Web Authorization Protocol                                       D. Fett
Internet-Draft                                                   yes.com
Intended status: Standards Track                             B. Campbell
Expires: 7 April 2022                                      Ping Identity
                                                              J. Bradley
                                                                  Yubico
                                                          T. Lodderstedt
                                                                 yes.com
                                                                M. Jones
                                                               Microsoft
                                                                D. Waite
                                                           Ping Identity
                                                          4 October 2021


  OAuth 2.0 Demonstrating Proof-of-Possession at the Application Layer
                                 (DPoP)
                        draft-ietf-oauth-dpop-04

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 7 April 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  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Conventions and Terminology . . . . . . . . . . . . . . .   4
   2.  Objectives  . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Concept . . . . . . . . . . . . . . . . . . . . . . . . . . .   5
   4.  DPoP Proof JWTs . . . . . . . . . . . . . . . . . . . . . . .   7
     4.1.  The DPoP HTTP Header  . . . . . . . . . . . . . . . . . .   7
     4.2.  DPoP Proof JWT Syntax . . . . . . . . . . . . . . . . . .   8
     4.3.  Checking DPoP Proofs  . . . . . . . . . . . . . . . . . .  10
   5.  DPoP Access Token Request . . . . . . . . . . . . . . . . . .  11
     5.1.  Authorization Server Metadata . . . . . . . . . . . . . .  13
   6.  Public Key Confirmation . . . . . . . . . . . . . . . . . . .  14
     6.1.  JWK Thumbprint Confirmation Method  . . . . . . . . . . .  14
     6.2.  JWK Thumbprint Confirmation Method in Token
           Introspection . . . . . . . . . . . . . . . . . . . . . .  15
   7.  Protected Resource Access . . . . . . . . . . . . . . . . . .  16
     7.1.  The DPoP Authentication Scheme  . . . . . . . . . . . . .  16
     7.2.  Compatibility with the Bearer Authentication Scheme . . .  19
   8.  Authorization Server-Provided Nonce . . . . . . . . . . . . .  20
     8.1.  Providing a New Nonce Value . . . . . . . . . . . . . . .  22
   9.  Resource Server-Provided Nonce  . . . . . . . . . . . . . . .  22
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  23
     10.1.  DPoP Proof Replay  . . . . . . . . . . . . . . . . . . .  23
     10.2.  DPoP Proof Pre-Generation  . . . . . . . . . . . . . . .  24
     10.3.  DPoP Nonce Downgrade . . . . . . . . . . . . . . . . . .  24
     10.4.  Untrusted Code in the Client Context . . . . . . . . . .  24
     10.5.  Signed JWT Swapping  . . . . . . . . . . . . . . . . . .  25
     10.6.  Signature Algorithms . . . . . . . . . . . . . . . . . .  25
     10.7.  Message Integrity  . . . . . . . . . . . . . . . . . . .  25
     10.8.  Access Token and Public Key Binding  . . . . . . . . . .  26
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  26
     11.1.  OAuth Access Token Type Registration . . . . . . . . . .  26
     11.2.  OAuth Extensions Error Registration  . . . . . . . . . .  27
     11.3.  HTTP Authentication Scheme Registration  . . . . . . . .  27
     11.4.  Media Type Registration  . . . . . . . . . . . . . . . .  27
     11.5.  JWT Confirmation Methods Registration  . . . . . . . . .  28
     11.6.  JSON Web Token Claims Registration . . . . . . . . . . .  28
     11.7.  HTTP Message Header Field Names Registration . . . . . .  29



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     11.8.  Authorization Server Metadata Registration . . . . . . .  29
   12. Normative References  . . . . . . . . . . . . . . . . . . . .  29
   13. Informative References  . . . . . . . . . . . . . . . . . . .  30
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .  33
   Appendix B.  Document History . . . . . . . . . . . . . . . . . .  34
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  36

1.  Introduction

   DPoP, an abbreviation for Demonstrating Proof-of-Possession at the
   Application Layer, is an application-level mechanism for sender-
   constraining OAuth access and refresh tokens.  It enables a client to
   prove the possession of a public/private key pair by including a DPoP
   header in an HTTP request.  The value of the header is a JWT
   [RFC7519] that enables the authorization server to bind issued tokens
   to the public part of a 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 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 are another example well
   positioned to benefit from DPoP-bound tokens to guard against misuse
   of tokens by a compromised or malicious resource.  Such applications
   often have dedicated protected storage for cryptographic keys.

