Web Authorization Protocol                                       D. Fett
Internet-Draft                                                   yes.com
Intended status: Standards Track                             B. Campbell
Expires: 2 November 2020 22 May 2021                                       Ping Identity
                                                              J. Bradley
                                                                  Yubico
                                                          T. Lodderstedt
                                                                 yes.com
                                                                M. Jones
                                                               Microsoft
                                                                D. Waite
                                                           Ping Identity
                                                              1 May
                                                        18 November 2020

  OAuth 2.0 Demonstration of Demonstrating Proof-of-Possession at the Application Layer
                                 (DPoP)
                        draft-ietf-oauth-dpop-01
                        draft-ietf-oauth-dpop-02

Abstract

   This document describes a mechanism for sender-constraining OAuth 2.0
   tokens via a proof-of-possession mechanism on the application level.
   This mechanism allows for the detection of replay attacks with access
   and refresh tokens.

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
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   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 2 November 2020. 22 May 2021.

Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   Please review these documents carefully, as they describe your rights
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   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Conventions and Terminology . . . . . . . . . . . . . . .   3
   2.  Main Objective  Objectives  . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Concept . . . . . . . . . . . . . . . . . . . . . . . . . . .   4   5
   4.  DPoP Proof JWTs . . . . . . . . . . . . . . . . . . . . . . .   6   7
     4.1.  The DPoP HTTP Header  . . . . . . . . . . . . . . . . . .   7
     4.2.  DPoP Proof JWT Syntax . . . . . . . . . . . . . . . . . .   6
     4.2.   8
     4.3.  Checking DPoP Proofs  . . . . . . . . . . . . . . . . . .   7   9
   5.  DPoP Access Token Request (Binding Tokens to a Public Key) . . . . . . .   8
   6.  Resource Access (Proof of Possession for Access Tokens) . . .   9
   7. . . . . . . . .  10
     5.1.  Authorization Server Metadata . . . . . . . . . . . . . .  13
   6.  Public Key Confirmation . . . . . . . . . . . . . . . . . . .  11
   8.  Authorization Server Metadata  13
     6.1.  JWK Thumbprint Confirmation Method  . . . . . . . . . . .  14
     6.2.  JWK Thumbprint Confirmation Method in Token
           Introspection . . . . . . . . . . .  12
   9. . . . . . . . . . . .  14
   7.  Protected Resource Access . . . . . . . . . . . . . . . . . .  16
     7.1.  The DPoP Authorization Request Header Scheme  . . . . . .  16
     7.2.  The Bearer Authorization Request Header Scheme  . . . . .  18
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
     9.1.  19
     8.1.  DPoP Proof Replay . . . . . . . . . . . . . . . . . . . .  13
     9.2.  19
     8.2.  Signed JWT Swapping . . . . . . . . . . . . . . . . . . .  13
     9.3.  19
     8.3.  Signature Algorithms  . . . . . . . . . . . . . . . . . .  13
     9.4.  19
     8.4.  Message Integrity . . . . . . . . . . . . . . . . . . . .  13
   10.  20
     8.5.  Public Key Binding  . . . . . . . . . . . . . . . . . . .  20
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
     10.1.  20
     9.1.  OAuth Access Token Type Registration  . . . . . . . . . .  14
     10.2.  20
     9.2.  HTTP Authentication Scheme Registration . . . . . . . .  14
     10.3. .  21
     9.3.  Media Type Registration . . . . . . . . . . . . . . . .  14
     10.4. .  21
     9.4.  JWT Confirmation Methods Registration . . . . . . . . .  15
     10.5. .  21
     9.5.  JSON Web Token Claims Registration  . . . . . . . . . . .  15
     10.6.  22
     9.6.  HTTP Message Header Field Names Registration  . . . . . .  15
     10.7.  22
     9.7.  Authorization Server Metadata Registration  . . . . . . .  16
   11.  22
   10. Normative References  . . . . . . . . . . . . . . . . . . . .  16
   12.  23
   11. Informative References  . . . . . . . . . . . . . . . . . . .  17  23
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .  19  26
   Appendix B.  Document History . . . . . . . . . . . . . . . . . .  19  27
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  21  29

1.  Introduction

   This document outlines a relatively simple

   DPoP, an abbreviation for Demonstrating Proof-of-Possession at the
   Application Layer, is an application-level mechanism for sender-constraining sender-
   constraining OAuth access and refresh tokens.  It enables a client to
   demonstrate proof-of-possession of a public/
   private public/private key pair by
   including the a "DPoP" header in an HTTP request.
   Using that header, an authorization server  The value of the
   header is able a JWT [RFC7519] that enables the authorization server to
   bind issued tokens to the public part of the client's key pair.
   Recipients of such tokens are then able to verify the binding of the
   token to the key pair that the client has demonstrated that it holds
   via the "DPoP" header, thereby providing some assurance that the
   client presenting the token also possesses the private key.  In other
   words, the legitimate presenter of the token is constrained to be the
   sender that holds and can prove possession of the private part of the
   key pair.

   The mechanism described herein can be used in cases where potentially
   stronger other
   methods of sender-constraining tokens that utilize elements of the
   underlying secure transport layer, such as [RFC8705] or
   [I-D.ietf-oauth-token-binding], are not available or desirable.  For
   example, due to a sub-par user experience of TLS client
   authentication in user agents and a lack of support for HTTP token
   binding, neither mechanism can be used if an OAuth client is a Single
   Page Application (SPA) running in a web browser.  Native applications
   installed and run on a user's device, which often have dedicated
   protected storage for cryptographic keys. are another example well
   positioned to benefit from DPoP-bound tokens to guard against misuse
   of tokens by a compromised or malicious resource.

   DPoP can be used with public clients to sender-constrain access tokens and refresh tokens.  With confidential clients, regardless of the
   client authentication method employed.  Furthermore, DPoP can also be
   used in conjunction with any client authentication method to sender-
   constrain access tokens. sender-constrain refresh tokens issued to public clients
   (those without authentication credentials associated with the
   "client_id").

1.1.  Conventions and 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 specification uses the terms "access token", "refresh token",
   "authorization server", "resource server", "authorization endpoint",
   "authorization request", "authorization response", "token endpoint",
   "grant type", "access token request", "access token response", and
   "client" defined by The OAuth 2.0 Authorization Framework [RFC6749].

2.  Main Objective

   Under the attacker model defined in [I-D.ietf-oauth-security-topics],
   the mechanism defined by this specification aims  Objectives

   The primary aim of DPoP is to prevent token
   replay at unauthorized or illegitimate
   parties from using leaked or stolen access tokens by binding a different endpoint.

   More precisely, if an adversary is able token
   to get hold a public key upon issuance and requiring that the client
   demonstrate possession of an access
   token or refresh the corresponding private key when using
   the token.  This constrains the legitimate sender of the token because it set up a counterfeit authorization
   server or resource server, to
   only the adversary is not able party with access to replay the
   respective private key and gives the server
   receiving the token at another authorization or resource server.

   Secondary objectives are discussed in Section 9.

3.  Concept

   The main data structure introduced by this specification added assurances that the sender is a legitimately
   authorized to use it.

   Access tokens that are sender-constrained via DPoP
   proof JWT, described in detail below, sent as a header thus stand in an HTTP
   request.  A client uses a DPoP proof JWT
   contrast to prove the typical bearer token, which can be used by any party
   in possession of such a
   private key corresponding token.  Although protections generally exist
   to a certain public key.  Roughly speaking,
   a DPoP proof is a signature over a timestamp and some data prevent unintended disclosure of the
   HTTP request bearer tokens, unforeseen vectors
   for leakage have occurred due to which it is attached.

   +--------+                                          +---------------+
   |        |--(A)-- Token Request ------------------->|               |
   | Client |        (DPoP Proof)                      | Authorization |
   |        |                                          |     Server    |
   |        |<-(B)-- DPoP-bound Access Token ----------|               |
   |        |        (token_type=DPoP)                 +---------------+
   |        |        PoP Refresh Token for public clients
   |        |
   |        |                                          +---------------+
   |        |--(C)-- DPoP-bound Access Token --------->|               |
   |        |        (DPoP Proof)                      |    Resource   |
   |        |                                          |     Server    |
   |        |<-(D)-- Protected Resource ---------------|               |
   |        |                                          +---------------+
   +--------+

                         Figure 1: Basic DPoP Flow

   The basic steps of an OAuth flow with DPoP are shown vulnerabilities and implementation
   issues in other layers in Figure 1:

   *  (A) In the Token Request, the client sends an authorization code
      to protocol or software stack (CRIME,
   BREACH, Heartbleed, and the authorization server in order to obtain an access Cloudflare parser bug are some examples).
   There have also been numerous published token
      (and potentially a refresh token).  The client attaches a theft attacks on OAuth
   implementations themselves.  DPoP
      proof to the request provides a general defense in an HTTP header.

   *  (B) The AS binds (sender-constrains) depth
   against the access impact of unanticipated token to the
      public key claimed by the client in the leakage.  DPoP proof; that is, the
      access token cannot is not,
   however, a substitute for a secure transport and MUST always be used without proving possession
   in conjunction with HTTPS.

