Grant Negotiation and Authorization Protocol
draft-ietf-gnap-core-protocol-06

Document Type Active Internet-Draft (gnap WG)
Authors Justin Richer  , Aaron Parecki  , Fabien Imbault 
Last updated 2021-07-12
Replaces draft-richer-transactional-authz
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GNAP                                                      J. Richer, Ed.
Internet-Draft                                       Bespoke Engineering
Intended status: Standards Track                              A. Parecki
Expires: 13 January 2022                                            Okta
                                                              F. Imbault
                                                                acert.io
                                                            12 July 2021

              Grant Negotiation and Authorization Protocol
                    draft-ietf-gnap-core-protocol-06

Abstract

   GNAP defines a mechanism for delegating authorization to a piece of
   software, and conveying that delegation to the software.  This
   delegation can include access to a set of APIs as well as information
   passed directly to the software.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Drafts is at https://datatracker.ietf.org/drafts/current/.

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   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 13 January 2022.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Simplified BSD License text
   as described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   5
     1.2.  Roles . . . . . . . . . . . . . . . . . . . . . . . . . .   5
     1.3.  Elements  . . . . . . . . . . . . . . . . . . . . . . . .   8
     1.4.  Sequences . . . . . . . . . . . . . . . . . . . . . . . .   9
       1.4.1.  Redirect-based Interaction  . . . . . . . . . . . . .  12
       1.4.2.  User-code Interaction . . . . . . . . . . . . . . . .  15
       1.4.3.  Asynchronous Authorization  . . . . . . . . . . . . .  17
       1.4.4.  Software-only Authorization . . . . . . . . . . . . .  19
       1.4.5.  Refreshing an Expired Access Token  . . . . . . . . .  20
       1.4.6.  Requesting User Information . . . . . . . . . . . . .  22
   2.  Requesting Access . . . . . . . . . . . . . . . . . . . . . .  23
     2.1.  Requesting Access to Resources  . . . . . . . . . . . . .  25
       2.1.1.  Requesting a Single Access Token  . . . . . . . . . .  25
       2.1.2.  Requesting Multiple Access Tokens . . . . . . . . . .  28
     2.2.  Requesting Subject Information  . . . . . . . . . . . . .  30
     2.3.  Identifying the Client Instance . . . . . . . . . . . . .  31
       2.3.1.  Identifying the Client Instance by Reference  . . . .  32
       2.3.2.  Providing Displayable Client Instance Information . .  33
       2.3.3.  Authenticating the Client Instance  . . . . . . . . .  33
     2.4.  Identifying the User  . . . . . . . . . . . . . . . . . .  34
       2.4.1.  Identifying the User by Reference . . . . . . . . . .  35
     2.5.  Interacting with the User . . . . . . . . . . . . . . . .  35
       2.5.1.  Start Mode Definitions  . . . . . . . . . . . . . . .  37
       2.5.2.  Finish Interaction Modes  . . . . . . . . . . . . . .  38
       2.5.3.  Hints . . . . . . . . . . . . . . . . . . . . . . . .  41
       2.5.4.  Extending Interaction Modes . . . . . . . . . . . . .  41
     2.6.  Extending The Grant Request . . . . . . . . . . . . . . .  41
   3.  Grant Response  . . . . . . . . . . . . . . . . . . . . . . .  42
     3.1.  Request Continuation  . . . . . . . . . . . . . . . . . .  43
     3.2.  Access Tokens . . . . . . . . . . . . . . . . . . . . . .  44
       3.2.1.  Single Access Token . . . . . . . . . . . . . . . . .  45
       3.2.2.  Multiple Access Tokens  . . . . . . . . . . . . . . .  48
     3.3.  Interaction Modes . . . . . . . . . . . . . . . . . . . .  49
       3.3.1.  Redirection to an arbitrary URL . . . . . . . . . . .  50
       3.3.2.  Launch of an application URL  . . . . . . . . . . . .  51
       3.3.3.  Display of a Short User Code  . . . . . . . . . . . .  51

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       3.3.4.  Interaction Finish  . . . . . . . . . . . . . . . . .  52
       3.3.5.  Extending Interaction Mode Responses  . . . . . . . .  53
     3.4.  Returning User Information  . . . . . . . . . . . . . . .  53
     3.5.  Returning Dynamically-bound Reference Handles . . . . . .  54
     3.6.  Error Response  . . . . . . . . . . . . . . . . . . . . .  56
     3.7.  Extending the Response  . . . . . . . . . . . . . . . . .  56
   4.  Determining Authorization and Consent . . . . . . . . . . . .  56
     4.1.  Interaction Start Methods . . . . . . . . . . . . . . . .  59
       4.1.1.  Interaction at a Redirected URI . . . . . . . . . . .  60
       4.1.2.  Interaction at the User Code URI  . . . . . . . . . .  60
       4.1.3.  Interaction through an Application URI  . . . . . . .  61
     4.2.  Post-Interaction Completion . . . . . . . . . . . . . . .  61
       4.2.1.  Completing Interaction with a Browser Redirect to the
               Callback URI  . . . . . . . . . . . . . . . . . . . .  62
       4.2.2.  Completing Interaction with a Direct HTTP Request
               Callback  . . . . . . . . . . . . . . . . . . . . . .  63
       4.2.3.  Calculating the interaction hash  . . . . . . . . . .  64
   5.  Continuing a Grant Request  . . . . . . . . . . . . . . . . .  65
     5.1.  Continuing After a Completed Interaction  . . . . . . . .  67
     5.2.  Continuing During Pending Interaction . . . . . . . . . .  68
     5.3.  Modifying an Existing Request . . . . . . . . . . . . . .  69
     5.4.  Canceling a Grant Request . . . . . . . . . . . . . . . .  75
   6.  Token Management  . . . . . . . . . . . . . . . . . . . . . .  75
     6.1.  Rotating the Access Token . . . . . . . . . . . . . . . .  75
     6.2.  Revoking the Access Token . . . . . . . . . . . . . . . .  77
   7.  Securing Requests from the Client Instance  . . . . . . . . .  78
     7.1.  Key Formats . . . . . . . . . . . . . . . . . . . . . . .  78
       7.1.1.  Key References  . . . . . . . . . . . . . . . . . . .  80
     7.2.  Presenting Access Tokens  . . . . . . . . . . . . . . . .  80
     7.3.  Proving Possession of a Key with a Request  . . . . . . .  81
       7.3.1.  HTTP Message Signing  . . . . . . . . . . . . . . . .  83
       7.3.2.  Mutual TLS  . . . . . . . . . . . . . . . . . . . . .  87
       7.3.3.  Detached JWS  . . . . . . . . . . . . . . . . . . . .  89
       7.3.4.  Attached JWS  . . . . . . . . . . . . . . . . . . . .  93
   8.  Resource Access Rights  . . . . . . . . . . . . . . . . . . .  97
     8.1.  Requesting Resources By Reference . . . . . . . . . . . . 100
   9.  Discovery . . . . . . . . . . . . . . . . . . . . . . . . . . 102
     9.1.  RS-first Method of AS Discovery . . . . . . . . . . . . . 103
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . 105
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 105
   12. Security Considerations . . . . . . . . . . . . . . . . . . . 105
   13. Privacy Considerations  . . . . . . . . . . . . . . . . . . . 105
   14. Normative References  . . . . . . . . . . . . . . . . . . . . 105
   Appendix A.  Document History . . . . . . . . . . . . . . . . . . 108
   Appendix B.  Compared to OAuth 2.0  . . . . . . . . . . . . . . . 110
   Appendix C.  Component Data Models  . . . . . . . . . . . . . . . 113
   Appendix D.  Example Protocol Flows . . . . . . . . . . . . . . . 113
     D.1.  Redirect-Based User Interaction . . . . . . . . . . . . . 113

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     D.2.  Secondary Device Interaction  . . . . . . . . . . . . . . 117
     D.3.  No User Involvement . . . . . . . . . . . . . . . . . . . 120
     D.4.  Asynchronous Authorization  . . . . . . . . . . . . . . . 121
     D.5.  Applying OAuth 2.0 Scopes and Client IDs  . . . . . . . . 124
   Appendix E.  JSON Structures and Polymorphism . . . . . . . . . . 126
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . 127

1.  Introduction

   This protocol allows a piece of software, the client instance, to
   request delegated authorization to resource servers and to request
   direct information.  This delegation is facilitated by an
   authorization server usually on behalf of a resource owner.  The end-
   user operating the software may interact with the authorization
   server to authenticate, provide consent, and authorize the request.

   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 also discusses discovery
   mechanisms for the client instance to configure itself dynamically.
   The means for an authorization server and resource server to
   interoperate are discussed in the companion document,
   [I-D.draft-ietf-gnap-resource-servers].

   The focus of this protocol is to provide interoperability between the
   different parties acting in each role, and is 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 B
   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 legacy systems to GNAP.  Some examples of these can
   be found in Appendix D.5.

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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, URLs, query components, keys, and
   other elements.  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.

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.

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+-------------+            +------------+
|             |            |            |
|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

   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).

   Client  application operated by an end-user that consumes resources
      from one or several RSs, possibly requiring access privileges from
      one or several ASs.

      Example: a client can be a mobile application, a web application,
      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.

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   Resource Server (RS)  server that provides operations on protected
      resources, where operations require a valid access token issued by
      an 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, the RO and end-user in many instances are the same person,
   where a user is authorizing the client instance to act on their own
   behalf at the RS.  In this case, one party fulfills both of the RO
   and end-user roles, but the roles themselves are still defined
   separately from each other to allow for other use cases where they
   are fulfilled by different parties.

   For 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 components that are installed
   on the end-user's device as well as a back-end system that it
   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.

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   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.

   For 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.

   Attribute  characteristics related to a subject.

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

      Note: an access token can be first issued to an 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 valid for a specific
      duration and/or to exercise some set of delegated rights to access
      a protected resource (noun): the act of granting.

   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 refered to as privilege delegation.

   Protected Resource  protected API (Application Programming Interface)
      served by an RS and that can be accessed by a client, if and only
      if a valid access token is provided.

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      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, organization or device.

   Subject Information  statement asserted by an AS about a subject.

1.4.  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 preconfigured between the components
   or entities performing the roles.  For example, one entity can fulfil
   multiple roles, and 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.

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       +------------+         +------------+
       | 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

   *  (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 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 some more
      dynamic processes are discussed in
      [I-D.draft-ietf-gnap-resource-servers].

   *  (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.

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   *  (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.  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.

   *  (4) The client instance continues the grant at the AS (Section 5).

   *  (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 [I-D.draft-ietf-gnap-resource-servers].

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

   *  (9) When the token no longer works, the client instance fetches an
      updated access token (Section 6.1) based on the rights granted in
      (5).

   *  (10) The AS issues a new access token (Section 3.2) to the client
      instance.

   *  (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.  The means of the RS determining this access are out of
      scope of this specification, but some options are discussed in
      [I-D.draft-ietf-gnap-resource-servers].