   DPoP can be used to sender-constrain access tokens regardless of the
   client authentication method employed, but DPoP itself is not used
   for client authentication.  DPoP can also be used to sender-constrain
   refresh tokens issued to public clients (those without authentication
   credentials associated with the client_id).










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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.  Objectives

   The primary aim of DPoP is to prevent unauthorized or illegitimate
   parties from using leaked or stolen access tokens by binding a token
   to a public key upon issuance and requiring that the client proves
   possession of the corresponding private key when using the token.
   This constrains the legitimate sender of the token to only the party
   with access to the private key and gives the server receiving the
   token added assurances that the sender is legitimately authorized to
   use it.

   Access tokens that are sender-constrained via DPoP thus stand in
   contrast to the typical bearer token, which can be used by any party
   in possession of such a token.  Although protections generally exist
   to prevent unintended disclosure of bearer tokens, unforeseen vectors
   for leakage have occurred due to vulnerabilities and implementation
   issues in other layers in the protocol or software stack (CRIME,
   BREACH, Heartbleed, and the Cloudflare parser bug are some examples).
   There have also been numerous published token theft attacks on OAuth
   implementations themselves.  DPoP provides a general defense in depth
   against the impact of unanticipated token leakage.  DPoP is not,
   however, a substitute for a secure transport and MUST always be used
   in conjunction with HTTPS.

   The very nature of the typical OAuth protocol interaction
   necessitates that the client discloses the access token to the
   protected resources that it accesses.  The attacker model in
   [I-D.ietf-oauth-security-topics] describes cases where a protected
   resource might be counterfeit, malicious or compromised and plays
   received tokens against other protected resources to gain
   unauthorized access.  Properly audience restricting access tokens can
   prevent such misuse, however, doing so in practice has proven to be
   prohibitively cumbersome for many deployments (even despite
   extensions such as [RFC8707]).  Sender-constraining access tokens is



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   a more robust and straightforward mechanism to prevent such token
   replay at a different endpoint and DPoP is an accessible application
   layer means of doing so.

   Due to the potential for cross-site scripting (XSS), browser-based
   OAuth clients bring to bear added considerations with respect to
   protecting tokens.  The most straightforward XSS-based attack is for
   an attacker to exfiltrate a token and use it themselves completely
   independent of the legitimate client.  A stolen access token is used
   for protected resource access and a stolen refresh token for
   obtaining new access tokens.  If the private key is non-extractable
   (as is possible with [W3C.WebCryptoAPI]), DPoP renders exfiltrated
   tokens alone unusable.

   XXS vulnerabilities also allow an attacker to execute code in the
   context of the browser-based client application and maliciously use a
   token indirectly through the client.  That execution context has
   access to utilize the signing key and thus can produce DPoP proofs to
   use in conjunction with the 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 scope for
   DPoP.

   Malicious XSS code executed in the context of the browser-based
   client application is also in a position to create DPoP proofs with
   timestamp values in the future and exfiltrate them in conjunction
   with a token.  These stolen artifacts can later be used together
   independent of the client application to access protected resources.
   To prevent this, servers can optionally require clients to include a
   server-chosen value into the proof that cannot be predicted by an
   attacker (nonce).  In the absence of the optional nonce, the impact
   of precomputed DPoP proofs is limited somewhat by the proof being
   bound to an access token on protected resource access.  Because a
   proof covering an access token that does not yet exist cannot
   feasibly be created, access tokens obtained with an exfiltrated
   refresh token and pre-computed proofs will be unusable.

   Additional security considerations are discussed in Section 10.

3.  Concept

   The main data structure introduced by this specification is a DPoP
   proof JWT, described in detail below, which is sent as a header in an
   HTTP request.  A client uses a DPoP proof JWT to prove the possession
   of a private key corresponding to a certain public key.