   The very nature of the
      respective private key.  This is signaled to typical OAuth protocol interaction
   necessitates that the client by using disclose the "token_type" value "DPoP".

   *  If a refresh access token is issued to a public client, it is bound to the public key of the DPoP proof
   protected resources that it accesses.  The attacker model in
   [I-D.ietf-oauth-security-topics] describes cases where a similar way.  Note that for
      confidential clients, refresh protected
   resource might be counterfeit, malicious or compromised and play
   received tokens are required by [RFC6749] against other protected resources to
      bound gain
   unauthorized access.  Properly audience restricting access tokens can
   prevent such misuse, however, doing so in practice has proven to the "client_id" and associated authentication
      credentials, which be
   prohibitively cumbersome (even despite extensions such as [RFC8707])
   for many deployments.  Sender-constraining access tokens is a sender-constraining mechanism that is more
      flexible than binding
   robust and straightforward mechanism to prevent such token replay at
   a particular public key.

   *  (C) If the client wants different endpoint and DPoP is an accessible application layer
   means of doing so.

   Due to use the access token, it has potential for cross-site scripting (XSS), browser-based
   OAuth clients bring to prove
      possession of the private key by, again, adding a header bear added considerations with respect to the
      request that carries the DPoP proof.
   protecting tokens.  The resource server needs to
      receive information about the public key most straightforward XSS-based attack is for
   an attacker to which exfiltrate a token and use it themselves completely
   independent from the legitimate client.  A stolen access token is bound.  This information is either encoded directly into the
   used for protected resource access and a stolen refresh token (for JWT structured for
   obtaining new access tokens), or provided at
      the token introspection endpoint of the authorization server (not
      shown).  The resource server verifies that tokens.  If the public private key to which
      the access token is bound matches the public key of the non-extractable
   (as is possible with [W3C.WebCryptoAPI]), DPoP
      proof.

   *  (D) The resource server refuses renders exfiltrated
   tokens alone unusable.

   XXS vulnerabilities also allow an attacker to serve the request if the
      signature check fails or the data execute code in the DPoP proof is wrong,
      e.g., the request URI does not match the URI claim in
   context of the DPoP
      proof JWT.

   *  When browser-based client application and maliciously use a refresh
   token that is sender-constrained using DPoP is used
      by indirectly through the client, the client client.  That execution context has
   access to provide a utilize the signing key and thus can produce DPoP proof just as proofs to
   use in conjunction with the case token.  At this application layer there
   is most likely no feasible defense against this threat except
   generally preventing XSS, therefore it is considered out of a resource access.  The new access token will be bound
      to scope for
   DPoP.

   Malicious XSS code executed in the same public key.

   The mechanism presented herein context of the browser-based
   client application is not also in a client authentication method.
   In fact, position to create DPoP proofs with
   timestamp values in the future and exfiltrate them in conjunction
   with a primary use case token.  These stolen artifacts can later be used together
   independent of DPoP is for public clients (e.g.,
   single page applications) that do not use the client authentication.
   Nonetheless, DPoP is designed application to access protected resources.
   The impact of such that it is compatible with
   "private_key_jwt" and all other precomputed DPoP proofs can be limited somewhat by
   a browser-based client authentication methods. generating and using a new DPoP does not directly ensure message integrity but relies on the TLS
   layer key for that purpose.  See each
   new authorization code grant.

   Additional security considerations are discussed in Section 9 for details.

4.  DPoP Proof JWTs

   DPoP introduces concept of 8.

3.  Concept

   The main data structure introduced by this specification is a DPoP
   proof JWT, described in detail below, which is used for
   binding public keys and proving knowledge about private keys.  The
   DPoP proof JWT is sent with as a header in an
   HTTP request using the "DPoP" header
   field.

4.1.  DPoP Proof JWT Syntax request.  A client uses a DPoP proof is a JWT ([RFC7519]) that is signed (using JWS,
   [RFC7515]) using to prove the possession
   of a private key chosen by the client (see below).  The
   header of corresponding to a certain public key.  Roughly
   speaking, a DPoP JWT contains at least the following parameters:

   *  "typ": type header, value "dpop+jwt" (REQUIRED).

   *  "alg": proof is a digital signature algorithm identifier as per [RFC7518]
      (REQUIRED).  MUST NOT be "none" or an identifier for over a symmetric
      algorithm (MAC).

   * timestamp and some data
   of the HTTP request to which it is attached.

   +--------+                                          +---------------+
   |        |--(A)-- Token Request ------------------->|               |
   | Client |        (DPoP Proof)                      | Authorization |
   |        |                                          |     Server    |
   |        |<-(B)-- DPoP-bound Access Token ----------|               |
   |        |        (token_type=DPoP)                 +---------------+
   |        |
   |        |
   |        |                                          +---------------+
   |        |--(C)-- DPoP-bound Access Token --------->|               |
   |        |        (DPoP Proof)                      |    Resource   |
   |        |                                          |     Server    |
   |        |<-(D)-- Protected Resource ---------------|               |
   |        |                                          +---------------+
   +--------+

                         Figure 1: Basic DPoP Flow

   The basic steps of an OAuth flow with DPoP are shown in Figure 1:

   *  (A) In the Token Request, the client sends an authorization grant
      (e.g., an authorization code, refresh token, etc.)
      to the authorization server in order to obtain an access token
      (and potentially a refresh token).  The client attaches a DPoP
      proof to the request in an HTTP header.

   *  (B) The authorization server binds (sender-constrains) the access
      token to the public key claimed by the client in the DPoP proof;
      that is, the access token cannot be used without proving
      possession of the respective private key.  If a refresh token is
      issued to a public client, it too is bound to the public key of
      the DPoP proof.

   *  (C) To use the access token the client has to prove possession of
      the private key by, again, adding a header to the request that
      carries the DPoP proof.  The resource server needs to receive
      information about the public key to which the access token is
      bound.  This information may be encoded directly into the access
      token (for JWT structured access tokens) or provided via token
      introspection endpoint (not shown).  The resource server verifies
      that the public key to which the access token is bound matches the
      public key of the DPoP proof.

   *  (D) The resource server refuses to serve the request if the
      signature check fails or the data in the DPoP proof is wrong,
      e.g., the request URI does not match the URI claim in the DPoP
      proof JWT.  The access token itself, of course, must also be valid
      in all other respects.

   The DPoP mechanism presented herein is not a client authentication
   method.  In fact, a primary use case of DPoP is for public clients
   (e.g., single page applications and native applications) that do not
   use client authentication.  Nonetheless, DPoP is designed such that
   it is compatible with "private_key_jwt" and all other client
   authentication methods.

   DPoP does not directly ensure message integrity but relies on the TLS
   layer for that purpose.  See Section 8 for details.

4.  DPoP Proof JWTs

   DPoP introduces the concept of a DPoP proof, which is a JWT created
   by the client and sent with an HTTP request using the "DPoP" header
   field.  A valid DPoP proof demonstrates to the server that the client
   holds the private key that was used to sign the JWT.  This enables
   authorization servers to bind issued tokens to the corresponding
   public key (as described in Section 5) and for resource servers to
   verify the key-binding of tokens that it receives (see Section 7.1),
   which prevents said tokens from being used by any entity that does
   not have access to the private key.

   The DPoP proof demonstrates possession of a key and, by itself, is
   not an authentication or access control mechanism.  When presented in
   conjunction with a key-bound access token as described in
   Section 7.1, the DPoP proof provides additional assurance about the
   legitimacy of the client to present the access token.  But a valid
   DPoP proof JWT is not sufficient alone to make access control
   decisions.

4.1.  The DPoP HTTP Header

   A DPoP proof is included in an HTTP request using the following
   message header field.

   "DPoP"  A JWT that adheres to the structure and syntax of
      Section 4.2.

   Figure 2 shows an example DPoP HTTP header field (line breaks and
   extra whitespace for display purposes only).

   DPoP: eyJ0eXAiOiJkcG9wK2p3dCIsImFsZyI6IkVTMjU2IiwiandrIjp7Imt0eSI6Ik
    VDIiwieCI6Imw4dEZyaHgtMzR0VjNoUklDUkRZOXpDa0RscEJoRjQyVVFVZldWQVdCR
    nMiLCJ5IjoiOVZFNGpmX09rX282NHpiVFRsY3VOSmFqSG10NnY5VERWclUwQ2R2R1JE
    QSIsImNydiI6IlAtMjU2In19.eyJqdGkiOiItQndDM0VTYzZhY2MybFRjIiwiaHRtIj
    oiUE9TVCIsImh0dSI6Imh0dHBzOi8vc2VydmVyLmV4YW1wbGUuY29tL3Rva2VuIiwia
    WF0IjoxNTYyMjYyNjE2fQ.2-GxA6T8lP4vfrg8v-FdWP0A0zdrj8igiMLvqRMUvwnQg
    4PtFLbdLXiOSsX0x7NVY-FNyJK70nfbV37xRZT3Lg
                      Figure 2: Example "DPoP" header

   Note that per [RFC7230] header field names are case-insensitive; so
   "DPoP", "DPOP", "dpop", etc., are all valid and equivalent header
   field names.  Case is significant in the header field value, however.

