   The client instance initiates the request to the AS.

   POST /tx HTTP/1.1
   Host: server.example.com
   Content-Type: application/json
   Signature-Input: sig1=...
   Signature: sig1=...
   Content-Digest: sha-256=...

   {
       "access_token": {
           "access": [
               "dolphin-metadata", "some other thing"
           ],
       },
       "client": "7C7C4AZ9KHRS6X63AJAO",
       "interact": {
           "start": ["redirect", "user_code"]
       }
   }

   The AS processes this and determines that the RO needs to interact.
   The AS supports both redirect URIs and user codes for interaction, so
   it includes both.  Since there is no interaction finish mode, the AS
   does not include a nonce but does include a "wait" parameter on the
   continuation section because it expects the client instance to poll
   for results.

   HTTP/1.1 200 OK
   Content-Type: application/json
   Cache-Control: no-store

   {
       "interact": {
           "redirect": "https://srv.ex/MXKHQ",
           "user_code": {
               "code": "A1BC3DFF"
           }
       },
       "continue": {
           "access_token": {
               "value": "80UPRY5NM33OMUKMKSKU"
           },
           "uri": "https://server.example.com/continue/VGJKPTKC50",
           "wait": 60
       }
   }

   The client instance saves the response and displays the user code
   visually on its screen along with the static device URI.  The client
   instance also displays the short interaction URI as a QR code to be
   scanned.

   If the user scans the code, they are taken to the interaction
   endpoint, and the AS looks up the current pending request based on
   the incoming URI.  If the user instead goes to the static page and
   enters the code manually, the AS looks up the current pending request
   based on the value of the user code.  In both cases, the user logs
   in, is identified as the RO for the resource being requested, and
   approves the request.  Once the request has been approved, the AS
   displays to the user a message to return to their device.

   Meanwhile, the client instance polls the AS every 60 seconds at the
   continuation URI.  The client instance signs the request using the
   same key and method that it did in the first request.

   POST /continue/VGJKPTKC50 HTTP/1.1
   Host: server.example.com
   Authorization: GNAP 80UPRY5NM33OMUKMKSKU
   Signature-Input: sig1=...
   Signature: sig1=...
   Content-Digest: sha-256=...

   The AS retrieves the pending request based on the pending grant
   request associated with the continuation access token and determines
   that it has not yet been authorized.  The AS indicates to the client
   instance that no access token has yet been issued but it can continue
   to call after another 60-second timeout.

   HTTP/1.1 200 OK
   Content-Type: application/json
   Cache-Control: no-store

   {
       "continue": {
           "access_token": {
               "value": "G7YQT4KQQ5TZY9SLSS5E"
           },
           "uri": "https://server.example.com/continue/ATWHO4Q1WV",
           "wait": 60
       }
   }

   Note that the continuation URI and access token have been rotated
   since they were used by the client instance to make this call.  The
   client instance polls the continuation URI after a 60-second timeout
   using this new information.

   POST /continue/ATWHO4Q1WV HTTP/1.1
   Host: server.example.com
   Authorization: GNAP G7YQT4KQQ5TZY9SLSS5E
   Signature-Input: sig1=...
   Signature: sig1=...
   Content-Digest: sha-256=...

   The AS retrieves the pending request based on the URI and access
   token, determines that it has been approved, and issues an access
   token for the client to use at the RS.

   NOTE: '\' line wrapping per RFC 8792

   HTTP/1.1 200 OK
   Content-Type: application/json
   Cache-Control: no-store

   {
       "access_token": {
           "value": "OS9M2PMHKUR64TB8N6BW7OZB8CDFONP219RP1LT0",
           "manage": "https://server.example.com/token/PRY5NM33O\
               M4TB8N6BW7OZB8CDFONP219RP1L",
           "access": [
               "dolphin-metadata", "some other thing"
           ]
       }
   }

B.3.  No User Involvement

   In this scenario, the client instance is requesting access on its own
   behalf, with no user to interact with.

   The client instance creates a request to the AS, identifying itself
   with its public key and using MTLS to make the request.

   POST /tx HTTP/1.1
   Host: server.example.com
   Content-Type: application/json

   {
       "access_token": {
           "access": [
               "backend service", "nightly-routine-3"
           ],
       },
       "client": {
         "key": {
           "proof": "mtls",
           "cert#S256": "bwcK0esc3ACC3DB2Y5_lESsXE8o9ltc05O89jdN-dg2"
         }
       }
   }

   The AS processes this, determines that the client instance can ask
   for the requested resources, and issues an access token.