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   *  (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 D 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 for there
   not to be a user involved in the delegation process.

1.4.1.  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 resource owner (RO).  Since the client
   instance is capable of directing the user to an arbitrary URL and
   receiving responses from the user's browser, interaction here is
   handled through front-channel redirects using the user's browser.
   The redirection URL 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.

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

   1.   The client instance establishes a verifiable session to 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 URL (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 URL to send the user to (Section 3.3.1) and
        information needed to verify the redirect (Section 3.3.4) 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).

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   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 URL (Section 4.1.1)
        given by the AS in (3).  The user's browser loads the
        interaction redirect URL.  The AS loads the pending request
        based on the incoming URL 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 URL provided in (2).  The redirect URL is augmented
        with an interaction reference that the AS associates with the
        ongoing request created in (2) and referenced in (4).  The
        redirect URL 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 D.1.

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1.4.2.  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 URL.  The URL the user enters
   the code at is an interactive service hosted by the AS in this
   example.  The client instance is not capable of presenting an
   arbitrary URL 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,
   though the user is not assumed to be interacting with the client
   instance through the same web browser used for interaction at the AS.

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

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   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).  This could optionally include a URL to direct
        the user to, but this URL should be static and so could be
        configured in the client instance's documentation.  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 4.1.1) given by the AS in (2).

   4.   The user's directs their browser to the user code URL.  This URL
        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 URL.

   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

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        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 D.2.

1.4.3.  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.

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 +--------+                                  +--------+         +------+
 | 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 ---------------------------|        |
 |        |                                  |        |     +--------+
 +--------+                                  +--------+

   1.   The client instance requests access to the resource (Section 2).
        The client instance does not send any interactions 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 user request section
        (Section 2.4).

   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 resources 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.

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   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.  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 D.4.

1.4.4.  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 a 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.

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

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

   2.  The AS determines that the request is been authorized, the AS
       grants access to the information 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 D.3.

1.4.5.  Refreshing an Expired Access Token

   In this example flow, the client instance receives an access token to
   access a resource server 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 new access
   token by rotating the expired access token at the AS using the
   token's management URL.

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   +--------+                                          +--------+
   | Client |                                          |   AS   |
   |Instance|--(1)--- Request Access ----------------->|        |
   |        |                                          |        |
   |        |<-(2)--- Grant Access --------------------|        |
   |        |                                          |        |
   |        |                             +--------+   |        |
   |        |--(3)--- Access Resource --->|   RS   |   |        |
   |        |                             |        |   |        |
   |        |<-(4)--- Success Response ---|        |   |        |
   |        |                             |        |   |        |
   |        |                             |        |   |        |
   |        |                             |        |   |        |
   |        |--(5)--- Access Resource --->|        |   |        |
   |        |                             |        |   |        |
   |        |<-(6)--- Error Response -----|        |   |        |
   |        |                             +--------+   |        |
   |        |                                          |        |
   |        |--(7)--- Rotate Token ------------------->|        |
   |        |                                          |        |
   |        |<-(8)--- Rotated 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 the token rotation section for
       details.

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   8.  The AS validates the rotation request including the signature and
       keys presented in (5) and returns a new access token
       (Section 3.2.1).  The response includes a new access token and
       can also include updated token management information, which the
       client instance will store in place of the values returned in
       (2).

1.4.6.  Requesting User Information

   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   |         | User |
 |Instance|                                  |        |         |      |
 |        |--(1)--- Request Access --------->|        |         |      |
 |        |                                  |        |         |      |
 |        |<-(2)--- Request Access ----------|        |         |      |
 |        |                                  |        |         |      |
 |        |+ (3) + Facilitate Interaction + + + + + + + + + + > |      |
 |        |                                  |        |         |      |
 |        |                                  |        |<+ (4) +>|      |
 |        |                                  |        |  AuthN  |      |
 |        |                                  |        |         |      |
 |        |                                  |        |<+ (5) +>|      |
 |        |                                  |        |  AuthZ  |      |
 |        |                                  |        |         |      |
 |        |< (6) + Signal Continuation + + + + + + + + + + + + +|      |
 |        |                                  |        |         +------+
 |        |--(7)--- Continue Request ------->|        |
 |        |                                  |        |
 |        |<-(8)----- Grant Access ----------|        |
 |        |                                  |        |
 +--------+                                  +--------+

   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).

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   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 the section on subject information
       for details.

2.  Requesting Access

   To start a request, the client instance sends JSON [RFC8259] document
   with an object as its root.  Each member of the request object
   represents a different aspect of the client instance's request.  Each
   field is described in detail in a section below.

   access_token (object / array of objects)  Describes the rights and
      properties associated with the requested access token.
      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.  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 and any continuation requests at
      the AS and any user-facing information about the client instance
      used in interactions.  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.  Section 2.4

   interact (object)  Describes the modes that the client instance has

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      for allowing the RO to interact with the AS and modes for the
      client instance to receive updates when interaction is complete.
      Section 2.5

   Additional members of this request object can be defined by
   extensions to this protocol as described in Section 2.6

   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": {

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           "start": ["redirect"],
           "finish": {
               "method": "redirect",
               "uri": "https://client.example.net/return/123455",
               "nonce": "LKLTI25DK82FX4T4QFZC"
           }
       },
       "subject": {
           "formats": ["iss_sub", "opaque"],
           "assertions": ["id_token"]
       }
   }

   The request and response MUST be sent as a JSON object in the body of
   the HTTP POST request with Content-Type "application/json", unless
   otherwise specified by the signature mechanism.

   The authorization server MUST include the HTTP "Cache-Control"
   response header field [RFC7234] with a value set to "no-store".

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 sections.

2.1.1.  Requesting a Single Access Token

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

   access (array of objects/strings)  Describes the rights that the
      client instance is requesting for one or more access tokens to be
      used at RS's.  This field is REQUIRED.  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.  If this field is included in the request, the AS MUST
      include the same label in the token response (Section 3.2).  This
      field is REQUIRED if used as part of a multiple access token
      request (Section 2.1.2), and is 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.  This field is OPTIONAL.

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   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
      the key's most recent rotation.  Methods for presenting bound and
      bearer access tokens are described in Section 7.2.  [[ See issue
      #38 (https://github.com/ietf-wg-gnap/gnap-core-protocol/issues/38)
      ]]

   "split"  If this flag is included, the client instance is capable of
      receiving a different number of tokens than specified in the token
      request (Section 2.1), including receiving multiple access tokens
      (Section 3.2.2) in response to any single token request
      (Section 2.1.1) or a different number of access tokens than
      requested in a multiple access token request (Section 2.1.2).  The
      "label" fields of the returned additional tokens are chosen by the
      AS.  The client instance MUST be able to tell from the token
      response where and how it can use each of the access tokens.  [[
      See issue #37 (https://github.com/ietf-wg-gnap/gnap-core-protocol/
      issues/37) ]]

   Flag values MUST NOT be included more than once.

   Additional flags can be defined by extensions using a registry TBD
   (Section 11).

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

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   "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",
       "flags": [ "split" ]
   }

   If access is approved, the resulting access token is valid for the
   described resource and is bound to the client instance's key (or its
   most recent rotation).  The token is labeled "token1-23" and could be
   split into multiple access tokens by the AS, if the AS chooses.  The
   token response structure is described in Section 3.2.1.

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2.1.2.  Requesting Multiple Access Tokens

   To request multiple access tokens to 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
   requesting a single access token (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 error.

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

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   "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" ]
       }
   ]

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   All approved access requests are returned in the multiple access
   token response (Section 3.2.2) structure 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 (or additional fields defined in a
   registry TBD (Section 11)).

   formats (array of strings)  An array of subject identifier subject
      types requested for the RO, as defined by
      [I-D.ietf-secevent-subject-identifiers].

   assertions (array of strings)  An array of requested assertion
      formats.  Possible values include "id_token" for an [OIDC] ID
      Token and "saml2" for a SAML 2 assertion.  Additional assertion
      values are defined by a registry TBD (Section 11).  [[ See issue
      #41 (https://github.com/ietf-wg-gnap/gnap-core-protocol/issues/41)
      ]]

   "subject": {
      "formats": [ "iss_sub", "opaque" ],
      "assertions": [ "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.

   The AS SHOULD NOT re-use subject identifiers for multiple different
   ROs.

   Note: the "formats" and "assertions" request fields are independent
   of each other, and a returned assertion MAY omit a requested subject
   identifier.

   [[ See issue #43 (https://github.com/ietf-wg-gnap/gnap-core-protocol/
   issues/43) ]]

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2.3.  Identifying the Client Instance

   When sending a non-continuation request to the AS, the client
   instance MUST identify itself by including the "client" field of the
   request and by signing the request as described in Section 7.3.  Note
   that for a continuation request (Section 5), the client instance is
   identified by its association with the request being continued and so
   this field is not sent under those circumstances.

   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.  This field is 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.  This field is
      OPTIONAL.

   display (object)  An object containing additional information that
      the AS MAY display to the RO during interaction, authorization,
      and management.  This field is OPTIONAL.

"client": {
    "key": {
        "proof": "httpsig",
        "jwk": {
                    "kty": "RSA",
                    "e": "AQAB",
                    "kid": "xyz-1",
                    "alg": "RS256",
                    "n": "kOB5rR4Jv0GMeLaY6_It_r3ORwdf8ci_JtffXyaSx8..."
        },
        "cert": "MIIEHDCCAwSgAwIBAgIBATANBgkqhkiG9w0BAQsFA..."
    },
    "class_id": "web-server-1234",
    "display": {
        "name": "My Client Display Name",
        "uri": "https://example.net/client"
    }
}

   Additional fields are defined in a registry TBD (Section 11).

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   The client instance MUST prove possession of any presented key by the
   "proof" mechanism associated with the key in the request.  Proof
   types are defined in a registry TBD (Section 11) and an initial set
   of methods is described in Section 7.3.

   Note that the AS MAY know the client instance's public key ahead of
   time, and the AS MAY apply different policies to the request
   depending on what has been registered against that key.  If the same
   public key is sent by value on subsequent access requests, the AS
   SHOULD 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 MAY accept or reject the request based on
   AS policy, attestations within the "client" request, and other
   mechanisms.

   [[ See issue #44 (https://github.com/ietf-wg-gnap/gnap-core-protocol/
   issues/44) ]]

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 the 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 error.

   If the client instance is identified in this manner, the registered
   key for the client instance MAY be a symmetric key known to the AS.
   The client instance MUST NOT send a symmetric key by value in the
   request, as doing so would expose the key directly instead of proving
   possession of it.

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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

   uri (string)  User-facing web page of the client software

   logo_uri (string)  Display image to represent the client software

       "display": {
           "name": "My Client Display Name",
           "uri": "https://example.net/client"
       }

   [[ See issue #48 (https://github.com/ietf-wg-gnap/gnap-core-protocol/
   issues/48) ]]

   Additional display fields are defined by a registry TBD (Section 11).

   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.