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   Roughly speaking, a DPoP proof is a signature over some data of the
   HTTP request to which it is attached, a timestamp, a unique
   identifier, an optional server-provided nonce, and a hash of the
   associated access token when an access token is present within the
   request.

   +--------+                                          +---------------+
   |        |--(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 (without the optional
   nonce) 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 a DPoP proof for that request.  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



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      matches the public key of the DPoP proof.  It also verifies that
      the access token hash in the DPoP proof matches the access token
      presented in the request.

   *  (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 10 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.  Each HTTP request requires a unique DPoP proof.

   A valid DPoP proof demonstrates to the server that the client holds
   the private key that was used to sign the DPoP proof 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.  However, 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.



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   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 the client, in JWK
      format, as defined in Section 4.1.3 of [RFC7515] (REQUIRED).  MUST
      NOT contain the private key.

   The payload 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 10.1.

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




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

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

   When the DPoP proof is used in conjunction with the presentation of
   an access token, see Section 7, the DPoP proof MUST also contain the
   following claim:

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

   A DPoP proof MAY contain other headers or claims as defined by
   extension, profile, or deployment specific requirements.

   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











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   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 10.7).

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.   that there is not more than one DPoP header in the request,

   2.   the string value of the header field is a well-formed JWT,

   3.   all required claims per Section 4.2 are contained in the JWT,

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

   5.   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,

   6.   the JWT signature verifies with the public key contained in the
        jwk header of the JWT,

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

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

   9.   if the server provided a nonce value to the client, the nonce
        claim matches the server-provided nonce value,

   10.  the token was issued within an acceptable timeframe and, within
        a reasonable consideration of accuracy and resource utilization,
        a proof JWT with the same jti value has not previously been
        received at the same resource during that time period (see
        Section 10.1).






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   11.  when presented to a protected resource in conjunction with an
        access token, ensure that the value of the ath claim equals the
        hash of the access token that has been presented alongside the
        DPoP proof.

   Servers SHOULD employ Syntax-Based Normalization and Scheme-Based
   Normalization in 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 JWT in a DPoP header when
   making an access token request to the 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 such an access token request
   using an authorization code grant with a DPoP proof JWT 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 DPoP HTTP header MUST contain a valid DPoP proof JWT.  If the
   DPoP proof 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.






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   To sender-constrain the access token, after checking the validity of
   the DPoP proof, the authorization server associates the issued access
   token with the public key from the DPoP proof, which can be
   accomplished as described in Section 6.  A token_type of DPoP MUST be
   included in the access token response to signal to the client that
   the access token was bound to its DPoP key and can be used as
   described in Section 7.1.  The example response shown in Figure 5
   illustrates such a response.

   HTTP/1.1 200 OK
   Content-Type: application/json
   Cache-Control: 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 includes a refresh token which the
   client can use to obtain a new access token when the previous one
   expires.  Refreshing an access token is a token request using the
   refresh_token grant type made to the authorization server's token
   endpoint.  As with all access token requests, the client makes it a
   DPoP request by including a DPoP proof, as shown in the Figure 6
   example (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
    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






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   When an authorization server supporting DPoP issues a refresh token
   to a public client that presents a valid DPoP proof at the token
   endpoint, the refresh token MUST be bound to the respective public
   key.  The binding MUST be validated when the refresh token is later
   presented to get new access tokens.  As a result, such a client MUST
   present a DPoP proof for the same key that was used to obtain the
   refresh token each time that refresh token is used to obtain a new
   access token.  The implementation details of the binding of the
   refresh token are at the discretion of the authorization server.  The
   server both produces and validates the refresh tokens that it issues
   so there is no interoperability consideration in the specific details
   of the binding.

   An authorization server MAY elect to issue access tokens which are
   not DPoP bound, which is signaled to the client with a value of
   Bearer in the token_type parameter of the access token response per
   [RFC6750].  For a public client that is also issued a refresh token,
   this has the effect of DPoP-binding the refresh token alone, which
   can improve the security posture even when protected resources are
   not updated to support DPoP.

   If a client receives a different token_type value than DPoP in the
   response, the access token protection provided by DPoP is not given.
   The client MUST discard the response in this case if this protection
   is deemed important for the security of the application and MAY
   continue as in a regular OAuth interaction otherwise.