   HTTP/1.1 200 OK
   Content-Type: application/json
   Cache-Control: no-store

   {
       "access_token": {
           "value": "OS9M2PMHKUR64TB8N6BW7OZB8CDFONP219RP1LT0",
           "manage": "https://server.example.com/token",
           "access": [
               "backend service", "nightly-routine-3"
           ]
       }
   }

B.4.  Asynchronous Authorization

   In this scenario, the client instance is requesting on behalf of a
   specific RO but has no way to interact with the user.  The AS can
   asynchronously reach out to the RO for approval in this scenario.

   The client instance starts the request at the AS by requesting a set
   of resources.  The client instance also identifies a particular user.

   POST /tx HTTP/1.1
   Host: server.example.com
   Content-Type: application/json
   Signature-Input: sig1=...
   Signature: sig1=...
   Content-Digest: sha-256=...

   {
       "access_token": {
           "access": [
               {
                   "type": "photo-api",
                   "actions": [
                       "read",
                       "write",
                       "dolphin"
                   ],
                   "locations": [
                       "https://server.example.net/",
                       "https://resource.local/other"
                   ],
                   "datatypes": [
                       "metadata",
                       "images"
                   ]
               },
               "read", "dolphin-metadata",
               {
                   "type": "financial-transaction",
                   "actions": [
                       "withdraw"
                   ],
                   "identifier": "account-14-32-32-3",
                   "currency": "USD"
               },
               "some other thing"
           ],
       },
       "client": "7C7C4AZ9KHRS6X63AJAO",
       "user": {
           "sub_ids": [ {
               "format": "opaque",
               "id": "J2G8G8O4AZ"
           } ]
     }
   }

   The AS processes this and determines that the RO needs to interact.
   The AS determines that it can reach the identified user
   asynchronously and that the identified user does have the ability to
   approve this request.  The AS indicates to the client instance that
   it can poll for continuation.

   HTTP/1.1 200 OK
   Content-Type: application/json
   Cache-Control: no-store

   {
       "continue": {
           "access_token": {
               "value": "80UPRY5NM33OMUKMKSKU"
           },
           "uri": "https://server.example.com/continue",
           "wait": 60
       }
   }

   The AS reaches out to the RO and prompts them for consent.  In this
   example scenario, the AS has an application that it can push
   notifications to for the specified account.

   Meanwhile, the client instance periodically polls the AS every 60
   seconds at the continuation URI.

   POST /continue HTTP/1.1
   Host: server.example.com
   Authorization: GNAP 80UPRY5NM33OMUKMKSKU
   Signature-Input: sig1=...
   Signature: sig1=...

   The AS retrieves the pending request based on the continuation access
   token and determines that it has not yet been authorized.  The AS
   indicates to the client instance that no access token has yet been
   issued but it can continue to call after another 60-second timeout.

   HTTP/1.1 200 OK
   Content-Type: application/json
   Cache-Control: no-store

   {
       "continue": {
           "access_token": {
               "value": "BI9QNW6V9W3XFJK4R02D"
           },
           "uri": "https://server.example.com/continue",
           "wait": 60
       }
   }

   Note that the continuation access token value has been rotated since
   it was used by the client instance to make this call.  The client
   instance polls the continuation URI after a 60-second timeout using
   the new token.

   POST /continue HTTP/1.1
   Host: server.example.com
   Authorization: GNAP BI9QNW6V9W3XFJK4R02D
   Signature-Input: sig1=...
   Signature: sig1=...

   The AS retrieves the pending request based on the handle, determines
   that it has been approved, and issues an access token.

   NOTE: '\' line wrapping per RFC 8792

   HTTP/1.1 200 OK
   Content-Type: application/json
   Cache-Control: no-store

   {
       "access_token": {
           "value": "OS9M2PMHKUR64TB8N6BW7OZB8CDFONP219RP1LT0",
           "manage": "https://server.example.com/token/PRY5NM33O\
               M4TB8N6BW7OZB8CDFONP219RP1L",
           "access": [
               "dolphin-metadata", "some other thing"
           ]
       }
   }

B.5.  Applying OAuth 2.0 Scopes and Client IDs

   While GNAP is not designed to be directly compatible with OAuth 2.0
   [RFC6749], considerations have been made to enable the use of OAuth
   2.0 concepts and constructs more smoothly within GNAP.

   In this scenario, the client developer has a client_id and set of
   scope values from their OAuth 2.0 system and wants to apply them to
   the new protocol.  In OAuth 2.0, the client developer would put their
   client_id and scope values as parameters into a redirect request to
   the authorization endpoint.

   NOTE: '\' line wrapping per RFC 8792

   HTTP 302 Found
   Location: https://server.example.com/authorize\
     ?client_id=7C7C4AZ9KHRS6X63AJAO\
     &scope=read%20write%20dolphin\
     &redirect_uri=https://client.example.net/return\
     &response_type=code\
     &state=123455

   Now the developer wants to make an analogous request to the AS using
   GNAP.  To do so, the client instance makes an HTTP POST and places
   the OAuth 2.0 values in the appropriate places.