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 D.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 also 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

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   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.

   sub_ids (array of objects)  An array of subject identifiers for the
      end-user, as defined by [I-D.ietf-secevent-subject-identifiers].

   assertions (object)  An object containing assertions as values keyed
      on the assertion type defined by a registry TBD (Section 11).
      Possible keys include "id_token" for an [OIDC] ID Token and
      "saml2" for a SAML 2 assertion.  Additional assertion values are
      defined by a registry TBD (Section 11).  [[ See issue #41
      (https://github.com/ietf-wg-gnap/gnap-core-protocol/issues/41) ]]

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

   Subject identifiers are hints to the AS in determining the RO and
   MUST NOT be taken as declarative statements that a particular RO is
   present at the client instance and acting as the end-user.
   Assertions SHOULD be validated by the AS.  [[ See issue #49
   (https://github.com/ietf-wg-gnap/gnap-core-protocol/issues/49) ]]

   If the identified end-user does not match the RO present at the AS
   during an interaction step, the AS SHOULD reject the request with an
   error.

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   [[ See issue #50 (https://github.com/ietf-wg-gnap/gnap-core-protocol/
   issues/50) ]]

   If the AS trusts the client instance to present verifiable
   assertions, 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.

2.4.1.  Identifying the User by Reference

   User reference identifiers can be dynamically issued by the AS
   (Section 3.5) to allow the client instance to represent the same end-
   user to the AS over subsequent 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"

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

   [[ See issue #51 (https://github.com/ietf-wg-gnap/gnap-core-protocol/
   issues/51) ]]

   If the AS does not recognize the user reference, it MUST return an
   error.

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 URL 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.

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   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 (list of strings/objects)  Indicates how the client instance
      can start an interaction.

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

   hints (object)  Provides additional information to inform the
      interaction process at the AS.

   The "interact" field MUST contain the "start" key, and MAY contain
   the "finish" and "hints" keys.  The value of each key is an array
   which contains strings or JSON objects as defined below.

   In this non-normative example, the client instance is indicating that
   it can redirect (Section 2.5.1.1) the end-user to an arbitrary URL
   and can receive a redirect (Section 2.5.2.1) through a browser
   request.

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

   In this 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 URL (Section 2.5.1.1) on a secondary device, but it
   cannot accept a redirect or push callback.

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

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   If the client instance does not provide a suitable interaction
   mechanism, the AS cannot contact the RO asynchronously, and the AS
   determines that interaction is required, then the AS SHOULD return an
   error since the client instance will be unable to complete the
   request without authorization.

   The AS SHOULD apply suitable timeouts to any interaction mechanisms
   provided, including user codes and redirection URLs.  The client
   instance SHOULD apply suitable timeouts to any callback URLs.

2.5.1.  Start Mode Definitions

   This specification defines the following interaction start modes as
   an array of string values under the "start" key:

   "redirect"  Indicates that the client instance can direct the end-
      user to an arbitrary URL for interaction.  Section 2.5.1.1

   "app"  Indicates that the client instance can launch an application
      on the end-user's device for interaction.  Section 2.5.1.2

   "user_code"  Indicates that the client instance can communicate a
      human-readable short code to the end-user for use with a stable
      URL.  Section 2.5.1.3

2.5.1.1.  Redirect to an Arbitrary URL

   If the client instance is capable of directing the end-user to a URL
   defined by the AS at runtime, the client instance indicates this by
   sending the "redirect" field with the boolean value "true".  The
   means by which the client instance will activate this URL is 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 URL to an interactive
   console.  While this URL is generally hosted at the AS, the client
   instance can make no assumptions about its contents, composition, or
   relationship to the AS grant URL.

   "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.

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2.5.1.2.  Open an Application-specific URL

   If the client instance can open a URL associated with an application
   on the end-user's device, the client instance indicates this by
   sending the "app" field with boolean value "true".  The means by
   which the client instance determines the application to open with
   this URL 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 URL
   payload Section 3.3.2.  The client instance manages this interaction
   method as described in Section 4.1.3.

   [[ See issue #54 (https://github.com/ietf-wg-gnap/gnap-core-protocol/
   issues/54) ]]

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 sending the "user_code" field with the
   boolean value "true".  This code is to be entered at a static URL
   that does not change at runtime.  While this URL is generally hosted
   at the AS, the client instance can make no assumptions about its
   contents, composition, or relationship to the AS grant URL.

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

   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 3.3.3.  The client instances manages this interaction
   method as described in Section 4.1.2

2.5.2.  Finish Interaction Modes

   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)  REQUIRED.  The callback method that the AS will use

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      to contact the client instance.  This specification defines the
      following interaction completion methods, with other values
      defined by a registry TBD (Section 11):

      "redirect"  Indicates that the client instance can receive a
         redirect from the end-user's device after interaction with the
         RO has concluded.  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.  Section 2.5.2.2

   uri (string)  REQUIRED.  Indicates the URI that the AS will either
      send the RO to after interaction or send an HTTP POST request.
      This URI MAY be unique per request and MUST be hosted by or
      accessible by the client instance.  This URI MUST NOT contain any
      fragment component.  This 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.  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.

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

   hash_method (string)  OPTIONAL.  The hash calculation mechanism to be
      used for the callback hash in Section 4.2.3.  Can be one of "sha3"
      or "sha2".  If absent, the default value is "sha3".  [[ See issue
      #56 (https://github.com/ietf-wg-gnap/gnap-core-protocol/issues/56)
      ]]

   If this interaction mode is supported for this client instance and
   request, the AS returns a nonce for use in validating the callback
   response (Section 3.3.4).  Requests to the callback URI MUST be
   processed as described in Section 4.2, and the AS MUST require
   presentation of an interaction callback reference as described in
   Section 5.1.

   [[ See issue #58 (https://github.com/ietf-wg-gnap/gnap-core-protocol/
   issues/58) ]]

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   [[ See issue #59 (https://github.com/ietf-wg-gnap/gnap-core-protocol/
   issues/59) ]]

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.

   "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 URL is from the RO's
   browser, this method is usually used when the RO and end-user are the
   same entity.  As such, the client instance MUST ensure the end-user
   is present on the request to prevent substitution attacks.

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.

   "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 URL is from the AS and not
   from the RO's browser, the client instance MUST NOT require the end-
   user to be present on the incoming HTTP request.

   [[ See issue #60 (https://github.com/ietf-wg-gnap/gnap-core-protocol/
   issues/60) ]]

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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 properties 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.  Section 2.5.3.1

   The following sections detail requests for interaction modes.
   Additional interaction modes are defined in a registry TBD
   (Section 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.

2.5.4.  Extending Interaction Modes

   Additional interaction start modes, finish modes, and hints are
   defined in a registry TBD (Section 11).

2.6.  Extending The Grant Request

   The request object MAY be extended by registering new items in a
   registry TBD (Section 11).  Extensions SHOULD be orthogonal to other
   parameters.  Extensions MUST document any aspects where the extension
   item affects or influences the values or behavior of other request
   and response objects.

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3.  Grant Response

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

   continue (object)  Indicates that the client instance can continue
      the request by making one or more continuation requests.
      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.  Section 3.2.1

   interact (object)  Indicates that interaction through some set of
      defined mechanisms needs to take place.  Section 3.3

   subject (object)  Claims about the RO as known and declared by the
      AS.  Section 3.4

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

   user_handle (string)  An identifier this client instance can use to
      identify its current end-user when making future requests.
      Section 3.5

   error (object)  An error code indicating that something has gone
      wrong.  Section 3.6

   In this example, the AS is returning an interaction URL
   (Section 3.3.1), a callback nonce (Section 3.3.4), and a continuation
   response (Section 3.1).

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

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   In this example, the AS is returning a bearer access token
   (Section 3.2.1) with a management URL and a subject identifier
   (Section 3.4) in the form of an opaque identifier.

   {
       "access_token": {
           "value": "OS9M2PMHKUR64TB8N6BW7OZB8CDFONP219RP1LT0",
           "flags": ["bearer"],
           "manage": "https://server.example.com/token/PRY5NM33O\
               M4TB8N6BW7OZB8CDFONP219RP1L",
       },
       "subject": {
           "sub_ids": [ {
              "format": "opaque",
              "id": "J2G8G8O4AZ"
           } ]
       }
   }

   In this 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).

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

3.1.  Request Continuation

   If the AS determines that the request can be continued with
   additional requests, it responds with the "continue" field.  This
   field contains a JSON object with the following properties.

   uri (string)  REQUIRED.  The URI at which the client instance can

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      make continuation requests.  This URI MAY vary per request, or MAY
      be stable at the AS if the AS includes an access token.  The
      client instance MUST use this value exactly as given when making a
      continuation request (Section 5).

   wait (integer)  RECOMMENDED.  The amount of time in integer seconds
      the client instance SHOULD wait after receiving this continuation
      handle and calling the URI.

   access_token (object)  REQUIRED.  A unique access token for
      continuing the request, 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 be usable at resources outside of the
      AS.  The client instance MUST present the access token in all
      requests to the continuation URI as described in Section 7.2.  [[
      See issue #66 (https://github.com/ietf-wg-gnap/gnap-core-protocol/
      issues/66) ]]

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

   The client instance can use the values of this field to continue the
   request as described in Section 5.  Note that the client instance
   MUST sign all continuation requests with its key as described in
   Section 7.3 and MUST present the access token in its continuation
   request.

   This field SHOULD be returned when interaction is expected, to allow
   the client instance to follow up after interaction has been
   concluded.

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.

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

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)  REQUIRED.  The value of the access token as a string.
      The value is opaque to the client instance.  The value SHOULD be
      limited to ASCII characters to facilitate transmission over HTTP
      headers within other protocols without requiring additional
      encoding.

   label (string)  REQUIRED for multiple access tokens, OPTIONAL for
      single access token.  The value of the "label" the client instance
      provided in the associated token request (Section 2.1), if
      present.  If the token has been split by the AS, the value of the
      "label" field is chosen by the AS and the "split" field is
      included and set to "true".

   manage (string)  OPTIONAL.  The management URI for this access token.
      If provided, the client instance MAY manage its access token as
      described in Section 6.  This management URI is a function of the
      AS and is separate from the RS the client instance is requesting
      access to.  This URI MUST NOT include the access token value and
      SHOULD be different for each access token issued in a request.

   access (array of objects/strings)  RECOMMENDED.  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.

   expires_in (integer)  OPTIONAL.  The number of seconds in which the
      access will expire.  The client instance MUST NOT use the access
      token past this time.  An RS MUST NOT accept an 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.

   key (object / string)  OPTIONAL.  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 the request.

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

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

   "bearer"  This flag indicates whether the token is bound to the
      client instance's key.  If the "bearer" flag is present, the
      access token is a bearer token, and the "key" field in this
      response MUST be omitted.  If the "bearer" flag is omitted and the
      "key" field in this response is omitted, the token is bound 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.

   "durable"  OPTIONAL.  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 will 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.