   Refresh tokens issued to confidential clients (those having
   established authentication credentials with the authorization server)
   are not bound to the 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 client to which they
   were issued and that confidential clients authenticate to the
   authorization server when presenting a refresh token.  As a result,
   such refresh tokens are sender-constrained by way of the client ID
   and 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 particular public key.

5.1.  Authorization Server Metadata

   This document introduces the following new authorization server
   metadata [RFC8414] parameter to signal support for DPoP in general
   and the specific JWS alg values the authorization server supports for
   DPoP proof JWTs.




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   dpop_signing_alg_values_supported  A JSON array containing a list of
      the 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 which the token is bound in order to verify the
   binding with respect to 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 directly, using
   the syntax described in Section 6.1, or through token introspection
   as described in Section 6.2.  Other methods of associating a public
   key with an access token are possible, per agreement by the
   authorization server and the protected resource, but are beyond the
   scope of this specification.

   Resource servers supporting DPoP MUST ensure that the public key from
   the DPoP proof matches the public key to which the access token is
   bound.

6.1.  JWK Thumbprint Confirmation Method

   When access tokens are represented as JSON Web Tokens (JWT)
   [RFC7519], the public key information SHOULD be represented using the
   jkt confirmation method member defined herein.  To convey the hash of
   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 base64url encoding (as defined in
      [RFC7515]) of the JWK SHA-256 Thumbprint (according to [RFC7638])
      of the DPoP public key (in JWK format) to which the access token
      is bound.

   The following example JWT in Figure 7 with decoded JWT payload shown
   in Figure 8 contains a cnf claim with the jkt JWK thumbprint
   confirmation method member.  The jkt value in these examples is the
   hash of the public key from the DPoP proofs in the examples in
   Section 5.








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

       Figure 7: JWT containing a 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 as well as to determine metainformation
   about the token.

   For a DPoP-bound access token, the hash of the public key to which
   the token is bound is conveyed to the protected resource as
   metainformation in a token introspection response.  The hash is
   conveyed using the same cnf content with jkt member structure as the
   JWK thumbprint confirmation method, described in Section 6.1, as a
   top-level member of the introspection response JSON.  Note that the
   resource server does not send a DPoP proof 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 data of the introspection response to
   validate the access token binding itself locally.

   If the token_type member is included in the introspection response,
   it MUST contain the value DPoP.

   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 in Figure 5.








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   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-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

   To make use of an access token that is bound to a public key using
   DPoP, a client MUST prove 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.  The DPoP proof MUST
   include the ath claim with a valid hash of the associated access
   token.

7.1.  The DPoP Authentication 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 authentication scheme credentials is as follows:






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

    credentials = "DPoP" 1*SP token68

                 Figure 11: DPoP Authentication Scheme ABNF

   For such an access token, a resource server MUST check that a DPoP
   proof was also received in the DPoP header field of the HTTP request,
   check the DPoP proof according to the rules in Section 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 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.  Following that is Figure 13, which shows
   the decoded content of that DPoP proof.  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 in both examples.

   GET /protectedresource HTTP/1.1
   Host: resource.example.org
   Authorization: DPoP Kz~8mXK1EalYznwH-LC-1fBAo.4Ljp~zsPE_NeO.gxU
   DPoP: eyJ0eXAiOiJkcG9wK2p3dCIsImFsZyI6IkVTMjU2IiwiandrIjp7Imt0eSI6Ik
    VDIiwieCI6Imw4dEZyaHgtMzR0VjNoUklDUkRZOXpDa0RscEJoRjQyVVFVZldWQVdCR
    nMiLCJ5IjoiOVZFNGpmX09rX282NHpiVFRsY3VOSmFqSG10NnY5VERWclUwQ2R2R1JE
    QSIsImNydiI6IlAtMjU2In19.eyJqdGkiOiJlMWozVl9iS2ljOC1MQUVCIiwiaHRtIj
    oiR0VUIiwiaHR1IjoiaHR0cHM6Ly9yZXNvdXJjZS5leGFtcGxlLm9yZy9wcm90ZWN0Z
    WRyZXNvdXJjZSIsImlhdCI6MTU2MjI2MjYxOCwiYXRoIjoiZlVIeU8ycjJaM0RaNTNF
    c05yV0JiMHhXWG9hTnk1OUlpS0NBcWtzbVFFbyJ9.2oW9RP35yRqzhrtNP86L-Ey71E
    OptxRimPPToA1plemAgR6pxHF8y6-yqyVnmcw6Fy1dqd-jfxSYoMxhAJpLjA