   POST /tx HTTP/1.1
   Host: server.example.com
   Content-Type: application/json
   Signature-Input: sig1=...
   Signature: sig1=...
   Content-Digest: sha-256=...

   {
       "access_token": {
           "access": [
               "read", "write", "dolphin"
           ],
           "flags": [ "bearer" ]
       },
       "client": "7C7C4AZ9KHRS6X63AJAO",
       "interact": {
           "start": ["redirect"],
           "finish": {
               "method": "redirect",
               "uri": "https://client.example.net/return?state=123455",
               "nonce": "LKLTI25DK82FX4T4QFZC"
           }
       }
   }

   The client_id can be used to identify the client instance's keys that
   it uses for authentication, the scopes represent resources that the
   client instance is requesting, and the redirect_uri and state value
   are pre-combined into a finish URI that can be unique per request.
   The client instance additionally creates a nonce to protect the
   callback, separate from the state parameter that it has added to its
   return URI.

   From here, the protocol continues as above.

Appendix C.  Interoperability Profiles

   The GNAP specification has many different modes, options, and
   mechanisms, allowing it to solve a wide variety of problems in a wide
   variety of deployments.  The wide applicability of GNAP makes it
   difficult, if not impossible, to define a set of mandatory-to-
   implement features, since one environment's required feature would be
   impossible to do in another environment.  While this is a large
   problem in many systems, GNAP's back-and-forth negotiation process
   allows parties to declare at runtime everything that they support and
   then have the other party select from that the subset of items that
   they also support, leading to functional compatibility in many parts
   of the protocol even in an open world scenario.

   In addition, GNAP defines a set of interoperability profiles that
   gather together core requirements to fix options into common
   configurations that are likely to be useful to large populations of
   similar applications.

   Conformant AS implementations of these profiles MUST implement at
   least the features as specified in the profile and MAY implement
   additional features or profiles.  Conformant client implementations
   of these profiles MUST implement at least the features as specified,
   except where a subset of the features allows the protocol to function
   (such as using polling instead of a push finish method for the
   Secondary Device profile).

C.1.  Web-Based Redirection

   Implementations conformant to the web-based redirection profile of
   GNAP MUST implement all of the following features:

   *  Interaction Start Methods: redirect

   *  Interaction Finish Methods: redirect

   *  Interaction Hash Algorithms: sha-256

   *  Key Proofing Methods: httpsig with no additional parameters

   *  Key Formats: jwks with signature algorithm included in the key's
      alg parameter

   *  JOSE Signature Algorithm: PS256

   *  Subject Identifier Formats: opaque

   *  Assertion Formats: id_token

C.2.  Secondary Device

   Implementations conformant to the Secondary Device profile of GNAP
   MUST implement all of the following features:

   *  Interaction Start Methods: user_code and user_code_uri

   *  Interaction Finish Methods: push

   *  Interaction Hash Algorithms: sha-256

   *  Key Proofing Methods: httpsig with no additional parameters

   *  Key Formats: jwks with signature algorithm included in the key's
      alg parameter

   *  JOSE Signature Algorithm: PS256

   *  Subject Identifier Formats: opaque

   *  Assertion Formats: id_token

Appendix D.  Guidance for Extensions

   Extensions to this specification have a variety of places to alter
   the protocol, including many fields and objects that can have
   additional values in a registry (Section 10) established by this
   specification.  For interoperability and to preserve the security of
   the protocol, extensions should register new values with IANA by
   following the specified mechanism.  While it may technically be
   possible to extend the protocol by adding elements to JSON objects
   that are not governed by an IANA registry, a recipient may ignore
   such values but is also allowed to reject them.

   Most object fields in GNAP are specified with types, and those types
   can allow different but related behavior.  For example, the access
   array can include either strings or objects, as discussed in
   Section 8.  The use of JSON polymorphism (Appendix E) within GNAP
   allows extensions to define new fields by not only choosing a new
   name but also by using an existing name with a new type.  However,
   the extension's definition of a new type for a field needs to fit the
   same kind of item being extended.  For example, a hypothetical
   extension could define a string value for the access_token request
   field, with a URL to download a hosted access token request.  Such an
   extension would be appropriate as the access_token field still
   defines the access tokens being requested.  However, if an extension
   were to define a string value for the access_token request field,
   with the value instead being something unrelated to the access token
   request such as a value or key format, this would not be an
   appropriate means of extension.  (Note that this specific extension
   example would create another form of SSRF attack surface as discussed
   in Section 11.34.)