   "split"  OPTIONAL.  Flag indicating that this token was generated by
      issuing multiple access tokens in response to one of the client
      instance's token request (Section 2.1) objects.  This behavior
      MUST NOT be used unless the client instance has specifically
      requested it by use of the "split" flag.

   Flag values MUST NOT be included more than once.

   Additional flags can be defined by extensions using a registry TBD
   (Section 11).

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

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   "access_token": {
       "value": "OS9M2PMHKUR64TB8N6BW7OZB8CDFONP219RP1LT0",
       "manage": "https://server.example.com/token/PRY5NM33O\
           M4TB8N6BW7OZB8CDFONP219RP1L",
       "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 multiple access
   token structure unless the client instance sends the "split" flag as
   described in Section 2.1.1.

   If the AS has split the access token response, the response MUST
   include the "split" flag.

   [[ See issue #69 (https://github.com/ietf-wg-gnap/gnap-core-protocol/
   issues/69) ]]

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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 multiple access token request (Section 2.1.2).

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

   "access_token": [
       {
           "label": "token1",
           "value": "OS9M2PMHKUR64TB8N6BW7OZB8CDFONP219RP1LT0",
           "manage": "https://server.example.com/token/PRY5NM33O\
               M4TB8N6BW7OZB8CDFONP219RP1L",
           "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 multiple access token response MUST be used when multiple access
   tokens are requested, even if only one access token is issued as a
   result of the request.  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 the requested names appropriate
   names.

   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 responds with a multiple access token structure
   containing one access token.

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   If the AS has split the access token response, the response MUST
   include the "split" flag in the "flags" array.

   "access_token": [
       {
           "label": "split-1",
           "value": "8N6BW7OZB8CDFONP219-OS9M2PMHKUR64TBRP1LT0",
           "flags": ["split"],
           "manage": "https://server.example.com/token/PRY5NM33O\
               M4TB8N6BW7OZB8CDFONP219RP1L",
           "access": [ "fruits" ]
       },
       {
           "label": "split-2",
           "value": "FG7VGZZPJ3IZEMN21EVU71FHCAR-UFGLO2FDAP4J1",
           "flags": ["split"],
           "access": [ "vegetables" ]
       }
   }

   Each access token MAY be bound to different keys with different
   proofing mechanisms.

   If token management (Section 6) is allowed, each access token SHOULD
   have different "manage" URIs.

   [[ See issue #70 (https://github.com/ietf-wg-gnap/gnap-core-protocol/
   issues/70) ]]

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 URL.  Section 3.3.1

   app (string)  Launch of an application URL.  Section 3.3.2

   finish (string)  A nonce used by the client instance to verify the
      callback after interaction is completed.  Section 3.3.4

   user_code (object)  Display a short user code.  Section 3.3.3

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   Additional interaction mode responses can be defined in a registry
   TBD (Section 11).

   The AS MUST NOT respond with any interaction mode that the client
   instance did not indicate in its request.  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).

3.3.1.  Redirection to an arbitrary URL

   If the client instance indicates that it can redirect to an arbitrary
   URL (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 URL to direct the end-user to.  This URL
   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 URL returned is a function of the AS, but the URL itself MAY be
   completely distinct from the URL the client instance uses to request
   access (Section 2), allowing an AS to separate its user-interactive
   functionality from its back-end security functionality.  If the AS
   does not directly host the functionality accessed through the given
   URL, then the means for the interaction functionality to communicate
   with the rest of the AS are out of scope for this specification.

   [[ See issue #72 (https://github.com/ietf-wg-gnap/gnap-core-protocol/
   issues/72) ]]

   The client instance sends the end-user to the URL to interact with
   the AS.  The client instance MUST NOT alter the URL in any way.  The
   means for the client instance to send the end-user to this URL is 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 URL in an interactive console.  See details of
   the interaction in Section 4.1.1.

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3.3.2.  Launch of an application URL

   If the client instance indicates that it can launch an application
   URL (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 URL for the client instance to launch.  This
   URL 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 URL as appropriate on its platform,
   and the means for the client instance to launch this URL is out of
   scope of this specification.  The client instance MUST NOT alter the
   URL in any way.  The client instance MAY attempt to detect if an
   installed application will service the URL being sent before
   attempting to launch the application URL.  See details of the
   interaction in Section 4.1.3.

   [[ See issue #71 (https://github.com/ietf-wg-gnap/gnap-core-protocol/
   issues/71) ]]

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 an object that contains the following members.

   code (string)  REQUIRED.  A unique short code that the user can type
      into an authorization server.  This string MUST be case-
      insensitive, MUST consist of only easily typeable characters (such
      as letters or numbers).  The time in which this code will be
      accepted SHOULD be short lived, such as several minutes.  It is
      RECOMMENDED that this code be no more than eight characters in
      length.

   url (string)  RECOMMENDED.  The interaction URL that the client
      instance will direct the RO to.  This URL MUST be stable such that
      client instances can be statically configured with it.

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   "interact": {
       "user_code": {
           "code": "A1BC-3DFF",
           "url": "https://srv.ex/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.

   The client instance SHOULD also communicate the URL if possible to
   facilitate user interaction, but since the URL should be stable, the
   client instance should be able to safely decide to not display this
   value.  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 URL given here at
   runtime.  Consequently, the URL needs to be stable enough that a
   client instance could be statically configured with it, perhaps
   referring the end-user to the URL via documentation instead of
   through an interactive means.  If the client instance is capable of
   communicating an arbitrary URL to the end-user, such as through a
   scannable code, the client instance can use the "redirect"
   (Section 2.5.1.1) mode for this purpose instead of or in addition to
   the user code mode.

   The URL returned is a function of the AS, but the URL itself MAY be
   completely distinct from the URL the client instance uses to request
   access (Section 2), allowing an AS to separate its user-interactive
   functionality from its back-end security functionality.  If the AS
   does not directly host the functionality accessed through the given
   URL, 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.

   [[ See issue #72 (https://github.com/ietf-wg-gnap/gnap-core-protocol/
   issues/72) ]]

3.3.4.  Interaction Finish

   If the client instance indicates that it can receive a
   post-interaction redirect or push at a URL (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.

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

   When the interaction is completed, the interaction component MUST
   contact the client instance using either a redirect or launch of the
   RO's browser or through an HTTP POST to the client instance's
   callback URL using the method indicated in the interaction request
   (Section 2.5.2) as described in Section 4.2.

   If the AS returns a nonce, 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.

3.3.5.  Extending Interaction Mode Responses

   Extensions to this specification can define new interaction mode
   responses in a registry TBD (Section 11).  Extensions MUST document
   the corresponding interaction request.

3.4.  Returning User 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.  This field is an object with the
   following OPTIONAL properties.

   sub_ids (array of objects)  An array of subject identifiers for the
      RO, as defined by [I-D.ietf-secevent-subject-identifiers].

   assertions (object)  An object containing assertions as values keyed
      on the assertion type defined by a registry TBD (Section 11).  [[
      See issue #41 (https://github.com/ietf-wg-gnap/gnap-core-protocol/
      issues/41) ]]

   updated_at (string)  Timestamp as an ISO8610 date string, 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.

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

   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).

   Subject identifiers returned by the AS SHOULD uniquely identify the
   RO at the AS.  Some forms of subject identifier are opaque to the
   client instance (such as the subject of an issuer and subject pair),
   while others 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 AS MUST
   ensure that the returned subject identifiers only apply to the
   authenticated end user.  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.

   Extensions to this specification MAY define additional response
   properties in a registry TBD (Section 11).

3.5.  Returning Dynamically-bound Reference Handles

   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.

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   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 some of these
   references dynamically to the client instance in its response to
   facilitate multiple interactions with the same software.  The client
   instance SHOULD use these references in future requests in lieu of
   sending the associated data value.  These handles are intended to be
   used on future requests.

   Dynamically generated handles are string values that MUST be
   protected by the client instance as secrets.  Handle values MUST be
   unguessable and MUST NOT contain any sensitive information.  Handle
   values are opaque to the client instance.

   All dynamically generated handles are returned as fields in the root
   JSON object of the response.  This specification defines the
   following dynamic handle returns, additional handles can be defined
   in a registry TBD (Section 11).

   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.

   user_handle (string)  A string value used to represent the current
      user.  The client instance can use in a future request, as
      described in Section 2.4.1.

   This non-normative example shows two handles along side an issued
   access token.

   {
       "user_handle": "XUT2MFM1XBIKJKSDU8QM",
       "instance_id": "7C7C4AZ9KHRS6X63AJAO",
       "access_token": {
           "value": "OS9M2PMHKUR64TB8N6BW7OZB8CDFONP219RP1LT0"
       }
   }

   [[ See issue #77 (https://github.com/ietf-wg-gnap/gnap-core-protocol/
   issues/77) ]]

   [[ See issue #78 (https://github.com/ietf-wg-gnap/gnap-core-protocol/
   issues/78) ]]

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3.6.  Error Response

   If the AS determines that the request cannot be issued for any
   reason, it responds to the client instance with an error message.

   error (string)  The error code.

   {

     "error": "user_denied"

   }

   The error code is one of the following, with additional values
   available in a registry TBD (Section 11):

   user_denied  The RO denied the request.

   too_fast  The client instance did not respect the timeout in the wait
      response.

   unknown_request  The request referenced an unknown ongoing access
      request.

   [[ See issue #79 (https://github.com/ietf-wg-gnap/gnap-core-protocol/
   issues/79) ]]

3.7.  Extending the Response

   Extensions to this specification MAY define additional fields for the
   grant response in a registry TBD (Section 11).

4.  Determining Authorization and Consent

   When the client instance makes its 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

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   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.

   The client instance can supply information directly to the AS in its
   request.  From this information, the AS can determine if the
   requested delegation can be granted immediately.  The client instance
   can send several kinds of things, including:

   *  the identity of the client instance, known from the presented keys
      or associated identifiers

   *  the identity of the end user presented in the "user" request
      parameter

   *  any additional information presented by the client instance in the
      request, including any extensions

   The AS will verify 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 the information.  If the
   AS determines that the information presented in the initial request
   is sufficient for granting the requested access, the AS MAY return
   the positive results immediately in its 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 or use a number
   of means at its disposal to gather that authorization from the
   appropriate ROs, including for example:

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

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

   The authorization and consent gathering process 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 as necessitated by the access that
   has been requested.  The AS can sometimes determine which RO needs to
   consent based on what has been requested by the client instance, such
   as a specific RS record, an identified user, or a request requiring
   specific access such as approval by an administrator.  If the AS 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 do anything it needs 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 RO's to approve a

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   given request in order.  The AS could also determine that the end
   user is not the appropriate RO for a given request and reach out to
   the appropriate RO asynchronously.  The details of determining which
   RO's are required for a given request are out of scope for this
   specification.

   The client instance can also indicate that it is capable of
   facilitating interaction with the end user, another party, or another
   piece of software through its interaction start (Section 2.5.1)
   request.  In many cases, the end user is delegating their own access
   as RO to the client instance.  Here, the AS needs to determine the
   identity of the end user and will often need to interact directly
   with the end user to 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 is out of scope of this specification.  The
   client instance MUST use each interaction method once at most.