                 Figure 12: DPoP Protected Resource Request














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   {
     "typ":"dpop+jwt",
     "alg":"ES256",
     "jwk": {
       "kty":"EC",
       "x":"l8tFrhx-34tV3hRICRDY9zCkDlpBhF42UQUfWVAWBFs",
       "y":"9VE4jf_Ok_o64zbTTlcuNJajHmt6v9TDVrU0CdvGRDA",
       "crv":"P-256"
     }
   }
   .
   {
     "jti":"e1j3V_bKic8-LAEB",
     "htm":"GET",
     "htu":"https://resource.example.org/protectedresource",
     "iat":1562262618,
     "ath":"fUHyO2r2Z3DZ53EsNrWBb0xWXoaNy59IiKCAqksmQEo"
   }

       Figure 13: Decoded Content of the DPoP Proof JWT in Figure 12

   Upon receipt of a request for a URI of a protected resource within
   the protection space requiring DPoP authentication, if the request
   does not include valid credentials 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 authentication.  The error
      parameter values described in Section 3.1 of [RFC6750] are
      suitable as are any appropriate values defined by extension.  The
      value use_dpop_nonce can be used as described in Section 9 to
      signal that a nonce is needed in the DPoP proof of subsequent



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      request(s).  And invalid_dpop_proof is used to indicate that the
      DPoP proof itself was deemed invalid based on the criteria of
      Section 4.3.

   *  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 algs="ES256 PS256"

    Figure 14: 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 error="invalid_token",
      error_description="Invalid DPoP key binding", algs="ES256"

     Figure 15: HTTP 401 Response to a Protected Resource Request with
                              an Invalid Token

7.2.  Compatibility with the Bearer Authentication Scheme

   Protected resources simultaneously supporting both the DPoP and
   Bearer schemes need to update how evaluation of bearer tokens is
   performed to prevent downgraded usage of a DPoP-bound access tokens.
   Specifically, such a protected resource MUST reject an access token
   received as a bearer token per [!@RFC6750], if that token is
   determined to be DPoP-bound.






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   Section 4.1 of [RFC7235] allows a protected resource to indicate
   support for multiple authentication schemes (i.e., Bearer and DPoP)
   with the WWW-Authenticate header field of a 401 (Unauthorized)
   response.

   A protected resource that supports only [RFC6750] and is unaware of
   DPoP 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 content of the access token as
   it relates to validity).  As such, a client MAY send a DPoP-bound
   access token using the Bearer scheme upon receipt of a WWW-
   Authenticate: Bearer challenge from a protected resource (or if it
   has prior such knowledge about the capabilities of the protected
   resource).  The effect of this likely simplifies the logistics of
   phased upgrades to protected resources in their support DPoP or even
   prolonged deployments of protected resources with mixed token type
   support.

8.  Authorization Server-Provided Nonce

   Including a nonce value contributed by the authorization server in
   the DPoP proof MAY be used by authorization servers to limit the
   lifetime of DPoP proofs.  The server is in control of when to require
   the use of a new nonce value in subsequent DPoP proofs.

   Without employing such a mechanism, a malicious party controlling the
   client (including potentially the end user) can create DPoP proofs
   for use arbitrarily far in the future.  This section specifies how
   server-provided nonces are used with DPoP.

   An authorization server MAY supply a nonce value to be included by
   the client in DPoP proofs sent to it by responding to requests not
   including a nonce with an error response per Section 5.2 of [RFC6749]
   using use_dpop_nonce as the error code value and including a DPoP-
   Nonce HTTP header in the response supplying a nonce value to be used
   when sending the subsequent request.