   As another example, both interaction start modes (Section 2.5.1) and
   key proofing methods (Section 7.3) can be defined as either strings
   or objects.  An extension could take a method defined as a string,
   such as app, and define an object-based version with additional
   parameters.  This extension should still define a method to launch an
   application on the end user's device, just like app does when
   specified as a string.

   Additionally, the ability to deal with different types for a field is
   not expected to be equal between an AS and client software, with the
   client software being assumed to be both more varied and more
   simplified than the AS.  Furthermore, the nature of the negotiation
   process in GNAP allows the AS more chance of recovery from unknown
   situations and parameters.  As such, any extensions that change the
   type of any field returned to a client instance should only do so
   when the client instance has indicated specific support for that
   extension through some kind of request parameter.

Appendix E.  JSON Structures and Polymorphism

   GNAP makes use of polymorphism within the JSON [RFC8259] structures
   used for the protocol.  Each portion of this protocol is defined in
   terms of the JSON data type that its values can take, whether it's a
   string, object, array, boolean, or number.  For some fields,
   different data types offer different descriptive capabilities and are
   used in different situations for the same field.  Each data type
   provides a different syntax to express the same underlying semantic
   protocol element, which allows for optimization and simplification in
   many common cases.

   Even though JSON is often used to describe strongly typed structures,
   JSON on its own is naturally polymorphic.  In JSON, the named members
   of an object have no type associated with them, and any data type can
   be used as the value for any member.  In practice, each member has a
   semantic type that needs to make sense to the parties creating and
   consuming the object.  Within this protocol, each object member is
   defined in terms of its semantic content, and this semantic content
   might have expressions in different concrete data types for different
   specific purposes.  Since each object member has exactly one value in
   JSON, each data type for an object member field is naturally mutually
   exclusive with other data types within a single JSON object.

   For example, a resource request for a single access token is composed
   of an object of resource request descriptions, while a request for
   multiple access tokens is composed of an array whose member values
   are all objects.  Both of these represent requests for access, but
   the difference in syntax allows the client instance and AS to
   differentiate between the two request types in the same request.

   Another form of polymorphism in JSON comes from the fact that the
   values within JSON arrays need not all be of the same JSON data type.
   However, within this protocol, each element within the array needs to
   be of the same kind of semantic element for the collection to make
   sense, even when the data types are different from each other.

   For example, each aspect of a resource request can be described using
   an object with multiple dimensional components, or the aspect can be
   requested using a string.  In both cases, the resource request is
   being described in a way that the AS needs to interpret, but with
   different levels of specificity and complexity for the client
   instance to deal with.  An API designer can provide a set of common
   access scopes as simple strings but still allow client software
   developers to specify custom access when needed for more complex
   APIs.

   Extensions to this specification can use different data types for
   defined fields, but each extension needs to not only declare what the
   data type means but also provide justification for the data type
   representing the same basic kind of thing it extends.  For example,
   an extension declaring an "array" representation for a field would
   need to explain how the array represents something akin to the non-
   array element that it is replacing.  See additional discussion in
   Appendix D.

Acknowledgements

   The authors would like to thank the following individuals for their
   reviews, implementations, and contributions: Åke Axeland, Aaron
   Parecki, Adam Omar Oueidat, Andrii Deinega, Annabelle Backman, Dick
   Hardt, Dmitri Zagidulin, Dmitry Barinov, Florian Helmschmidt, Francis
   Pouatcha, George Fletcher, Haardik Haardik, Hamid Massaoud, Jacky
   Yuan, Joseph Heenan, Kathleen Moriarty, Leif Johansson, Mike Jones,
   Mike Varley, Nat Sakimura, Takahiko Kawasaki, Takahiro Tsuchiya, and
   Yaron Sheffer.

   The authors would also like to thank the GNAP Working Group design
   team (Kathleen Moriarty, Dick Hardt, Mike Jones, and the authors),
   who incorporated elements from the XAuth and XYZ proposals to create
   the first draft version of this document.

   In addition, the authors would like to thank Aaron Parecki and Mike
   Jones for insights into how to integrate identity and authentication
   systems into the core protocol.  Both Justin Richer and Dick Hardt
   developed the use cases, diagrams, and insights provided in the XYZ
   and XAuth proposals that have been incorporated here.  The authors
   would like to especially thank Mike Varley and the team at SecureKey
   for feedback and development of early versions of the XYZ protocol
   that fed into this standards work.

   Finally, the authors want to acknowledge the immense contributions of
   Aaron Parecki to the content of this document.  We thank him for his
   insight, input, and hard work, without which GNAP would not have
   grown to what it is.

Authors' Addresses

   Justin Richer (editor)
   Bespoke Engineering
   Email: ietf@justin.richer.org
   URI:   https://bspk.io/


   Fabien Imbault
   acert.io
   Email: fabien.imbault@acert.io
   URI:   https://acert.io/