4.2.  DPoP Proof JWT Syntax

   A DPoP proof is a JWT ([RFC7519]) that is signed (using JWS,
   [RFC7515]) with a private key chosen by the client (see below).  The
   header of a DPoP JWT contains at least the following parameters:

   *  "typ": type header, value "dpop+jwt" (REQUIRED).

   *  "alg": a digital signature algorithm identifier as per [RFC7518]
      (REQUIRED).  MUST NOT be "none" or an identifier for a symmetric
      algorithm (MAC).

   *  "jwk": representing the public key chosen by public key chosen by the client, in JWK
      format, as defined in [RFC7515] (REQUIRED)

   The body of a DPoP proof contains at least the following claims:

   *  "jti": Unique identifier for the DPoP proof JWT (REQUIRED).  The
      value MUST be assigned such that there is a negligible probability
      that the same value will be assigned to any other DPoP proof used
      in the same context during the time window of validity.  Such
      uniqueness can be accomplished by encoding (base64url or any other
      suitable encoding) at least 96 bits of pseudorandom data or by
      using a version 4 UUID string according to [RFC4122].  The "jti"
      can be used by the server for replay detection and prevention, see
      Section 8.1.

   *  "htm": The HTTP method for the request to which the JWT is
      attached, as defined in [RFC7231] (REQUIRED).

   *  "htu": The HTTP URI used for the request, without query and
      fragment parts (REQUIRED).

   *  "iat": Time at which the JWT was created (REQUIRED).

   Figure 3 is a conceptual example showing the decoded content of the
   DPoP proof in Figure 2.  The JSON of the JOSE header and payload are
   shown but the signature part is omitted.  As usual, line breaks and
   extra whitespace are included for formatting and readability.

   {
     "typ":"dpop+jwt",
     "alg":"ES256",
     "jwk": {
       "kty":"EC",
       "x":"l8tFrhx-34tV3hRICRDY9zCkDlpBhF42UQUfWVAWBFs",
       "y":"9VE4jf_Ok_o64zbTTlcuNJajHmt6v9TDVrU0CdvGRDA",
       "crv":"P-256"
     }
   }
   .
   {
     "jti":"-BwC3ESc6acc2lTc",
     "htm":"POST",
     "htu":"https://server.example.com/token",
     "iat":1562262616
   }

              Figure 3: Example JWT content of a "DPoP" proof

   Of the HTTP content in the request, only the HTTP method and URI are
   included in the DPoP JWT, and therefore only these 2 headers of the
   request are covered by the DPoP proof and its signature.  The idea is
   sign just enough of the HTTP data to provide reasonable proof-of-
   possession with respect to the HTTP request.  But that it be a
   minimal subset of the HTTP data so as to avoid the substantial
   difficulties inherent in attempting to normalize HTTP messages.
   Nonetheless, DPoP proofs can be extended to contain other information
   of the HTTP request (see also Section 8.4).

4.3.  Checking DPoP Proofs

   To check if a string that was received as part of an HTTP Request is
   a valid DPoP proof, the receiving server MUST ensure that

   1.  the string value is a well-formed JWT,

   2.  all required claims are contained in the JWT,

   3.  the "typ" field in the header has the value "dpop+jwt",

   4.  the algorithm in the header of the JWT indicates an asymmetric
       digital signature algorithm, is not "none", is supported by the
       application, and is deemed secure,

   5.  that the JWT is signed using the public key contained in the
       "jwk" header of the JWT,

   6.  the "htm" claim matches the HTTP method value of the HTTP request
       in which the JWT was received,

   7.  the "htu" claims matches the HTTPS URI value for the HTTP request
       in which the JWT was received, ignoring any query and fragment
       parts,

   8.  the token was issued within an acceptable timeframe (see
       Section 8.1), and

   9.  that, within a reasonable consideration of accuracy and resource
       utilization, a JWT with the same "jti" value has not previously
       been received at the client, in JWK
      format, as defined same URI (see Section 8.1).

   Servers SHOULD employ Syntax-Based Normalization and Scheme-Based
   Normalization in [RFC7515] (REQUIRED)

   The body accordance with Section 6.2.2. and Section 6.2.3. of
   [RFC3986] before comparing the "htu" claim.

5.  DPoP Access Token Request

   To request an access token that is bound to a public key using DPoP,
   the client MUST provide a valid DPoP proof contains at least JWT in a "DPoP" header
   when making an access token request to the following claims:

   *  "jti": Unique identifier authorization server's
   token endpoint.  This is applicable for all access token requests
   regardless of grant type (including, for example, the common
   "authorization_code" and "refresh_token" grant types but also
   extension grants such as the JWT authorization grant [RFC7523]).  The
   HTTPS request shown in Figure 4 illustrates an such an access token
   request using an an authorization code grant with a DPoP proof JWT (REQUIRED). in
   the "DPoP" header (extra line breaks and whitespace for display
   purposes only).

   POST /token HTTP/1.1
   Host: server.example.com
   Content-Type: application/x-www-form-urlencoded;charset=UTF-8
   DPoP: eyJ0eXAiOiJkcG9wK2p3dCIsImFsZyI6IkVTMjU2IiwiandrIjp7Imt0eSI6Ik
    VDIiwieCI6Imw4dEZyaHgtMzR0VjNoUklDUkRZOXpDa0RscEJoRjQyVVFVZldWQVdCR
    nMiLCJ5IjoiOVZFNGpmX09rX282NHpiVFRsY3VOSmFqSG10NnY5VERWclUwQ2R2R1JE
    QSIsImNydiI6IlAtMjU2In19.eyJqdGkiOiItQndDM0VTYzZhY2MybFRjIiwiaHRtIj
    oiUE9TVCIsImh0dSI6Imh0dHBzOi8vc2VydmVyLmV4YW1wbGUuY29tL3Rva2VuIiwia
    WF0IjoxNTYyMjYyNjE2fQ.2-GxA6T8lP4vfrg8v-FdWP0A0zdrj8igiMLvqRMUvwnQg
    4PtFLbdLXiOSsX0x7NVY-FNyJK70nfbV37xRZT3Lg

   grant_type=authorization_code
   &code=SplxlOBeZQQYbYS6WxSbIA
   &redirect_uri=https%3A%2F%2Fclient%2Eexample%2Ecom%2Fcb
   &code_verifier=bEaL42izcC-o-xBk0K2vuJ6U-y1p9r_wW2dFWIWgjz-
    Figure 4: Token Request for a DPoP sender-constrained token using an
                             authorization code

   The
      value "DPoP" HTTP header MUST be assigned such that there is contain a negligible probability
      that valid DPoP proof JWT.  If the same value will be assigned to any other
   DPoP proof used
      in is invalid, the authorization server issues an error
   response per Section 5.2 of [RFC6749] with "invalid_dpop_proof" as
   the value of the "error" parameter.

   To sender-constrain the access token, after checking the validity of
   the DPoP proof, the authorization server associates the issued access
   token with the same context during public key from the time window of validity.  Such
      uniqueness DPoP proof, which can be
   accomplished by encoding (base64url or any other
      suitable encoding) at least 96 bits as described in Section 6.  A "token_type" of pseudorandom data or by
      using a version 4 UUID string according "DPoP" in
   the access token response signals to [RFC4122].  The "jti"
      SHOULD be used by the server for replay detection client that the access token
   was bound to its DPoP key and prevention,
      see can used as described in Section 9.1.

   *  "htm": 7.1.
   The HTTP method for example response shown in Figure 5 illustrates such a response.

   HTTP/1.1 200 OK
   Content-Type: application/json
   Cache-Control: no-cache, no-store

   {
    "access_token": "Kz~8mXK1EalYznwH-LC-1fBAo.4Ljp~zsPE_NeO.gxU",
    "token_type": "DPoP",
    "expires_in": 2677,
    "refresh_token": "Q..Zkm29lexi8VnWg2zPW1x-tgGad0Ibc3s3EwM_Ni4-g"
   }

                      Figure 5: Access Token Response

   The example response in Figure 5 included a refresh token, which the request
   client can use to which obtain a new access token when the JWT the previous one
   expires.  Refreshing an access token is
      attached, as defined in [RFC7231] (REQUIRED).

   *  "htu": The HTTP URI used for a token request using the request, without query and
      fragment parts (REQUIRED).

   *  "iat": Time at which
   "refresh_token" grant type made to the JWT was created (REQUIRED).

   Figure 2 shows the JSON header and payload of authorization server's
   token endpoint.  As with all access token requests, the client makes
   it a DPoP proof JWT.

   {
     "typ":"dpop+jwt",
     "alg":"ES256",
     "jwk": {
       "kty":"EC",
       "x":"l8tFrhx-34tV3hRICRDY9zCkDlpBhF42UQUfWVAWBFs",
       "y":"9VE4jf_Ok_o64zbTTlcuNJajHmt6v9TDVrU0CdvGRDA",
       "crv":"P-256"
     }
   }.{
     "jti":"-BwC3ESc6acc2lTc",
     "htm":"POST",
     "htu":"https://server.example.com/token",
     "iat":1562262616
   } request by including a DPoP proof, which is shown in the
   Figure 2: Example JWT content 6 example (extra line breaks and whitespace for "DPoP" proof header

   Note: To keep display
   purposes only).