   For example, in response to a token request without a nonce when the
   authorization server requires one, the authorization server can
   respond with a DPoP-Nonce value such as the following to provide a
   nonce value to include in the DPoP proof:








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    HTTP/1.1 400 Bad Request
    DPoP-Nonce: eyJ7S_zG.eyJH0-Z.HX4w-7v

    {
     "error": "use_dpop_nonce"
     "error_description":
       "Authorization server requires nonce in DPoP proof"
    }

      Figure 16: HTTP 400 Response to a Token Request without a Nonce

   Other HTTP headers and JSON fields MAY also be included in the error
   response, but there MUST NOT be more than one DPoP-Nonce header.

   Upon receiving the nonce, the client is expected to retry its token
   request using a DPoP proof including the supplied nonce value in the
   nonce claim of the DPoP proof.  An example unencoded JWT Payload of
   such a DPoP proof including a nonce is:

    {
     "jti": "-BwC3ESc6acc2lTc",
     "htm": "POST",
     "htu": "https://server.example.com/token",
     "iat": 1562262616,
     "nonce": "eyJ7S_zG.eyJH0-Z.HX4w-7v"
    }

           Figure 17: DPoP Proof Payload Including a Nonce Value

   The nonce syntax in ABNF as used by [RFC6749] (which is the same as
   the scope-token syntax) is:

     nonce = 1*NQCHAR

                           Figure 18: Nonce ABNF

   The nonce is opaque to the client.

   If the nonce claim in the DPoP proof of a token request does not
   exactly match the nonce supplied by the authorization server to the
   client, the authorization server MUST reject the request.  The
   rejection response MAY include a DPoP-Nonce HTTP header providing a
   new nonce value to use for subsequent requests.








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8.1.  Providing a New Nonce Value

   It is up to the authorization server when to supply a new nonce value
   for the client to use.  The client is expected to use the existing
   supplied nonce in DPoP proofs until the server supplies a new nonce
   value.

   The authorization server MAY supply the new nonce in the same way
   that the initial one was supplied: by using a DPoP-Nonce HTTP header
   in the response.  Of course, each time this happens it requires an
   extra protocol round trip.

   A more efficient manner of supplying a new nonce value is also
   defined -- by including a DPoP-Nonce HTTP header in the 200 OK
   response from the previous request.  The client MUST use the new
   nonce value supplied for the next token request, and for all
   subsequent token requests until the authorization server supplies a
   new nonce.

   An example 200 OK response providing a new nonce value is:

    HTTP/1.1 200 OK
    DPoP-Nonce: eyJ7S_zG.eyJbYu3.xQmBj-1

        Figure 19: HTTP 200 Response Providing the Next Nonce Value

9.  Resource Server-Provided Nonce

   Resource servers can also choose to provide a nonce value to be
   included in DPoP proofs sent to them.  They provide the nonce using
   the DPoP-Nonce header in same way that authorization servers do.  The
   error signaling is performed as described in Section 7.1.

   For example, in response to a resource request without a nonce when
   the resource server requires one, the resource server can respond
   with a DPoP-Nonce value such as the following to provide a nonce
   value to include in the DPoP proof:

    HTTP/1.1 401 Unauthorized
    WWW-Authenticate: DPoP error="use_dpop_nonce",
      error_description="Resource server requires nonce in DPoP proof"
    DPoP-Nonce: eyJ7S_zG.eyJH0-Z.HX4w-7v

     Figure 20: HTTP 401 Response to a Resource Request without a Nonce

   Note that the nonces provided by the two kinds of servers are
   different and MUST not be confused with one another.  In particular,
   a nonce provided to the client by a particular server MUST only be



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   used with that server and no other.  Developers should also take care
   to not confuse this nonce with the OpenID Connect [OpenID.Core] ID
   Token nonce, should one also be present.

10.  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 plus the optional server-provided nonce.
   DPoP, however, has a somewhat different nature of protection than
   TLS-based methods such as OAuth Mutual TLS [RFC8705] or OAuth Token
   Binding [I-D.ietf-oauth-token-binding] (see also Section 10.1 and
   Section 10.7).  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 with respect to the transport layer
   to which the token is bound and replay protection in general.

10.1.  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 (on the order of a few seconds).