   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), through
   polling, or through some other method defined by an extension of this
   specification.

   If the AS and client instance have not reached a state where the
   delegation can be granted, the AS and client instance can repeat the
   interaction process as long as the AS supplies the client instance
   with continuation information (Section 3.1) to facilitate the ongoing
   requests.

4.1.  Interaction Start Methods

   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 method.  The actions of
   the client instance required for the interaction start modes defined
   in this specification are described in the following sections.

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4.1.1.  Interaction at a Redirected URI

   When the end user is directed to an arbitrary URI through the
   "redirect" (Section 3.3.1) mode, 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.  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.  If the URI is hosted by the AS, the
   AS MUST determine the grant request being referenced from the URL
   value itself.  If the URL 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 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 MUST
   be protected by HTTPS or equivalent means.

4.1.2.  Interaction at the User Code URI

   When the end user is directed to enter a short code through the
   "user_code" (Section 3.3.3) mode, the client instance communicates
   the user code to the end-user and directs the end user to enter that
   code at an associated URI.  This mode is used when the client
   instance is not able to facilitate launching an arbitrary URI.  The
   associated URI could be statically configured with the client
   instance or communicated in the response from the AS, but the client
   instance communicates that URL to the end user.  As a consequence,
   these URIs SHOULD be short.

   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.  If the URI is hosted by the AS, the
   AS MUST determine the grant request being referenced from the user
   code.  If the user code cannot be associated with a currently active

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   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 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.

   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 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 MUST be protected by HTTPS or equivalent means.

4.1.3.  Interaction through an Application URI

   When the client instance is directed to launch an application through
   the "app" (Section 3.3.2) mode, the client launches the URL 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" (Section 3.3.4) method is associated with
   the current request, the AS MUST follow the appropriate method at
   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.

   The AS MUST create an interaction reference and associate that
   reference with the current interaction and the underlying pending
   request.  This 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.

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   The AS MUST calculate a hash value based on the client instance and
   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.

   The AS MUST send the hash and interaction reference based on the
   interaction finish mode as described in the following sections.

   Note that the "finish" method still occurs in many error cases, such
   as when the RO has denied access.  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 cancelled or otherwise blocked.

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

   When using the "redirect" interaction finish method (Section 3.3.4),
   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 URL sent in the
   callback request (Section 2.5.2.1).

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

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

   interact_ref  REQUIRED.  The interaction reference generated for this
      interaction.

   The means of directing the RO to this URL 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 URL.

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   https://client.example.net/return/123455\
     ?hash=p28jsq0Y2KK3WS__a42tavNC64ldGTBroywsWxT4md_jZQ1R2\
       HZT8BOWYHcLmObM7XHPAdJzTZMtKBsaraJ64A\
     &interact_ref=4IFWWIKYBC2PQ6U56NL1

   When receiving the request, the client instance MUST parse the query
   parameters to 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.

4.2.2.  Completing Interaction with a Direct HTTP Request Callback

   When using the "callback" interaction mode (Section 3.3.4) with the
   "push" method, 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 URL sent in the callback
   request (Section 2.5.2.2).

   The entity message body is a JSON object consisting of the following
   two fields:

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

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

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

   {
     "hash": "p28jsq0Y2KK3WS__a42tavNC64ldGTBroywsWxT4md_jZQ1R\
       2HZT8BOWYHcLmObM7XHPAdJzTZMtKBsaraJ64A",
     "interact_ref": "4IFWWIKYBC2PQ6U56NL1"
   }

   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
   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.

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4.2.3.  Calculating the interaction hash

   The "hash" parameter in the request to the client instance's callback
   URL 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.  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 string by concatenating the following values in the
   following order using a single newline ("\\n") character to separate
   them:

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

   *  the AS's nonce value from the interaction finish response
      (Section 3.3.4)

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

   *  the grant endpoint URL 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.

   VJLO6A4CAYLBXHTR0KRO
   MBDOFXG4Y5CVJCX821LH
   4IFWWIKYBC2PQ6U56NL1
   https://server.example.com/tx

   The party then hashes this string with the appropriate algorithm
   based on the "hash_method" parameter of the "callback".  If the
   "hash_method" value is not present in the client instance's request,
   the algorithm defaults to "sha3".

   [[ See issue #56 (https://github.com/ietf-wg-gnap/gnap-core-protocol/
   issues/56) ]]

4.2.3.1.  SHA3-512

   The "sha3" hash method consists of hashing the input string with the
   512-bit SHA3 algorithm.  The byte array is then encoded using URL
   Safe Base64 with no padding.  The resulting string is the hash value.

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   p28jsq0Y2KK3WS__a42tavNC64ldGTBroywsWxT4md_jZQ1R2HZT8BOWYHcLmObM\
     7XHPAdJzTZMtKBsaraJ64A

4.2.3.2.  SHA2-512

   The "sha2" hash method consists of hashing the input string with the
   512-bit SHA2 algorithm.  The byte array is then encoded using URL
   Safe Base64 with no padding.  The resulting string is the hash value.

   62SbcD3Xs7L40rjgALA-ymQujoh2LB2hPJyX9vlcr1H6ecChZ8BNKkG_HrOKP_Bp\
     j84rh4mC9aE9x7HPBFcIHw

5.  Continuing a Grant Request

   While it is possible for the AS to return a Section 3 with all the
   client instance's requested information (including access tokens
   (Section 3.2) and direct user information (Section 3.4)), it's more
   common that the AS and the client instance will need to communicate
   several times over the lifetime of an access grant.  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).

   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 an
   access token to use during the continued requests.

   The access token is initially bound to the same key and method the
   client instance used to make the initial request.  As a consequence,
   when the client instance makes any calls to the continuation URL, the
   client instance MUST present the 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 all keys presented by the client
   instance or referenced in an ongoing request for each call within
   that request.

   [[ See issue #85 (https://github.com/ietf-wg-gnap/gnap-core-protocol/
   issues/85) ]]

   For example, here the client instance makes a POST request to a
   unique URI and signs the request with HTTP Message Signatures:

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   POST /continue/KSKUOMUKM HTTP/1.1
   Authorization: GNAP 80UPRY5NM33OMUKMKSKU
   Host: server.example.com
   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 URL, the provided access token, and the client
   instance identified by the key signature.  If the AS cannot determine
   a single active grant request to map the continuation request to, the
   AS MUST return an error.

   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 getting the current state of the request.

   All requests to the continuation API are protected by this bound
   access token.  For example, here the client instance makes a POST
   request to a stable continuation endpoint URL 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=...
   Digest: sha256=...

   {
     "interact_ref": "4IFWWIKYBC2PQ6U56NL1"
   }

   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 SHOULD wait at least 5
   seconds.  If the client instance does not respect the given wait
   period, the AS MUST return an error. [[ See issue #86
   (https://github.com/ietf-wg-gnap/gnap-core-protocol/issues/86) ]]

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

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   If the AS determines that the client instance can make a further
   continuation request, the AS MUST include a new "continue" response
   (Section 3.1).  The new "continue" response MUST include a bound
   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 "continue" response, the client instance MUST NOT make an
   additional continuation request.  If a client instance does so, the
   AS MUST return an error. [[ See issue #87 (https://github.com/ietf-
   wg-gnap/gnap-core-protocol/issues/87) ]]

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

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=...
   Digest: sha256=...

   {
     "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.  If the AS detects a client
   instance submitting the same interaction reference multiple times,
   the AS MUST return an error and SHOULD invalidate the ongoing
   request.

   The 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 "continue" response (Section 3.1) as
   described above.  The response SHOULD NOT contain any interaction
   responses (Section 3.3). [[ See issue #89 (https://github.com/ietf-
   wg-gnap/gnap-core-protocol/issues/89) ]]

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   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:

   {
       "access_token": {
           "value": "OS9M2PMHKUR64TB8N6BW7OZB8CDFONP219RP1LT0",
           "manage": "https://server.example.com/token/PRY5NM33O\
               M4TB8N6BW7OZB8CDFONP219RP1L",
       },
       "subject": {
           "sub_ids": [ {
              "format": "opaque",
              "id": "J2G8G8O4AZ"
           } ]
       }
   }

   With this example, the client instance can not make an additional
   continuation request because a "continue" field is not included.

   [[ See issue #88 (https://github.com/ietf-wg-gnap/gnap-core-protocol/
   issues/88) ]]

5.2.  Continuing During Pending Interaction

   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 a message body.

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

   The 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 "continue" 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).

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   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 this 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
       }
   }

   [[ See issue #90 (https://github.com/ietf-wg-gnap/gnap-core-protocol/
   issues/90) ]]

   [[ See issue #91 (https://github.com/ietf-wg-gnap/gnap-core-protocol/
   issues/91) ]]

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

   {
       "access_token": {
           "value": "OS9M2PMHKUR64TB8N6BW7OZB8CDFONP219RP1LT0",
           "manage": "https://server.example.com/token/PRY5NM33O\
               M4TB8N6BW7OZB8CDFONP219RP1L",
       },
       "subject": {
           "sub_ids": [ {
              "format": "opaque",
              "id": "J2G8G8O4AZ"
           } ]
       }
   }

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 claims have 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.

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   The client instance MAY include the "access_token" and "subject"
   fields as described in Section 2.1 and Section 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.  [[ See issue #92 (https://github.com/ietf-wg-gnap/gnap-
   core-protocol/issues/92) ]]

   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 RO, 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.  [[ See issue #93 (https://github.com/ietf-wg-gnap/gnap-core-
   protocol/issues/93) ]]

   The client instance MUST NOT include the "client" section of the
   request. [[ See issue #94 (https://github.com/ietf-wg-gnap/gnap-core-
   protocol/issues/94) ]]

   The client instance MAY include post-interaction responses such as
   described in Section 5.1. [[ See issue #95 (https://github.com/ietf-
   wg-gnap/gnap-core-protocol/issues/95) ]]

   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 existing access
   tokens after a modification has occurred.  [[ See issue #96
   (https://github.com/ietf-wg-gnap/gnap-core-protocol/issues/96) ]]

   If the modified request can be granted immediately by the AS, the
   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 "continue" 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:

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   POST /tx HTTP/1.1
   Host: server.example.com
   Content-Type: application/json
   Signature-Input: sig1=...
   Signature: sig1=...
   Digest: sha256=...

   {
       "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"
           ]
       }
   }

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   This "continue" field allows the client instance to make an eventual
   continuation call.  In the future, 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=...
   Digest: sha256=...

   {
       "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 likely determine that
   reducing the breadth of the requested access means that new access
   tokens can be issued to the client instance.  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"
           ]
       }
   }

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   For another example, the client instance initially requests read-only
   access but later needs to step up its access.  The initial request
   could look like this example.

   POST /tx HTTP/1.1
   Host: server.example.com
   Content-Type: application/json
   Signature-Input: sig1=...
   Signature: sig1=...
   Digest: sha256=...