   POST /token HTTP/1.1
   Host: server.example.com
   Content-Type: application/x-www-form-urlencoded;charset=UTF-8
   DPoP: eyJ0eXAiOiJkcG9wK2p3dCIsImFsZyI6IkVTMjU2IiwiandrIjp7Imt0eSI6Ik
    VDIiwieCI6Imw4dEZyaHgtMzR0VjNoUklDUkRZOXpDa0RscEJoRjQyVVFVZldWQVdCR
    nMiLCJ5IjoiOVZFNGpmX09rX282NHpiVFRsY3VOSmFqSG10NnY5VERWclUwQ2R2R1JE
    QSIsImNydiI6IlAtMjU2In19.eyJqdGkiOiItQndDM0VTYzZhY2MybFRjIiwiaHRtIj
    oiUE9TVCIsImh0dSI6Imh0dHBzOi8vc2VydmVyLmV4YW1wbGUuY29tL3Rva2VuIiwia
    WF0IjoxNTYyMjY1Mjk2fQ.pAqut2IRDm_De6PR93SYmGBPXpwrAk90e8cP2hjiaG5Qs
    GSuKDYW7_X620BxqhvYC8ynrrvZLTk41mSRroapUA

   grant_type=refresh_token
   &refresh_token=Q..Zkm29lexi8VnWg2zPW1x-tgGad0Ibc3s3EwM_Ni4-g

    Figure 6: Token Request for a DPoP-bound token using a refresh token

   When an authorization server supporting DPoP simple issues a refresh token
   to implement, only the HTTP method and URI
   are signed in a public client that presents a valid DPoP proofs.  The idea is sign just enough of proof at the HTTP
   data to provide reasonable proof-of-possession with respect to token
   endpoint, the
   HTTP request.  But that it refresh token MUST be a minimal subset of the HTTP data so as bound to avoid the substantial difficulties inherent in attempting to
   normalize HTTP messages.  Nonetheless, DPoP proofs can respective public
   key.  The binding MUST be extended to
   contain other information of validated when the HTTP request (see also Section 9.4).

4.2.  Checking DPoP Proofs

   To check if a string that was received as part of an HTTP Request refresh token is later
   presented to get new access tokens.  As a result, such a client MUST
   present a valid DPoP proof, proof for the receiving server MUST ensure same key that

   1. was used to obtain the string value
   refresh token each time that refresh token is used to obtain a well-formed JWT,

   2.  all required claims are contained in new
   access token.  The implementation details of the JWT,

   3. binding of the "typ" field in
   refresh token are at the header has discretion of the value "dpop+jwt",

   4. authorization server.  The
   server both produces and validates the algorithm refresh tokens that it issues
   so there's no interoperability consideration in the header specific details
   of the JWT indicates an asymmetric
       digital signature algorithm, is binding.

   An authorization server MAY elect to issue access tokens which are
   not "none", is supported by the
       application, and is deemed secure,

   5.  that the JWT DPoP bound, which is signed using signaled to the public key contained client with a value of
   "Bearer" in the
       "jwk" header "token_type" parameter of the JWT,

   6.  the "htm" claim matches access token response
   per [RFC6750].  For a public client that is also issued a refresh
   token, this has the HTTP method value effect of DPoP-binding the HTTP request
       in refresh token alone,
   which can improve the JWT was received (case-insensitive),

   7. security posture even when protected resources
   are not updated to support DPoP.

   Refresh tokens issued to confidential clients (those having
   established authentication credentials with the "htu" claims matches authorization server)
   are not bound to the HTTP URI value for DPoP proof public key because they are already
   sender-constrained with a different existing mechanism.  The OAuth
   2.0 Authorization Framework [RFC6749] already requires that an
   authorization server bind refresh tokens to the HTTP request
       in client to which the JWT was received, ignoring any query and fragment
       parts,

   8.  the token was they
   were issued within an acceptable timeframe (see
       Section 9.1), and

   9.  that, within that confidential clients authenticate to the
   authorization server when presenting a reasonable consideration refresh token.  As a result,
   such refresh tokens are sender-constrained by way of accuracy the client ID
   and resource
       utilization, the associated authentication requirement.  This existing sender-
   constraining mechanism is more flexible (e.g., it allows credential
   rotation for the client without invalidating refresh tokens) than
   binding directly to a JWT with particular public key.

5.1.  Authorization Server Metadata

   This document introduces the same "jti" value has not been
       received previously (see Section 9.1).

   Servers SHOULD employ Syntax-Based Normalization and Scheme-Based
   Normalization following new authorization server
   metadata [RFC8414] parameter to signal support for DPoP in accordance with Section 6.2.2. general
   and Section 6.2.3. the specific JWS "alg" values the authorization server supports
   for DPoP proof JWTs.

   "dpop_signing_alg_values_supported"  A JSON array containing a list
      of
   [RFC3986] before comparing the "htu" claim.

5.  Token Request (Binding Tokens JWS "alg" values supported by the authorization server for
      DPoP proof JWTs.

6.  Public Key Confirmation

   Resource servers MUST be able to reliably identify whether an access
   token is bound using DPoP and ascertain sufficient information about
   the public key to a Public Key)

   To bind a which the token is bound in order to verify the
   binding with respect to the the presented DPoP proof (see
   Section 7.1).  Such a binding is accomplished by associating the
   public key with the token in a way that can be accessed by the
   protected resource, such as embedding the JWK hash in the issued
   access token request, directly, using the client MUST
   provide a valid DPoP proof JWT syntax described in a "DPoP" header.  The HTTPS request
   shown Section 6.1, or
   through token introspection as described in Figure 3 illustrates Section 6.2.  Other
   methods of associating a public key with an access token are
   possible, per agreement by the protocol for authorization server and the protected
   resource, but are beyond the scope of this (with extra line
   breaks for display purposes only).

   POST /token HTTP/1.1
   Host: server.example.com
   Content-Type: application/x-www-form-urlencoded;charset=UTF-8
   DPoP: eyJ0eXAiOiJkcG9wK2p3dCIsImFsZyI6IkVTMjU2IiwiandrIjp7Imt0eSI6Ik
    VDIiwieCI6Imw4dEZyaHgtMzR0VjNoUklDUkRZOXpDa0RscEJoRjQyVVFVZldWQVdCR
    nMiLCJ5IjoiOVZFNGpmX09rX282NHpiVFRsY3VOSmFqSG10NnY5VERWclUwQ2R2R1JE
    QSIsImNydiI6IlAtMjU2In19.eyJqdGkiOiItQndDM0VTYzZhY2MybFRjIiwiaHRtIj
    oiUE9TVCIsImh0dSI6Imh0dHBzOi8vc2VydmVyLmV4YW1wbGUuY29tL3Rva2VuIiwia
    WF0IjoxNTYyMjYyNjE2fQ.2-GxA6T8lP4vfrg8v-FdWP0A0zdrj8igiMLvqRMUvwnQg
    4PtFLbdLXiOSsX0x7NVY-FNyJK70nfbV37xRZT3Lg
   grant_type=authorization_code
   &code=SplxlOBeZQQYbYS6WxSbIA
   &redirect_uri=https%3A%2F%2Fclient%2Eexample%2Ecom%2Fcb
   &code_verifier=bEaL42izcC-o-xBk0K2vuJ6U-y1p9r_wW2dFWIWgjz-

        Figure 3: Token Request for a specification.

   Resource servers supporting DPoP sender-constrained token

   The "DPoP" HTTP header MUST contain a valid DPoP proof JWT.  If ensure that the the public key
   from the DPoP proof matches the pubic key to which the access token
   is invalid, bound.

6.1.  JWK Thumbprint Confirmation Method

   When access tokens are represented as JSON Web Tokens (JWT)
   [RFC7519], the authorization server issues an error
   response per Section 5.2 public key information SHOULD be represented using the
   "jkt" confirmation method member defined herein.  To convey the hash
   of [RFC6749] with "invalid_dpop_proof" as a public key in a JWT, this specification introduces the following
   new JWT Confirmation Method [RFC7800] member for use under the "cnf"
   claim.

   "jkt"  JWK SHA-256 Thumbprint Confirmation Method.  The value of the
      "jkt" member MUST be the value base64url encoding (as defined in
      [RFC7515]) of the "error" parameter.

   The authorization server, after checking the validity JWK SHA-256 Thumbprint (according to [RFC7638])
      of the DPoP
   proof, associates public key (in JWK format) to which the access token issued at the token endpoint
      is bound.

   The following example JWT in Figure 7 with decoded JWT payload shown
   in Figure 8 contains a "cnf" claim with the public key.  It then sets "token_type" to "DPoP" "jkt" JWK thumbprint
   confirmation method member.  The "jkt" value in these examples is the token
   response, which signals to
   hash of the client that public key from the access token was bound
   to its DPoP key and can used as described proofs in the examples in
   Section 6.