   Servers SHOULD 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., a
   few seconds in the future).  Because clock skews between servers and
   clients may be large, servers may choose to limit DPoP proof
   lifetimes by using server-provided nonce values rather than clock
   times, yielding intended results even in the face of arbitrarily
   large clock skews.

   Server-provided nonces are an effective means of preventing DPoP
   proof replay.







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10.2.  DPoP Proof Pre-Generation

   An attacker in control of the client can pre-generate DPoP proofs for
   use arbitrarily far into the future by choosing the iat value in the
   DPoP proof to be signed by the proof-of-possession key.  Note that
   one such attacker is the person who is the legitimate user of the
   client.  The user may pre-generate DPoP proofs to exfiltrate from the
   machine possessing the proof-of-possession key upon which they were
   generated and copy them to another machine that does not possess the
   key.  For instance, a bank employee might pre-generate DPoP proofs on
   a bank computer and then copy them to another machine for use in the
   future, thereby bypassing bank audit controls.  When DPoP proofs can
   be pre-generated and exfiltrated, all that is actually being proved
   in DPoP protocol interactions is possession of a DPoP proof -- not of
   the proof-of-possession key.

   Use of server-provided nonce values that are not predictable by
   attackers can prevent this attack.  By providing new nonce values at
   times of its choosing, the server can limit the lifetime of DPoP
   proofs, preventing pre-generated DPoP proofs from being used.  When
   server-provided nonces are used, possession of the proof-of-
   possession key is being demonstrated -- not just possession of a DPoP
   proof.

10.3.  DPoP Nonce Downgrade

   A server MUST NOT accept any DPoP proofs without the nonce claim when
   a DPoP nonce has been provided to the client.

10.4.  Untrusted Code in the Client Context

   If an adversary is able to run code in the client's execution
   context, the security of DPoP is no longer guaranteed.  Common issues
   in web applications leading to the execution of untrusted code are
   cross-site scripting and remote code inclusion attacks.

   If the private key used for DPoP is stored in such a way that it
   cannot be exported, e.g., in a hardware or software security module,
   the adversary cannot exfiltrate the key and use it to create
   arbitrary DPoP proofs.  The adversary can, however, create new DPoP
   proofs as long as the client is online, and use these proofs
   (together with the respective tokens) either on the victim's device
   or on a device under the attacker's control to send arbitrary
   requests that will be accepted by servers.

   To send requests even when the client is offline, an adversary can
   try to pre-compute DPoP proofs using timestamps in the future and
   exfiltrate these together with the access or refresh token.



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   An adversary might further try to associate tokens issued from the
   token endpoint with a key pair under the adversary's control.  One
   way to achieve this is to modify existing code, e.g., by replacing
   cryptographic APIs.  Another way is to launch a new authorization
   grant between the client and the authorization server in an iframe.
   This grant needs to be "silent", i.e., not require interaction with
   the user.  With code running in the client's origin, the adversary
   has access to the resulting authorization code and can use it to
   associate their own DPoP keys with the tokens returned from the token
   endpoint.  The adversary is then able to use the resulting tokens on
   their own device even if the client is offline.

   Therefore, protecting clients against the execution of untrusted code
   is extremely important even if DPoP is used.  Besides secure coding
   practices, Content Security Policy [W3C.CSP] can be used as a second
   layer of defense against cross-site scripting.

10.5.  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.

10.6.  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.

10.7.  Message Integrity

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




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

10.8.  Access Token and Public Key Binding

   The binding of the access token to the DPoP public key, 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 an additional related JWT confirmation method member be defined
   for that purpose, registered in the respective IANA registry, and
   used in place of the jkt confirmation method defined herein.

   Similarly, the binding of the DPoP proof to the access token uses a
   hash of that access token as the value of the ath claim in the DPoP
   proof (see Section 4.2).  This relies on the value of the hash being
   sufficiently unique so as to reliably identify the access token.  The
   collision resistance of SHA-256 meets that requirement.  If, in the
   future, access token digests need be computed using hash function(s)
   other than SHA-256, it is suggested that an additional related JWT
   claim be defined for that purpose, registered in the respective IANA
   registry, and used in place of the ath claim defined herein.