   {
       "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"
           ]
       }
   }

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   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 section 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.

   [[ See issue #97 (https://github.com/ietf-wg-gnap/gnap-core-protocol/
   issues/97) ]]

   PATCH /continue HTTP/1.1
   Host: server.example.com
   Content-Type: application/json
   Authorization: GNAP 80UPRY5NM33OMUKMKSKU
   Signature-Input: sig1=...
   Signature: sig1=...
   Digest: sha256=...

   {
       "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.

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5.4.  Canceling a Grant Request

   If the client instance wishes to cancel an ongoing grant request, it
   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 cancelled, the AS responds with an
   HTTP 202.  The AS SHOULD revoke all associated access tokens.

6.  Token Management

   If an access token response includes the "manage" parameter as
   described in Section 3.2.1, the client instance MAY call this URL to
   manage the access token with any of the actions defined in the
   following sections.  Other actions are undefined by this
   specification.

   The access token being managed acts as the access element for its own
   management API.  The client instance MUST present proof of an
   appropriate key along with the access token.

   If the token is sender-constrained (i.e., not a bearer token), it
   MUST be sent with the appropriate binding for the access token
   (Section 7.2).

   If the token is a bearer token, the client instance MUST present
   proof of the same key identified in the initial request (Section 2.3)
   as described in Section 7.3.

   The AS MUST validate the proof and assure that it is associated with
   either the token itself or the client instance the token was issued
   to, as appropriate for the token's presentation type.

6.1.  Rotating the Access Token

   The client instance makes an HTTP POST to the token management URI,
   sending the access token in the appropriate header and signing the
   request with the appropriate key.

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   POST /token/PRY5NM33OM4TB8N6BW7OZB8CDFONP219RP1L HTTP/1.1
   Host: server.example.com
   Authorization: GNAP OS9M2PMHKUR64TB8N6BW7OZB8CDFONP219RP1LT0
   Signature-Input: sig1=...
   Signature: sig1=...
   Digest: sha256=...

   The AS validates that the token presented is associated with the
   management URL, that the AS issued the token to the given client
   instance, and that the presented key is appropriate to the token.

   If the access token has expired, the AS SHOULD honor the rotation
   request to the token management URL since it is likely that the
   client instance is attempting to refresh the expired token.  To
   support this, the AS MAY apply different lifetimes for the use of the
   token in management vs. its use at an RS.  An AS MUST NOT honor a
   rotation request for an access token that has been revoked, either by
   the AS or by the client instance through the token management URI
   (Section 6.2).

   If the token is validated and the key is appropriate for the request,
   the AS MUST invalidate the current access token associated with this
   URL, if possible, and return a new access token response as described
   in Section 3.2.1, unless the "multi_token" flag is specified in the
   request.  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 MAY include an updated access token management URL as
   well, and if so, the client instance MUST use this new URL to manage
   the new access token. [[ See issue #101 (https://github.com/ietf-wg-
   gnap/gnap-core-protocol/issues/101) ]]

   [[ See issue #102 (https://github.com/ietf-wg-gnap/gnap-core-
   protocol/issues/102) ]]

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   {
       "access_token": {
           "value": "FP6A8H6HY37MH13CK76LBZ6Y1UADG6VEUPEER5H2",
           "manage": "https://server.example.com/token/PRY5NM33O\
               M4TB8N6BW7OZB8CDFONP219RP1L",
           "access": [
               {
                   "type": "photo-api",
                   "actions": [
                       "read",
                       "write",
                       "dolphin"
                   ],
                   "locations": [
                       "https://server.example.net/",
                       "https://resource.local/other"
                   ],
                   "datatypes": [
                       "metadata",
                       "images"
                   ]
               },
               "read", "dolphin-metadata"
           ]
       }
   }

   [[ See issue #103 (https://github.com/ietf-wg-gnap/gnap-core-
   protocol/issues/103) ]]

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/PRY5NM33OM4TB8N6BW7OZB8CDFONP219RP1L HTTP/1.1
   Host: server.example.com
   Authorization: GNAP OS9M2PMHKUR64TB8N6BW7OZB8CDFONP219RP1LT0
   Signature-Input: sig1=...
   Signature: sig1=...

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   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 204 response code.

   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 URL as valid, since the end result is
   still the token not being usable.

7.  Securing Requests from the Client Instance

   In GNAP, the client instance secures its requests to the AS and RS by
   presenting an access token, presenting proof of a key that it
   possesses, 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 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 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.  Proof of this key material MUST be bound to a
   request, the nature of which varies with the location in the protocol
   the key is used.  For a key used as part of a client instance's
   initial request in Section 2.3, the key value is 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 presenting the
   access token.

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   A key presented by value MUST be a public key in at least one
   supported format.  If a key is sent in multiple formats, all the key
   format values MUST be equivalent.  Note that while most formats
   present the full value of the public key, some formats present a
   value cryptographically derived from the public key.

   proof (string)  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.  The
      "proof" field is REQUIRED.

   jwk (object)  The public key and its properties represented as a JSON
      Web Key [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.

   cert (string)  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.

   cert#S256 (string)  The certificate thumbprint calculated as per
      OAuth-MTLS [RFC8705] in base64 URL encoding.  Note that this
      format does not include the full public key.

   Additional key formats are defined in a registry TBD (Section 11).

   This non-normative example shows a single key presented in multiple
   formats.  This example key is intended to be used with the HTTP
   Message Signatures ({{httpsig-binding}}) proofing mechanism, as
   indicated by the "httpsig" value of the "proof" field.

  "key": {
      "proof": "httpsig",
      "jwk": {
                  "kty": "RSA",
                  "e": "AQAB",
                  "kid": "xyz-1",
                  "alg": "RS256",
                  "n": "kOB5rR4Jv0GMeLaY6_It_r3ORwdf8ci_JtffXyaSx8xY..."
      },
      "cert": "MIIEHDCCAwSgAwIBAgIBATANBgkqhkiG9w0BAQsFA..."
  }

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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.

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

   Keys referenced in this manner MAY be shared symmetric keys.  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 value are out of scope for this
   specification.

7.2.  Presenting Access Tokens

   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 in
   the "bound" and "key" parameters in the single (Section 3.2.1) and
   multiple access tokens (Section 3.2.2) responses.

   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 "httpsig"-bound access token is sent as
   follows:

   Authorization: GNAP OS9M2PMHKUR64TB8N6BW7OZB8CDFONP219RP1LT0
   Signature-Input: sig1=(authorization);...
   Signature: sig1=...

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   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.

   [[ See issue #104 (https://github.com/ietf-wg-gnap/gnap-core-
   protocol/issues/104) ]]

   The client software MUST reject as an error a situation where the
   "flags" field contains the "bearer" flag and the "key" field is
   present with any value.

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.  Values defined by this specification are as
   follows:

   httpsig  HTTP Signing signature header

   mtls  Mutual TLS certificate verification

   jwsd  A detached JWS signature header

   jws  Attached JWS payload

   Additional proofing methods are defined by a registry TBD
   (Section 11).

   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 body 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.  Key binding method definitions SHOULD
   enumerate how these requirements are fulfilled.

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   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 {{I-D.ietf-gnap-
   resource-servers}}. To facilitate this extended use, the sections
   below are defined in generic terms of the "sender" and "verifier" of
   the HTTP message.  In the core functions of GNAP, the "sender" is the
   client instance and the "verifier" is the AS or RS, 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.

   [[ See issue #105 (https://github.com/ietf-wg-gnap/gnap-core-
   protocol/issues/105) ]]

   In the following sections, unless otherwise noted, the "RS256" JOSE
   Signature Algorithm is applied using the following RSA key (presented
   here in JWK format):

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   {
       "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"
   }

7.3.1.  HTTP Message Signing

   This method is indicated by "httpsig" in the "proof" field.  The
   sender creates an HTTP Message Signature as described in
   [I-D.ietf-httpbis-message-signatures].

   The covered content of the signature MUST include the following:

   @request-target:  the target of the HTTP request

   digest:  The Digest header as defined in [RFC3230].  When the request
      message has a body, the signer MUST calculate this header value
      and the verifier MUST validate this header.

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

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

   Other covered content MAY also be included.

   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, the signing
   algorithm used MUST be the JWS algorithm denoted by the key's "alg"
   field, and the explicit "alg" signature parameter MUST NOT be
   included.

   In this example, the message body is the following JSON object:

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   {
       "access_token": {
           "access": [
               "dolphin-metadata"
           ]
       },
       "interact": {
           "start": ["redirect"],
           "finish": {
               "method": "redirect",
               "uri": "https://client.foo/callback",
               "nonce": "VJLO6A4CAYLBXHTR0KRO"
           }
       },
       "client": {
         "proof": "httpsig",
         "key": {
           "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 body is hashed for the Digest header using SHA-256 into the
   following encoded value:

   SHA-256=98QzyNVYpdgTrWBKpC4qFSCmmR+CrwwvUoiaDCSjKxw=

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

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   "@request-target": post /gnap
   "host": server.example.com
   "content-type": application/json
   "digest": SHA-256=98QzyNVYpdgTrWBKpC4qFSCmmR+CrwwvUoiaDCSjKxw=
   "content-length": 986
   "@signature-params": ("@request-target" "host" "content-type" \
     "digest" "content-length");created=1618884475;keyid="gnap-rsa"

   This leads to the following full HTTP message request:

   POST /gnap HTTP/1.1
   Host: server.example.com
   Content-Type: application/json
   Content-Length: 986
   Digest: SHA-256=98QzyNVYpdgTrWBKpC4qFSCmmR+CrwwvUoiaDCSjKxw=
   Signature-Input: sig1=("@request-target" "host" "content-type" \
     "digest" "content-length");created=1618884475;keyid="gnap-rsa"
   Signature: \
     sig1=:axj8FLOvEWBcwh+Xk6VTTKXxqo4XNygleTDJ8h3ZJfi1sSmWrRtyo9RG/dc\
     miZmdszRjWbg+/ixVZpA4BL3AOwEOxxtmHAXNB8uJ0I3tfbs6Suyk4sEo8zPr+MJq\
     MjxdJEUgAQAy2AH+wg5a7CKq4IdLTulFK9njUIeG7MygHumeiumM3DbDQAHgF46dV\
     q5UC6KJnqhGM1rFC128jd2D0sgWKCUgKGCHtfR159zfKWcEO9krsLoOnCdTzm1UyD\
     DMjkIjqeN/1j8PdMJaRAwV4On079O0DVu6bl1jVtkzo/e/ZmwPr/X436V4xiw/hZt\
     w4sfNsSbmsT0+UAQ20X/xaw==:

   {
       "access_token": {
           "access": [
               "dolphin-metadata"
           ]
       },
       "interact": {
           "start": ["redirect"],
           "finish": {
               "method": "redirect",
               "uri": "https://client.foo/callback",
               "nonce": "VJLO6A4CAYLBXHTR0KRO"
           }
       },
       "client": {
         "proof": "httpsig",
         "key": {
           "jwk": {
               "kid": "gnap-rsa",
               "kty": "RSA",
               "e": "AQAB",
               "alg": "RS256",

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               "n": "hYOJ-XOKISdMMShn_G4W9m20mT0VWtQBsmBBkI2cmRt4Ai8Bf\
     YdHsFzAtYKOjpBR1RpKpJmVKxIGNy0g6Z3ad2XYsh8KowlyVy8IkZ8NMwSrcUIBZG\
     YXjHpwjzvfGvXH_5KJlnR3_uRUp4Z4Ujk2bCaKegDn11V2vxE41hqaPUnhRZxe0jR\
     ETddzsE3mu1SK8dTCROjwUl14mUNo8iTrTm4n0qDadz8BkPo-uv4BC0bunS0K3bA_\
     3UgVp7zBlQFoFnLTO2uWp_muLEWGl67gBq9MO3brKXfGhi3kOzywzwPTuq-cVQDyE\
     N7aL0SxCb3Hc4IdqDaMg8qHUyObpPitDQ"
           }
         }
         "display": {
           "name": "My Client Display Name",
           "uri": "https://client.foo/"
         },
       }
   }

   If the HTTP Message includes a message body, the verifier MUST
   calculate and verify the value of the "Digest" header.  The verifier
   MUST ensure that the signature includes all required covered content.
   The verifier MUST validate the signature against the expected key of
   the signer.