   If 5.

   eyJhbGciOiJFUzI1NiIsImtpZCI6IkJlQUxrYiJ9.eyJzdWIiOiJzb21lb25lQGV4YW1
   wbGUuY29tIiwiaXNzIjoiaHR0cHM6Ly9zZXJ2ZXIuZXhhbXBsZS5jb20iLCJuYmYiOjE
   1NjIyNjI2MTEsImV4cCI6MTU2MjI2NjIxNiwiY25mIjp7ImprdCI6IjBaY09DT1JaTll
   5LURXcHFxMzBqWnlKR0hUTjBkMkhnbEJWM3VpZ3VBNEkifX0.3Tyo8VTcn6u_PboUmAO
   YUY1kfAavomW_YwYMkmRNizLJoQzWy2fCo79Zi5yObpIzjWb5xW4OGld7ESZrh0fsrA

       Figure 7: JWT containing a refresh JWK SHA-256 Thumbprint Confirmation

   {
     "sub":"someone@example.com",
     "iss":"https://server.example.com",
     "nbf":1562262611,
     "exp":1562266216,
     "cnf":{"jkt":"0ZcOCORZNYy-DWpqq30jZyJGHTN0d2HglBV3uiguA4I"}
   }

    Figure 8: JWT Claims Set with a JWK SHA-256 Thumbprint Confirmation

6.2.  JWK Thumbprint Confirmation Method in Token Introspection

   OAuth 2.0 Token Introspection [RFC7662] defines a method for a
   protected resource to query an authorization server about the active
   state of an access token is issued as well as to determine metainformation
   about the token.

   For a DPoP-bound access token, the hash of the public client at key to which
   the token endpoint
   and a valid DPoP proof is presented, the refresh token MUST be bound is conveyed to the public key contained protected resource as
   metainformation in the header of the DPoP proof JWT.

   When a DPoP-bound refresh token introspection response.  The hash is used at
   conveyed using the token endpoint by same "cnf" content with "jkt" member structure as
   the JWK thumbprint confirmation method, described in Section 6.1, as
   a
   public client, top-level member of the AS MUST ensure introspection response JSON.  Note that the
   resource server does not send a DPoP proof contains with the introspection
   request and the authorization server does not validate an access
   token's DPoP binding at the introspection endpoint.  Rather the
   resource server uses the
   same public key as data of the one introspection response to
   validate the refresh access token is bound to. binding itself locally.

   The example introspection request in Figure 9 and corresponding
   response in Figure 10 illustrate an introspection exchange for the
   example DPoP-bound access token that was issued MUST be bound to the public key contained in the DPoP
   proof.

6.  Resource Access (Proof of Possession Figure 5.

   POST /as/introspect.oauth2 HTTP/1.1
   Host: server.example.com
   Content-Type: application/x-www-form-urlencoded
   Authorization: Basic cnM6cnM6TWt1LTZnX2xDektJZHo0ZnNON2tZY3lhK1Rp

   token=Kz~8mXK1EalYznwH-LC-1fBAo.4Ljp~zsPE_NeO.gxU

                  Figure 9: Example Introspection Request

   HTTP/1.1 200 OK
   Content-Type: application/json
   Cache-Control: no-cache, no-store

   {
     "active": true,
     "sub": "someone@example.com",
     "iss": "https://server.example.com",
     "nbf": 1562262611,
     "exp": 1562266216,
     "cnf": {"jkt": "0ZcOCORZNYy-DWpqq30jZyJGHTN0d2HglBV3uiguA4I"}
   }

     Figure 10: Example Introspection Response for a DPoP-Bound Access
                                   Token

7.  Protected Resource Access Tokens)

   To make use of an access token that is bound to a public key using
   DPoP, a client MUST prove the possession of the corresponding private key
   by providing a DPoP proof in the "DPoP" request header.  As such,
   protected resource requests with a DPoP-bound access token
   necessarily must include both a DPoP proof as per Section 4 and the
   access token as described in Section 7.1.

7.1.  The DPoP Authorization Request Header Scheme

   A DPoP-bound access token is sent using the "Authorization" request
   header field per Section 2 of [RFC7235] using an authentication
   scheme of "DPoP".  The syntax of the "Authorization" header field for
   the "DPoP" scheme uses the "token68" syntax defined in Section 2.1 of
   [RFC7235] (repeated below for ease of reference) for credentials.
   The Augmented Backus-Naur Form (ABNF) notation [RFC5234] syntax for
   DPoP Authorization scheme credentials is as follows:

    token68    = 1*( ALPHA / DIGIT /
                      "-" / "." / "_" / "~" / "+" / "/" ) *"="

    credentials = "DPoP" 1*SP token68

                 Figure 4: 11: DPoP Authorization Scheme ABNF

   For such an access token, a resource server MUST check that a "DPoP"
   header DPoP
   proof was also received in the "DPoP" header field of the HTTP
   request, check the header's contents DPoP proof according to the rules in Section 4.2, 4.3,
   and check that the public key of the DPoP proof matches the public
   key to which the access token is bound per Section 7. 6.

   The resource server MUST NOT grant access to the resource unless all
   checks are successful.

   Figure 12 shows an example request to a protected resource with a
   DPoP-bound access token in the "Authorization" header and the DPoP
   proof in the "DPoP" header (line breaks and extra whitespace for
   display purposes only).

   GET /protectedresource HTTP/1.1
   Host: resource.example.org
   Authorization: DPoP eyJhbGciOiJFUzI1NiIsImtpZCI6IkJlQUxrYiJ9.eyJzdWI
    iOiJzb21lb25lQGV4YW1wbGUuY29tIiwiaXNzIjoiaHR0cHM6Ly9zZXJ2ZXIuZXhhbX
    BsZS5jb20iLCJhdWQiOiJodHRwczovL3Jlc291cmNlLmV4YW1wbGUub3JnIiwibmJmI
    joxNTYyMjYyNjExLCJleHAiOjE1NjIyNjYyMTYsImNuZiI6eyJqa3QiOiIwWmNPQ09S
    Wk5ZeS1EV3BxcTMwalp5SkdIVE4wZDJIZ2xCVjN1aWd1QTRJIn19.vsFiVqHCyIkBYu
    50c69bmPJsj8qYlsXfuC6nZcLl8YYRNOhqMuRXu6oSZHe2dGZY0ODNaGg1cg-kVigzY
    hF1MQ Kz~8mXK1EalYznwH-LC-1fBAo.4Ljp~zsPE_NeO.gxU
   DPoP: eyJ0eXAiOiJkcG9wK2p3dCIsImFsZyI6IkVTMjU2IiwiandrIjp7Imt0eSI6Ik
    VDIiwieCI6Imw4dEZyaHgtMzR0VjNoUklDUkRZOXpDa0RscEJoRjQyVVFVZldWQVdCR
    nMiLCJ5IjoiOVZFNGpmX09rX282NHpiVFRsY3VOSmFqSG10NnY5VERWclUwQ2R2R1JE
    QSIsImNydiI6IlAtMjU2In19.eyJqdGkiOiJlMWozVl9iS2ljOC1MQUVCIiwiaHRtIj
    oiR0VUIiwiaHR1IjoiaHR0cHM6Ly9yZXNvdXJjZS5leGFtcGxlLm9yZy9wcm90ZWN0Z
    WRyZXNvdXJjZSIsImlhdCI6MTU2MjI2MjYxOH0.lNhmpAX1WwmpBvwhok4E74kWCiGB
    NdavjLAeevGy32H3dbF0Jbri69Nm2ukkwb-uyUI4AUg1JSskfWIyo4UCbQ

                 Figure 5: 12: DPoP Protected Resource Request with a DPoP sender-
                          constrained access token

   Upon receipt of a request for a URI of a protected resource within
   the protection space requiring DPoP authorization, if the request
   does not include valid credentials or or does not contain an access
   token sufficient for access to the protected resource, the server can
   reply with a challenge using the 401 (Unauthorized) status code
   ([RFC7235], Section 3.1) and the "WWW-Authenticate" header field
   ([RFC7235], Section 4.1).  The server MAY include the "WWW-
   Authenticate" header in response to other conditions as well.

   In such challenges:

   *  The scheme name is "DPoP".

   *  The authentication parameter "realm" MAY be included to indicate
      the scope of protection in the manner described in [RFC7235],
      Section 2.2.

   *  A "scope" authentication parameter MAY be included as defined in
      [RFC6750], Section 3.

   *  An "error" parameter ([RFC6750], Section 3) SHOULD be included to
      indicate the reason why the request was declined, if the request
      included an access token but failed authorization.  Parameter
      values are described in Section 3.1 of [RFC6750].

   *  An "error_description" parameter ([RFC6750], Section 3) MAY be
      included along with the "error" parameter to provide developers a
      human-readable explanation that is not meant to be displayed to
      end-users.

   *  An "algs" parameter SHOULD be included to signal to the client the
      JWS algorithms that are acceptable for the DPoP proof JWT.  The
      value of the parameter is a space-delimited list of JWS "alg"
      (Algorithm) header values ([RFC7515], Section 4.1.1).