7.3.2.  Mutual TLS

   This method is indicated by "mtls" in the "proof" field.  The signer
   presents its TLS client certificate during TLS negotiation with the
   verifier.

   In this example, the certificate is communicated to the application
   through the "Client-Cert" header from a TLS reverse proxy, 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 \

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     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": {
         "proof": "jws",
         "key": {
           "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"]
       }

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   }

   The verifier compares the TLS client certificate presented during
   mutual TLS 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 issue #110 (https://github.com/ietf-wg-gnap/gnap-core-
   protocol/issues/110) ]]

7.3.3.  Detached JWS

   This method is indicated by "jwsd" in the "proof" field.  A JWS
   [RFC7515] object is created as follows:

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

   kid (string)  The key identifier.  RECOMMENDED.  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.  REQUIRED.
      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".

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

   htm (string)  The HTTP Method used to make this request, as an
      uppercase ASCII string.  REQUIRED

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

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

   ath (string)  When a request is bound to an access token, the access

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      token hash value.  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
      request protects an access token.

   If the HTTP request has a message body, such as an HTTP POST or PUT
   method, the payload of the JWS object is the Base64url encoding
   (without padding) of the SHA256 digest of the bytes of the body.  If
   the request being made does not have a message body, such as an HTTP
   GET, OPTIONS, or DELETE method, the JWS signature is calculated over
   an empty payload.

   The client instance presents the signed object in compact form
   [RFC7515] in the Detached-JWS HTTP Header field.

   In this 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 body is the following JSON object:

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   {
       "access_token": {
           "access": [
               "dolphin-metadata"
           ]
       },
       "interact": {
           "start": ["redirect"],
           "finish": {
               "method": "redirect",
               "uri": "https://client.foo/callback",
               "nonce": "VJLO6A4CAYLBXHTR0KRO"
           }
       },
       "client": {
         "proof": "jwsd",
         "key": {
           "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:

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   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": {
         "proof": "jwsd",
         "key": {
           "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/"

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         },
       }
   }

   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.  All required fields MUST be present and
   their values MUST be valid.  If the HTTP message request contains a
   body, the verifier MUST calculate the hash of body just as the signer
   does, with no normalization or transformation of the request.

7.3.4.  Attached JWS

   This method is indicated by "jws" in the "proof" field.  A JWS
   [RFC7515] object is created as follows:

   The JOSE header MUST contain the "kid" parameter of the key bound to
   this client instance for this request.  The "alg" parameter MUST be
   set to a value appropriate for the key identified by kid and MUST NOT
   be "none".

   To protect the request, the JWS header MUST contain the following
   additional parameters.

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

   htm (string)  The HTTP Method used to make this request, as an
      uppercase ASCII string.

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

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

   ath (string)  When a request is bound to an access token, the access
      token hash value.  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.

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

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   If the request being made does not have a message body, 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 this 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 body, used as the JWS Payload, is the following JSON
   object:

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   {
       "access_token": {
           "access": [
               "dolphin-metadata"
           ]
       },
       "interact": {
           "start": ["redirect"],
           "finish": {
               "method": "redirect",
               "uri": "https://client.foo/callback",
               "nonce": "VJLO6A4CAYLBXHTR0KRO"
           }
       },
       "client": {
         "proof": "jws",
         "key": {
           "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:

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   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

   [[ See issue #109 (https://github.com/ietf-wg-gnap/gnap-core-
   protocol/issues/109) ]]

   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.  If the HTTP message request contains a
   body, the verifier MUST decode the payload of the JWS object and
   treat this as the HTTP message body.

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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).

   The root of this structure is a JSON array.  The elements of the JSON
   array represent rights of access that are associated with the the
   access token.  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.

   type (string)  The type of resource request as a string.  This field
      MAY define which other fields are allowed in the request object.
      This field is 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, a
   set of common properties are defined here.  Specific API
   implementations SHOULD NOT re-use these fields with different
   semantics or syntax.  The available values for these properties are
   determined by the API being protected at the RS.

   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.

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   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 resource owner
      is no longer online.

   The following non-normative example is describing three kinds of
   access (read, write, delete) to each of two different locations and
   two different data types (metadata, 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"
           ]
       }
   ]

   The access requested for a given object when using these fields 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 following non-

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   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 while simultaneously requesting "write" and "delete" access
   for "metadata" on a different server, but 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.

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   "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
   (Section 3.2.1)'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 or RS representing the access being requested.  Each string
   SHOULD correspond to a specific expanded object representation at the
   AS.

   "access": [
       "read", "dolphin-metadata", "some other thing"
   ]

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   This value is opaque to the client instance and MAY be any valid JSON
   string, and therefore 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 D.5.  Note that the reference string here is not bound to
   the same character restrictions as in 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".

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   "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.

9.  Discovery

   By design, the protocol minimizes the need for any pre-flight
   discovery.  To begin a request, the client instance only needs to
   know the endpoint of the AS 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 containing the following information:

   grant_request_endpoint (string)  REQUIRED.  The location of the AS's

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      grant request endpoint.  The location MUST be a URL [RFC3986] with
      a scheme component that 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.

   interaction_start_modes_supported (array of strings)  OPTIONAL.  A
      list of the AS's interaction start methods.  The values of this
      list correspond to the possible values for the interaction start
      section (Section 2.5.1) of the request.

   interaction_finish_methods_supported (array of strings)  OPTIONAL.  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 section (Section 2.5.2) of the request.

   key_proofs_supported (array of strings)  OPTIONAL.  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 (Section 7.1) of the request.

   subject_formats_supported (array of strings)  OPTIONAL.  A list of
      the AS's supported subject identifier types.  The values of this
      list correspond to possible values of the subject identifier
      section (Section 2.2) of the request.

   assertions_supported (array of strings)  OPTIONAL.  A list of the
      AS's supported assertion formats.  The values of this list
      correspond to possible values of the subject assertion section
      (Section 2.2) of the request.

   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, a
   given client instance can be registered with the "mtls" key proofing
   mechanism, but the AS also returns other proofing methods, then the
   AS will deny a request from that client instance using a different
   proofing mechanism.

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 MAY respond to the client instance
   with an authentication header indicating that GNAP needs to be used
   to access the resource.  The address of the GNAP endpoint MUST be
   sent in the "as_uri" parameter.  The RS MAY additionally return a
   resource reference that the client instance MAY use in its access
   token request.  This resource reference MUST be sufficient for at

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   least the action the client instance was attempting to take at the RS
   and MAY be more powerful.  The means for the RS to determine the
   resource reference are out of scope of this specification, but some
   dynamic methods are discussed in
   [I-D.draft-ietf-gnap-resource-servers].  The content of the resource
   handle is opaque to the client instance.

   WWW-Authenticate: \
     GNAP as_uri=https://server.example/tx,access=FWWIKYBQ6U56NL1

   The client instance then makes a request to the "as_uri" as described
   in Section 2, with the value of "access" as one of the members of the
   "access" array in the "access_token" portion of the request.  The
   client instance MAY request additional resources and other
   information.  The client instance MAY request multiple access tokens.

   In this non-normative example, the client instance is requesting a
   single access token using the resource reference "FWWIKYBQ6U56NL1"
   received from the RS in addition to the "dolphin-metadata" resource
   reference that the client instance has been configured with out of
   band.

   POST /tx HTTP/1.1
   Host: server.example.com
   Content-Type: application/json
   Signature-Input: sig1=...
   Signature: sig1=...
   Digest: sha256=...

   {
       "access_token": {
           "access": [
               "FWWIKYBQ6U56NL1",
               "dolphin-metadata"
           ]
       },
       "client": "KHRS6X63AJ7C7C4AZ9AO"
   }

   If issued, the resulting access token would contain sufficient access
   to be used at both referenced resources.

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10.  Acknowledgements

   The editors would like to thank the feedback of the following
   individuals for their reviews, implementations, and contributions:
   Aeke Axeland, Aaron Parecki, Adam Omar Oueidat, Annabelle Backman,
   Dick Hardt, Dmitri Zagidulin, Dmitry Barinov, Fabien Imbault, Francis
   Pouatcha, George Fletcher, Haardik Haardik, Hamid Massaoud, Jacky
   Yuan, Joseph Heenan, Justin Richer, Kathleen Moriarty, Mike Jones,
   Mike Varley, Nat Sakimura, Takahiko Kawasaki, Takahiro Tsuchiya.

   The editors would also like to thank the GNAP working group design
   team of Kathleen Moriarty, Fabien Imbault, Dick Hardt, Mike Jones,
   and Justin Richer, who incorporated elements from the XAuth and XYZ
   proposals to create the first version of this document.

   In addition, the editors would like to thank Aaron Parecki and Mike
   Jones for insights into how to integrate identity and authentication
   systems into the core protocol, and Justin Richer and Dick Hardt for
   the use cases, diagrams, and insights provided in the XYZ and XAuth
   proposals that have been incorporated here.  The editors 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.

11.  IANA Considerations

   [[ TBD: There are a lot of items in the document that are expandable
   through the use of value registries. ]]

12.  Security Considerations

   [[ TBD: There are a lot of security considerations to add. ]]

   All requests have to be over TLS or equivalent as per [BCP195].  Many
   handles act as shared secrets, though they can be combined with a
   requirement to provide proof of a key as well.

13.  Privacy Considerations

   [[ TBD: There are a lot of privacy considerations to add. ]]

   Handles are passed between parties and therefore should not contain
   any private data.

   When user information is passed to the client instance, the AS needs
   to make sure that it has the permission to do so.