   *  Additional authentication parameters MAY be used and unknown
      parameters MUST be ignored by recipients

   For example, in response to a protected resource request without
   authentication:

    HTTP/1.1 401 Unauthorized
    WWW-Authenticate: DPoP realm="WallyWorld", algs="ES256 PS256"

    Figure 6 13: HTTP 401 Response To A Protected Resource Request Without
                               Authentication

   And in response to a protected resource request that was rejected
   because the confirmation of the DPoP binding in the access token
   failed:

    HTTP/1.1 401 Unauthorized
    WWW-Authenticate: DPoP realm="WallyWorld", error="invalid_token",
      error_description="Invalid DPoP key binding", algs="ES256"

                                  Figure 7

7.  Public Key Confirmation

   It MUST be ensured that resource servers can reliably identify
   whether a token is bound using DPoP and learn the public key to which
   the token is bound.

   Access tokens that are represented as JSON Web Tokens (JWT) [RFC7519]
   MUST contain information about the DPoP public key (in JWK format) in
   the member "jkt" of the "cnf" claim, as shown DPoP binding in the access token
   failed:

    HTTP/1.1 401 Unauthorized
    WWW-Authenticate: DPoP realm="WallyWorld", error="invalid_token",
      error_description="Invalid DPoP key binding", algs="ES256"

     Figure 8. 14: HTTP 401 Response To A Protected Resource Request With
                              An Invalid Token

7.2.  The value in "jkt" MUST be Bearer Authorization Request Header Scheme

   Protected resources simultaneously supporting both the base64url encoding [RFC7515] "DPoP" and
   "Bearer" schemes need to update how evaluation of the
   JWK SHA-256 Thumbprint (according bearer tokens is
   performed to [RFC7638]) prevent downgraded usage of the public key to
   which the a DPoP-bound access token is bound.

   {
     "sub":"someone@example.com",
     "iss":"https://server.example.com",
     "aud":"https://resource.example.org",
     "nbf":1562262611,
     "exp":1562266216,
     "cnf":{
         "jkt":"0ZcOCORZNYy-DWpqq30jZyJGHTN0d2HglBV3uiguA4I"
     }
   }

            Figure 8: Example tokens.
   Specifically, such a protected resource MUST reject an access token body with "cnf" claim

   When access
   received as a bearer token per [!@RFC6750], if that token introspection is used, the same "cnf" claim as
   above MUST
   determined to be contained in the introspection response.

   Resource servers MUST ensure DPoP-bound.

   A protected resource that the fingerprint supports only [RFC6750] and is unaware of the public key
   in the
   DPoP proof JWT equals the value in would most presumably accept a DPoP-bound access token as a
   bearer token (JWT [RFC7519] says to ignore unrecognized claims,
   Introspection [RFC7662] says that other parameters might be present
   while placing no functional requirements on their presence, and
   [RFC6750] is effectively silent on the "jkt" claim in content of the access token or introspection response.

8.  Authorization Server Metadata

   This document introduces the following new authorization server
   metadata [RFC8414] parameter as
   it relates to signal the JWS "alg" values validity).  As such, a client MAY send a DPoP-bound
   access token using the
   authorization server supports for DPoP proof JWTs:

   "dpop_signing_alg_values_supported"  OPTIONAL.  JSON array containing "Bearer" scheme upon receipt of a list "WWW-
   Authenticate: Bearer" challenge from a protected resource (or if it
   has prior such knowledge about the capabilities of the JWS "alg" values supported by protected
   resource).  The effect of this likely simplifies the authorization
      server for logistics of
   phased upgrades to protected resources in their support DPoP proof JWTs

9. or even
   prolonged deployments of protected resources with mixed token type
   support.

8.  Security Considerations

   In DPoP, the prevention of token replay at a different endpoint (see
   Section 2) is achieved through the binding of the DPoP proof to a
   certain URI and HTTP method.  DPoP does not,  DPoP, however, achieve the
   same level has a somewhat different
   nature of protection as than TLS-based methods such as OAuth Mutual TLS
   [RFC8705] or OAuth Token Binding [I-D.ietf-oauth-token-binding] (see
   also Section 9.1 8.1 and Section 9.4). 8.4).  TLS-based mechanisms can leverage
   a tight integration between the TLS layer and the application layer
   to achieve a very high level of message integrity
   and replay protection.  Therefore, it is RECOMMENDED with respect to prefer TLS-
   based methods over DPoP if such methods are suitable for the scenario
   at hand.

9.1.
   transport layer to which the token is bound and replay protection in
   general.

8.1.  DPoP Proof Replay

   If an adversary is able to get hold of a DPoP proof JWT, the
   adversary could replay that token at the same endpoint (the HTTP
   endpoint and method are enforced via the respective claims in the
   JWTs).  To prevent this, servers MUST only accept DPoP proofs for a
   limited time window after their "iat" time, preferably only for a
   relatively brief period.  Servers SHOULD store store, in the context of the
   request URI, the "jti" value of each DPoP proof for the time window
   in which the respective DPoP proof JWT would be accepted and decline
   HTTP requests to the same URI for which the "jti" value has been seen
   before.  In order to guard against memory exhaustion attacks a server
   SHOULD reject DPoP proof JWTs with unnecessarily large "jti" values
   or store only a hash thereof.

   Note: To accommodate for clock offsets, the server MAY accept DPoP
   proofs that carry an "iat" time in the reasonably near future (e.g., up to
   a few seconds in the future).

9.2.

8.2.  Signed JWT Swapping

   Servers accepting signed DPoP proof JWTs MUST check the "typ" field
   in the headers of the JWTs to ensure that adversaries cannot use JWTs
   created for other purposes.

9.3.

8.3.  Signature Algorithms

   Implementers MUST ensure that only asymmetric digital signature
   algorithms that are deemed secure can be used for signing DPoP
   proofs.  In particular, the algorithm "none" MUST NOT be allowed.

9.4.

8.4.  Message Integrity

   DPoP does not ensure the integrity of the payload or headers of
   requests.  The signature of DPoP proofs proof only contains claims for the HTTP URI and
   method, but not, for example, the message body or other general request
   headers.

   This is an intentional design decision intended to keep DPoP simple
   to use, but as described, makes DPoP potentially susceptible to
   replay attacks where an attacker is able to modify message contents
   and headers.  In many setups, the message integrity and
   confidentiality provided by TLS is sufficient to provide a good level
   of protection.

   Implementers that have stronger requirements on the integrity of
   messages are encouraged to either use TLS-based mechanisms or signed
   requests.  TLS-based mechanisms are in particular OAuth Mutual TLS
   [RFC8705] and OAuth Token Binding [I-D.ietf-oauth-token-binding].

   Note: While signatures on (parts of) covering other parts of requests are out of
   the scope of this specification, signatures or additional information to be signed
   can be added into DPoP proofs.

10.

8.5.  Public Key Binding

   The binding between the DPoP public key and the access token, which
   is specified in Section 6, uses a cryptographic hash of the JWK
   representation of the public key.  It relies on the hash function
   having sufficient second-preimage resistance so as to make it
   computationally infeasible to find or create another key that
   produces to the same hash output value.  The SHA-256 hash function
   was used because it meets the aforementioned requirement while being
   widely available.  If, in the future, JWK thumbprints need to be
   computed using hash function(s) other than SHA-256, it is suggested
   that, for additional related JWT confirmation methods, members be
   defined for that purpose and registered in the IANA "JWT Confirmation
   Methods" registry [IANA.JWT.Claims] for JWT "cnf" member values.

9.  IANA Considerations

10.1.

9.1.  OAuth Access Token Type Registration

   This specification requests registration of the following access
   token type in the "OAuth Access Token Types" registry
   [IANA.OAuth.Params] established by [RFC6749].

   *  Type name: "DPoP"
   *  Additional Token Endpoint Response Parameters: (none)

   *  HTTP Authentication Scheme(s): "DPoP"

   *  Change controller: IESG

   *  Specification document(s): [[ this specification ]]

10.2.

9.2.  HTTP Authentication Scheme Registration

   This specification requests registration of the following scheme in
   the "Hypertext Transfer Protocol (HTTP) Authentication Scheme
   Registry" [RFC7235][IANA.HTTP.AuthSchemes]:

   *  Authentication Scheme Name: "DPoP"

   *  Reference: [[ Section 6 7.1 of this specification ]]

10.3.

9.3.  Media Type Registration

   [[ Is a media type registration at [IANA.MediaTypes] necessary for
   "application/dpop+jwt"?  There is a "+jwt" structured syntax suffix
   registered already at [IANA.MediaType.StructuredSuffixs] [IANA.MediaType.StructuredSuffix] by
   Section 7.2 of [RFC8417], which is maybe sufficient?  A fullblown full-blown
   registration of "application/dpop+jwt" seems like it'd be overkill.
   The "dpop+jwt" is used in the JWS/JWT "typ" header for explicit
   typing of the JWT per Section 3.11 of [RFC8725] but it is not used
   anywhere else (such as the "Content-Type" of HTTP messages).