14.  Normative References

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   [BCP195]   Sheffer, Y., Holz, R., and P. Saint-Andre,
              "Recommendations for Secure Use of Transport Layer
              Security (TLS) and Datagram Transport Layer Security
              (DTLS)", May 2015,
              <https://www.rfc-editor.org/info/bcp195>.

   [I-D.draft-ietf-gnap-resource-servers]
              Richer, J., Parecki, A., and F. Imbault, "Grant
              Negotiation and Authorization Protocol Resource Server
              Connections", Work in Progress, Internet-Draft, draft-
              ietf-gnap-resource-servers-00, 28 April 2021,
              <https://www.ietf.org/archive/id/draft-ietf-gnap-resource-
              servers-00.txt>.

   [I-D.ietf-httpbis-message-signatures]
              Backman, A., Richer, J., and M. Sporny, "Signing HTTP
              Messages", Work in Progress, Internet-Draft, draft-ietf-
              httpbis-message-signatures-05, 8 June 2021,
              <https://www.ietf.org/archive/id/draft-ietf-httpbis-
              message-signatures-05.txt>.

   [I-D.ietf-oauth-rar]
              Lodderstedt, T., Richer, J., and B. Campbell, "OAuth 2.0
              Rich Authorization Requests", Work in Progress, Internet-
              Draft, draft-ietf-oauth-rar-05, 15 May 2021,
              <https://www.ietf.org/archive/id/draft-ietf-oauth-rar-
              05.txt>.

   [I-D.ietf-oauth-signed-http-request]
              Richer, J., Bradley, J., and H. Tschofenig, "A Method for
              Signing HTTP Requests for OAuth", Work in Progress,
              Internet-Draft, draft-ietf-oauth-signed-http-request-03, 8
              August 2016, <https://www.ietf.org/archive/id/draft-ietf-
              oauth-signed-http-request-03.txt>.

   [I-D.ietf-secevent-subject-identifiers]
              Backman, A. and M. Scurtescu, "Subject Identifiers for
              Security Event Tokens", Work in Progress, Internet-Draft,
              draft-ietf-secevent-subject-identifiers-08, 24 May 2021,
              <https://www.ietf.org/archive/id/draft-ietf-secevent-
              subject-identifiers-08.txt>.

   [OIDC]     Sakimura, N., Bradley, J., Jones, M., de Medeiros, B., and
              C. Mortimore, "OpenID Connect Core 1.0 incorporating
              errata set 1", November 2014,
              <https://openiD.net/specs/openiD-connect-core-1_0.html>.

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   [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>.

   [RFC3230]  Mogul, J. and A. Van Hoff, "Instance Digests in HTTP",
              RFC 3230, DOI 10.17487/RFC3230, January 2002,
              <https://www.rfc-editor.org/info/rfc3230>.

   [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>.

   [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>.

   [RFC7234]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
              RFC 7234, DOI 10.17487/RFC7234, June 2014,
              <https://www.rfc-editor.org/info/rfc7234>.

   [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>.

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   [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>.

   [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>.

Appendix A.  Document History

   *  -06

      -  Removed "capabilities" and "existing_grant" protocol fields.

      -  Removed separate "instance_id" field.

      -  Split "interaction_methods_supported" into
         "interaction_start_modes_supported" and
         "interaction_finish_methods_supported".

      -  Added AS endpoint to hash calculation to fix mix-up attack.

      -  Added "privileges" field to resource access request object.

      -  Moved client-facing RS response back from GNAP-RS document.

      -  Removed oauthpop key binding.

      -  Removed dpop key binding.

      -  Added example DID identifier.

      -  Changed token response booleans to flag structure to match
         request.

      -  Updated signature examples to use HTTP Message Signatures.

   *  -05

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      -  Changed "interaction_methods" to
         "interaction_methods_supported".

      -  Changed "key_proofs" to "key_proofs_supported".

      -  Changed "assertions" to "assertions_supported".

      -  Updated discovery and field names for subject formats.

      -  Add an appendix to provide protocol rationale, compared to
         OAuth2.

      -  Updated subject information definition.

      -  Refactored the RS-centric components into a new document.

      -  Updated cryptographic proof of possession methods to match
         current reference syntax.

      -  Updated proofing language to use "signer" and "verifier"
         generically.

      -  Updated cryptographic proof of possession examples.

      -  Editorial cleanup and fixes.

      -  Diagram cleanup and fixes.

   *  -04

      -  Updated terminology.

      -  Refactored key presentation and binding.

      -  Refactored "interact" request to group start and end modes.

      -  Changed access token request and response syntax.

      -  Changed DPoP digest field to 'htd' to match proposed FAPI
         profile.

      -  Include the access token hash in the DPoP message.

      -  Removed closed issue links.

      -  Removed function to read state of grant request by client.

      -  Closed issues related to reading and updating access tokens.

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   *  -03

      -  Changed "resource client" terminology to separate "client
         instance" and "client software".

      -  Removed OpenID Connect "claims" parameter.

      -  Dropped "short URI" redirect.

      -  Access token is mandatory for continuation.

      -  Removed closed issue links.

      -  Editorial fixes.

   *  -02

      -  Moved all "editor's note" items to GitHub Issues.

      -  Added JSON types to fields.

      -  Changed "GNAP Protocol" to "GNAP".

      -  Editorial fixes.

   *  -01

      -  "updated_at" subject info timestamp now in ISO 8601 string
         format.

      -  Editorial fixes.

      -  Added Aaron and Fabien as document authors.

   *  -00

      -  Initial working group draft.

Appendix B.  Compared to 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 the a web
       browser.  The type of interaction available is fixed by the grant
       type, and the most common interactive grant types start in the

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       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 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 resource owner to be two
       different people, but 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.

   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.

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       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, including the return of data
       apart from the access token.  The "resource" parameter and RAR
       extensions (as defined in [I-D.ietf-oauth-rar]) expand on the
       "scope" concept in similar but different ways.

       GNAP defines a rich structure for requesting access, with string
       references as an optimization.  GNAP defines methods for
       requesting directly-returned user information, separate from API
       access.  This information includes identifiers for the current
       user and structured assertions.  The core GNAP protocol 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
       constraint 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 a 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.

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       GNAP is designed to be interoperable with decentralized identity
       standards and to provide a human-centric authorization layer.  In
       addition to the core protocol, GNAP that supports various
       patterns of communication between RSs and ASs through extensions.
       GNAP tries to limit the odds of a consolidation to just a handful
       of super-popular AS services.

Appendix C.  Component Data Models

   While different implementations of this protocol will have different
   realizations of all the components and artifacts enumerated here, the
   nature of the protocol implies some common structures and elements
   for certain components.  This appendix seeks to enumerate those
   common elements.

   TBD: Client has keys, allowed requested resources, identifier(s),
   allowed requested subjects, allowed

   TBD: AS has "grant endpoint", interaction endpoints, store of trusted
   client keys, policies

   TBD: Token has RO, user, client, resource list, RS list,

Appendix D.  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 would be applied for different situations.

   Some longer fields, particularly cryptographic information, have been
   truncated for display purposes in these examples.

D.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.

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   POST /tx HTTP/1.1
   Host: server.example.com
   Content-Type: application/json
   Signature-Input: sig1=...
   Signature: sig1=...
   Digest: sha256=...

   {
       "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"
           }
       }
   }

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   The AS processes the request and determines that the RO needs to
   interact.  The AS returns the following response giving 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",
          "push": "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_url by sending the following HTTP message to the user's
   browser.

   HTTP 302 Found
   Location: https://server.example.com/interact/4CF492MLVMSW9MKM

   The user's browser fetches the AS's interaction URL.  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, 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.

   HTTP 302 Found
   Location: https://client.example.net/return/123455\
     ?hash=p28jsq0Y2KK3WS__a42tavNC64ldGTBroywsWxT4md_jZQ1R2\
       HZT8BOWYHcLmObM7XHPAdJzTZMtKBsaraJ64A\
     &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

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   pending request.  The client instance uses the values in this to
   validate the hash parameter.  The client instance then calls the
   continuation URL and presents the handle and interaction reference in
   the request body.  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=...
   Digest: sha256=...

   {
       "interact_ref": "4IFWWIKYBC2PQ6U56NL1"
   }

   The AS retrieves the pending request based on the handle and issues a
   bearer access token and returns this to the client instance.

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   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"
       }
   }

D.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.

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   POST /tx HTTP/1.1
   Host: server.example.com
   Content-Type: application/json
   Signature-Input: sig1=...
   Signature: sig1=...
   Digest: sha256=...

   {
       "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 "callback" 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": "A1BC-3DFF",
               "url": "https://srv.ex/device"
           }
       },
       "continue": {
           "access_token": {
               "value": "80UPRY5NM33OMUKMKSKU"
           },
           "uri": "https://server.example.com/continue/VGJKPTKC50",
           "wait": 60
       }
   }

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   The client instance saves the response and displays the user code
   visually on its screen along with the static device URL.  The client
   instance also displays the short interaction URL 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 URL.  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 periodically polls the AS every 60
   seconds at the continuation URL.  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=...
   Digest: sha256=...

   The AS retrieves the pending request based on the handle 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
       }
   }

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   Note that the continuation URL and access token have been rotated
   since they were used by the client instance to make this call.  The
   client instance polls the continuation URL 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=...
   Digest: sha256=...

   The AS retrieves the pending request based on the URL and access
   token, determines that it has been approved, and issues an access
   token for the client to use at the RS.

   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"
           ]
       }
   }

D.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.

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   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 and 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"
           ]
       }
   }

D.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.

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   POST /tx HTTP/1.1
   Host: server.example.com
   Content-Type: application/json
   Signature-Input: sig1=...
   Signature: sig1=...
   Digest: sha256=...

   {
       "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"
           } ]
      }
   }

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   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, the AS has an application that it can push notifications in
   to for the specified account.

   Meanwhile, the client instance periodically polls the AS every 60
   seconds at the continuation URL.

   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 handle 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.

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   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 handle has been rotated since it was used
   by the client instance to make this call.  The client instance polls
   the continuation URL after a 60 second timeout using the new handle.

   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 and
   determines that it has been approved and it 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/PRY5NM33O\
               M4TB8N6BW7OZB8CDFONP219RP1L",
           "access": [
               "dolphin-metadata", "some other thing"
           ]
       }
   }

D.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.

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   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.  Traditionally, the OAuth 2.0 client developer
   would put their "client_id" and "scope" values as parameters into a
   redirect request to the authorization endpoint.

   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=...
   Digest: sha256=...

   {
       "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"
           }
       }
   }

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   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 URL.

   From here, the protocol continues as above.

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 array of resource request descriptions while a request for
   multiple access tokens is composed of an object whose member values
   are all arrays.  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.

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   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.

Authors' Addresses

   Justin Richer (editor)
   Bespoke Engineering

   Email: ietf@justin.richer.org
   URI:   https://bspk.io/

   Aaron Parecki
   Okta

   Email: aaron@parecki.com
   URI:   https://aaronparecki.com

   Fabien Imbault
   acert.io

   Email: fabien.imbault@acert.io
   URI:   https://acert.io/

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