   Note that there does seem to be some precedence for [IANA.MediaTypes]
   registration with [I-D.ietf-oauth-access-token-jwt],
   [I-D.ietf-oauth-jwsreq], [RFC8417], and of course [RFC7519].  But
   precedence isn't always right. ]]

10.4.

9.4.  JWT Confirmation Methods Registration

   This specification requests registration of the following value in
   the IANA "JWT Confirmation Methods" registry [IANA.JWT] for JWT "cnf"
   member values established by [RFC7800].

   *  Confirmation Method Value: "jkt"

   *  Confirmation Method Description: JWK SHA-256 Thumbprint

   *  Change Controller: IESG

   *  Specification Document(s): [[ Section 7 6 of this specification ]]

10.5.

9.5.  JSON Web Token Claims Registration

   This specification requests registration of the following Claims in
   the IANA "JSON Web Token Claims" registry [IANA.JWT] established by
   [RFC7519].

   HTTP method:

   *  Claim Name: "htm"

   *  Claim Description: The HTTP method of the request

   *  Change Controller: IESG

   *  Specification Document(s): [[ Section 4.1 4.2 of this specification ]]

   HTTP URI:

   *  Claim Name: "htu"

   *  Claim Description: The HTTP URI of the request (without query and
      fragment parts)

   *  Change Controller: IESG

   *  Specification Document(s): [[ Section 4.1 4.2 of this specification ]]

10.6.

9.6.  HTTP Message Header Field Names Registration

   This document specifies the following new HTTP header fields,
   registration of which is requested in the "Permanent Message Header
   Field Names" registry [IANA.Headers] defined in [RFC3864].

   *  Header Field Name: "DPoP"

   *  Applicable protocol: HTTP

   *  Status: standard

   *  Author/change Controller: IETF

   *  Specification Document(s): [[ this specification ]]

10.7.

9.7.  Authorization Server Metadata Registration

   This specification requests registration of the following values in
   the IANA "OAuth Authorization Server Metadata" registry
   [IANA.OAuth.Parameters] established by [RFC8414].

   *  Metadata Name: "dpop_signing_alg_values_supported"

   *  Metadata Description: JSON array containing a list of the JWS
      algorithms supported for DPoP proof JWTs

   *  Change Controller: IESG

   *  Specification Document(s): [[ Section 8 5.1 of this specification ]]

11.

10.  Normative References

   [RFC7800]  Jones, M., Bradley, J., and H. Tschofenig, "Proof-of-
              Possession Key Semantics for JSON Web Tokens (JWTs)",
              RFC 7800, DOI 10.17487/RFC7800, April 2016,
              <https://www.rfc-editor.org/info/rfc7800>.

   [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>.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <https://www.rfc-editor.org/info/rfc5234>.

   [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>.

   [RFC6750]

   [RFC7515]  Jones, M. M., Bradley, J., and D. Hardt, "The OAuth 2.0 Authorization
              Framework: Bearer Token Usage", N. Sakimura, "JSON Web
              Signature (JWS)", RFC 6750, 7515, DOI 10.17487/RFC6750, October 2012,
              <https://www.rfc-editor.org/info/rfc6750>. 10.17487/RFC7515, May
              2015, <https://www.rfc-editor.org/info/rfc7515>.

   [RFC7518]  Jones, M., "JSON Web Algorithms (JWA)", RFC 7518,
              DOI 10.17487/RFC7518, May 2015,
              <https://www.rfc-editor.org/info/rfc7518>.

   [RFC7638]  Jones, M. and N. Sakimura, "JSON Web Key (JWK)
              Thumbprint", RFC 7638, DOI 10.17487/RFC7638, September
              2015, <https://www.rfc-editor.org/info/rfc7638>.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <https://www.rfc-editor.org/info/rfc5234>.

   [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>.

   [RFC7231]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
              DOI 10.17487/RFC7231, June 2014,
              <https://www.rfc-editor.org/info/rfc7231>.

   [RFC7515]

   [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>.

   [RFC7800]  Jones, M., Bradley, J., and N. Sakimura, "JSON H. Tschofenig, "Proof-of-
              Possession Key Semantics for JSON Web
              Signature (JWS)", Tokens (JWTs)",
              RFC 7515, 7800, DOI 10.17487/RFC7515, May
              2015, <https://www.rfc-editor.org/info/rfc7515>.

12. 10.17487/RFC7800, April 2016,
              <https://www.rfc-editor.org/info/rfc7800>.

11.  Informative References

   [I-D.ietf-oauth-token-binding]
              Jones, M., Campbell, B.,

   [I-D.ietf-oauth-security-topics]
              Lodderstedt, T., Bradley, J., Labunets, A., and W. Denniss, D. Fett,
              "OAuth 2.0 Token Binding", Security Best Current Practice", Work in
              Progress, Internet-
              Draft, draft-ietf-oauth-token-binding-08, 19 Internet-Draft, draft-ietf-oauth-security-
              topics-16, 5 October 2020, <https://tools.ietf.org/html/
              draft-ietf-oauth-security-topics-16>.

   [RFC7662]  Richer, J., Ed., "OAuth 2.0 Token Introspection",
              RFC 7662, DOI 10.17487/RFC7662, October 2015,
              <https://www.rfc-editor.org/info/rfc7662>.

   [IANA.OAuth.Params]
              IANA, "OAuth Parameters",
              <https://www.iana.org/assignments/oauth-parameters>.

   [RFC8725]  Sheffer, Y., Hardt, D., and M. Jones, "JSON Web Token Best
              Current Practices", BCP 225, RFC 8725,
              DOI 10.17487/RFC8725, February 2020,
              <https://www.rfc-editor.org/info/rfc8725>.

   [RFC3864]  Klyne, G., Nottingham, M., and J. Mogul, "Registration
              Procedures for Message Header Fields", BCP 90, RFC 3864,
              DOI 10.17487/RFC3864, September 2004,
              <https://www.rfc-editor.org/info/rfc3864>.

   [RFC8414]  Jones, M., Sakimura, N., and J. Bradley, "OAuth 2.0
              Authorization Server Metadata", RFC 8414,
              DOI 10.17487/RFC8414, June 2018,
              <https://tools.ietf.org/html/draft-ietf-oauth-token-
              binding-08>.
              <https://www.rfc-editor.org/info/rfc8414>.

   [I-D.ietf-oauth-access-token-jwt]
              Bertocci, V., "JSON Web Token (JWT) Profile for OAuth 2.0
              Access Tokens", Work in Progress, Internet-Draft, draft-
              ietf-oauth-access-token-jwt-07, 27 April 2020,
              <https://tools.ietf.org/html/draft-ietf-oauth-access-
              token-jwt-07>.

   [I-D.ietf-oauth-jwsreq]
              Sakimura, N. and J. Bradley, "The OAuth 2.0 Authorization
              Framework: JWT Secured Authorization Request (JAR)", Work
              in Progress, Internet-Draft, draft-ietf-oauth-jwsreq-21,
              19 April draft-
              ietf-oauth-access-token-jwt-10, 23 September 2020,
              <https://tools.ietf.org/html/draft-ietf-oauth-jwsreq-21>.

   [RFC8705]  Campbell, B.,
              <https://tools.ietf.org/html/draft-ietf-oauth-access-
              token-jwt-10>.

   [IANA.Headers]
              IANA, "Message Headers",
              <https://www.iana.org/assignments/message-headers>.

   [RFC7519]  Jones, M., Bradley, J., Sakimura, N., and T.
              Lodderstedt, "OAuth 2.0 Mutual-TLS Client Authentication
              and Certificate-Bound Access Tokens", N. Sakimura, "JSON Web Token
              (JWT)", RFC 8705, 7519, DOI 10.17487/RFC8705, February 2020,
              <https://www.rfc-editor.org/info/rfc8705>. 10.17487/RFC7519, May 2015,
              <https://www.rfc-editor.org/info/rfc7519>.

   [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>.

   [IANA.MediaTypes]
              IANA, "Media Types",
              <https://www.iana.org/assignments/media-types>.

   [RFC8414]  Jones,

   [W3C.WebCryptoAPI]
              Watson, M., Sakimura, N., "Web Cryptography API", 26 January 2017,
              <https://www.w3.org/TR/2017/REC-WebCryptoAPI-20170126>.

   [RFC7230]  Fielding, R., Ed. and J. Bradley, "OAuth 2.0
              Authorization Server Metadata", Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing",
              RFC 8414, 7230, DOI 10.17487/RFC8414, 10.17487/RFC7230, June 2018,
              <https://www.rfc-editor.org/info/rfc8414>.

   [RFC8417]  Hunt, 2014,
              <https://www.rfc-editor.org/info/rfc7230>.

   [RFC4122]  Leach, P., Ed., Jones, Mealling, M., Denniss, W., and M. Ansari,
              "Security Event Token (SET)", R. Salz, "A Universally
              Unique IDentifier (UUID) URN Namespace", RFC 8417, 4122,
              DOI 10.17487/RFC8417, 10.17487/RFC4122, July 2018,
              <https://www.rfc-editor.org/info/rfc8417>.

   [IANA.JWT] IANA, "JSON Web Token Claims",
              <http://www.iana.org/assignments/jwt>.

   [RFC3864]  Klyne, G., Nottingham, M., 2005,
              <https://www.rfc-editor.org/info/rfc4122>.

   [RFC8705]  Campbell, B., Bradley, J., Sakimura, N., and J. Mogul, "Registration
              Procedures for Message Header Fields", BCP 90, T.
              Lodderstedt, "OAuth 2.0 Mutual-TLS Client Authentication
              and Certificate-Bound Access Tokens", RFC 3864, 8705,
              DOI 10.17487/RFC3864, September 2004,
              <https://www.rfc-editor.org/info/rfc3864>.

   [RFC2119]  Bradner, S., "Key words 10.17487/RFC8705, February 2020,
              <https://www.rfc-editor.org/info/rfc8705>.

   [RFC8707]  Campbell, B., Bradley, J., and H. Tschofenig, "Resource
              Indicators for use in RFCs to Indicate
              Requirement Levels", BCP 14, OAuth 2.0", RFC 2119, 8707, DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC7519] 10.17487/RFC8707,
              February 2020, <https://www.rfc-editor.org/info/rfc8707>.

   [RFC7523]  Jones, M., Bradley, J., Campbell, B., and N. Sakimura, C. Mortimore, "JSON Web Token
              (JWT)",
              (JWT) Profile for OAuth 2.0 Client Authentication and
              Authorization Grants", RFC 7519, 7523, DOI 10.17487/RFC7519, 10.17487/RFC7523, May
              2015,
              <https://www.rfc-editor.org/info/rfc7519>.

   [IANA.OAuth.Params]
              IANA, "OAuth Parameters",
              <https://www.iana.org/assignments/oauth-parameters>.

   [IANA.MediaType.StructuredSuffixs]
              IANA, "Structured Syntax Suffix Registry",
              <https://www.iana.org/assignments/media-type-structured-
              suffixs>.

   [IANA.Headers]
              IANA, "Message Headers",
              <https://www.iana.org/assignments/message-headers>. <https://www.rfc-editor.org/info/rfc7523>.

   [RFC7235]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Authentication", RFC 7235,
              DOI 10.17487/RFC7235, June 2014,
              <https://www.rfc-editor.org/info/rfc7235>.

   [IANA.JWT] IANA, "JSON Web Token Claims",
              <http://www.iana.org/assignments/jwt>.

   [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>.

   [IANA.HTTP.AuthSchemes]
              IANA, "Hypertext Transfer Protocol (HTTP) Authentication
              Scheme Registry",
              <https://www.iana.org/assignments/http-authschemes>.

   [RFC8725]  Sheffer, Y., Hardt, D.,
              <https://www.iana.org/assignments/http-authschemes>.

   [IANA.MediaType.StructuredSuffix]
              IANA, "Structured Syntax Suffix Registry",
              <https://www.iana.org/assignments/media-type-structured-
              suffix>.

   [RFC8417]  Hunt, P., Ed., Jones, M., Denniss, W., and M. Jones, "JSON Web Ansari,
              "Security Event Token Best
              Current Practices", BCP 225, (SET)", RFC 8725, 8417,
              DOI 10.17487/RFC8725, February 2020,
              <https://www.rfc-editor.org/info/rfc8725>.

   [I-D.ietf-oauth-security-topics]
              Lodderstedt, T., 10.17487/RFC8417, July 2018,
              <https://www.rfc-editor.org/info/rfc8417>.

   [I-D.ietf-oauth-token-binding]
              Jones, M., Campbell, B., Bradley, J., Labunets, A., and D. Fett, W. Denniss,
              "OAuth 2.0 Security Best Current Practice", Token Binding", Work in Progress, Internet-Draft, draft-ietf-oauth-security-
              topics-15, 5 April 2020, <https://tools.ietf.org/html/
              draft-ietf-oauth-security-topics-15>.

   [RFC4122]  Leach, P., Mealling, M., Internet-
              Draft, draft-ietf-oauth-token-binding-08, 19 October 2018,
              <https://tools.ietf.org/html/draft-ietf-oauth-token-
              binding-08>.

   [RFC6750]  Jones, M. and R. Salz, "A Universally
              Unique IDentifier (UUID) URN Namespace", D. Hardt, "The OAuth 2.0 Authorization
              Framework: Bearer Token Usage", RFC 4122, 6750,
              DOI 10.17487/RFC4122, July 2005,
              <https://www.rfc-editor.org/info/rfc4122>. 10.17487/RFC6750, October 2012,
              <https://www.rfc-editor.org/info/rfc6750>.

   [IANA.MediaTypes]
              IANA, "Media Types",
              <https://www.iana.org/assignments/media-types>.

   [I-D.ietf-oauth-jwsreq]
              Sakimura, N., Bradley, J., and M. Jones, "The OAuth 2.0
              Authorization Framework: JWT Secured Authorization Request
              (JAR)", Work in Progress, Internet-Draft, draft-ietf-
              oauth-jwsreq-30, 10 September 2020,
              <https://tools.ietf.org/html/draft-ietf-oauth-jwsreq-30>.

Appendix A.  Acknowledgements

   We would like to thank Filip Skokan, Mike Engan, Justin Richer,
   Michael Peck, Vladimir Dzhuvinov, Rob Otto, Dominick Baier, Jim
   Willeke, Annabelle Backman, Dominick Baier, William
   Denniss, Vladimir Dzhuvinov, Mike Engan, Nikos Fotiou, Mark Haine,
   Dick Hardt, Bjorn Hjelm, Jared Jennings, Steinar Noem, Aaron Parecki, Neil Madden,
   Rob Otto, Aaron Parecki, Michael Peck, Paul Querna, Dick Hardt, Justin Richer,
   Filip Skokan, Dave Tonge, Jared Jennings,
   Mark Haine Jim Willeke, and others (please let us
   know, if you've been mistakenly omitted) for their valuable input,
   feedback and general support of this work.

   This document resulted from discussions at the 4th OAuth Security
   Workshop in Stuttgart, Germany.  We thank the organizers of this
   workshop (Ralf Kusters, Guido Schmitz).

Appendix B.  Document History

   [[ To be removed from the final specification ]]

   -02

   *  Lots of editorial updates and additions including expanding on the
      objectives, better defining the key confirmation representations,
      example updates and additions, better describing mixed bearer/dpop
      token type deployments, clarify RT binding only being done for
      public clients and why, more clearly allow for a bound RT but with
      bearer AT, explain/justify the choice of SHA-256 for key binding,
      and more

   *  Require that a protected resource supporting bearer and DPoP at
      the same time must reject an access token received as bearer, if
      that token is DPoP-bound

   *  Remove the case-insensitive qualification on the "htm" claim check

   *  Relax the jti tracking requirements a bit and qualify it by URI

   -01

   *  Editorial updates

   *  Attempt to more formally define the DPoP Authorization header
      scheme

   *  Define the 401/WWW-Authenticate challenge

   *  Added "invalid_dpop_proof" error code for DPoP errors in token
      request

   *  Fixed up and added to the IANA section

   *  Added "dpop_signing_alg_values_supported" authorization server
      metadata

   *  Moved the Acknowledgements into an Appendix and added a bunch of
      names (best effort)

   -00 [[ Working Group Draft ]]

   *  Working group draft

   -04
   *  Update OAuth MTLS reference to RFC 8705

   *  Use the newish RFC v3 XML and HTML format

   -03

   *  rework the text around uniqueness requirements on the jti claim in
      the DPoP proof JWT

   *  make tokens a bit smaller by using "htm", "htu", and "jkt" rather
      than "http_method", "http_uri", and "jkt#S256" respectively

   *  more explicit recommendation to use mTLS if that is available

   *  added David Waite as co-author

   *  editorial updates

   -02

   *  added normalization rules for URIs

   *  removed distinction between proof and binding

   *  "jwk" header again used instead of "cnf" claim in DPoP proof

   *  renamed "Bearer-DPoP" token type to "DPoP"

   *  removed ability for key rotation

   *  added security considerations on request integrity

   *  explicit advice on extending DPoP proofs to sign other parts of
      the HTTP messages

   *  only use the jkt#S256 in ATs

   *  iat instead of exp in DPoP proof JWTs

   *  updated guidance on token_type evaluation

   -01

   *  fixed inconsistencies

   *  moved binding and proof messages to headers instead of parameters

   *  extracted and unified definition of DPoP JWTs
   *  improved description

   -00

   *  first draft

Authors' Addresses

   Daniel Fett
   yes.com

   Email: mail@danielfett.de

   Brian Campbell
   Ping Identity

   Email: bcampbell@pingidentity.com

   John Bradley
   Yubico

   Email: ve7jtb@ve7jtb.com

   Torsten Lodderstedt
   yes.com

   Email: torsten@lodderstedt.net

   Michael Jones
   Microsoft

   Email: mbj@microsoft.com
   URI:   https://self-issued.info/

   David Waite
   Ping Identity

   Email: david@alkaline-solutions.com