11.  IANA Considerations

11.1.  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 ]]





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11.2.  OAuth Extensions Error Registration

   This specification requests registration of the following error
   values in the "OAuth Extensions Error" registry [IANA.OAuth.Params]
   established by [RFC6749].

   Invalid DPoP proof:

   *  Name: invalid_dpop_proof

   *  Usage Location: token error response, resource error response

   *  Protocol Extension: Demonstrating Proof of Possession (DPoP)

   *  Change controller: IETF

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

   Use DPoP nonce:

   *  Name: use_dpop_nonce

   *  Usage Location: token error response, resource error response

   *  Protocol Extension: Demonstrating Proof of Possession (DPoP)

   *  Change controller: IETF

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

11.3.  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 7.1 of this specification ]]

11.4.  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.StructuredSuffix] by
   Section 7.2 of [RFC8417], which is maybe sufficient?  A 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



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   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. ]]

11.5.  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 6 of this specification ]]

11.6.  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 4.2 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 4.2 of this specification ]]



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   Access token hash:

   *  Claim Name: ath

   *  Claim Description: The base64url encoded SHA-256 hash of the ASCII
      encoding of the associated access token's value

   *  Change Controller: IESG

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

11.7.  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 ]]

11.8.  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 5.1 of this specification ]]

12.  Normative References

   [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>.



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   [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]  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>.

   [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>.

   [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>.

13.  Informative References

   [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-13, 25 May 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-oauth-
              access-token-jwt-13>.

   [I-D.ietf-oauth-jwsreq]
              Sakimura, N., Bradley, J., and M. B. Jones, "The OAuth 2.0
              Authorization Framework: JWT-Secured Authorization Request
              (JAR)", Work in Progress, Internet-Draft, draft-ietf-
              oauth-jwsreq-34, 8 April 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-oauth-
              jwsreq-34>.




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   [I-D.ietf-oauth-security-topics]
              Lodderstedt, T., Bradley, J., Labunets, A., and D. Fett,
              "OAuth 2.0 Security Best Current Practice", Work in
              Progress, Internet-Draft, draft-ietf-oauth-security-
              topics-18, 13 April 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-oauth-
              security-topics-18>.

   [I-D.ietf-oauth-token-binding]
              Jones, M. B., Campbell, B., Bradley, J., and W. Denniss,
              "OAuth 2.0 Token Binding", Work in Progress, Internet-
              Draft, draft-ietf-oauth-token-binding-08, 19 October 2018,
              <https://datatracker.ietf.org/doc/html/draft-ietf-oauth-
              token-binding-08>.

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

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

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

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

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

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

   [OpenID.Core]
              Sakimura, N., Bradley, J., Jones, M.B., Medeiros, B.d.,
              and C. Mortimore, "OpenID Connect Core 1.0", November
              2014,
              <http://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>.

   [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>.

   [RFC4122]  Leach, P., Mealling, M., and R. Salz, "A Universally
              Unique IDentifier (UUID) URN Namespace", RFC 4122,
              DOI 10.17487/RFC4122, July 2005,
              <https://www.rfc-editor.org/info/rfc4122>.

   [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>.

   [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing",
              RFC 7230, DOI 10.17487/RFC7230, June 2014,
              <https://www.rfc-editor.org/info/rfc7230>.

   [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>.

   [RFC7519]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
              (JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,
              <https://www.rfc-editor.org/info/rfc7519>.

   [RFC7523]  Jones, M., Campbell, B., and C. Mortimore, "JSON Web Token
              (JWT) Profile for OAuth 2.0 Client Authentication and
              Authorization Grants", RFC 7523, DOI 10.17487/RFC7523, May
              2015, <https://www.rfc-editor.org/info/rfc7523>.

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

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

   [RFC8417]  Hunt, P., Ed., Jones, M., Denniss, W., and M. Ansari,
              "Security Event Token (SET)", RFC 8417,
              DOI 10.17487/RFC8417, July 2018,
              <https://www.rfc-editor.org/info/rfc8417>.

   [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>.

   [RFC8707]  Campbell, B., Bradley, J., and H. Tschofenig, "Resource
              Indicators for OAuth 2.0", RFC 8707, DOI 10.17487/RFC8707,
              February 2020, <https://www.rfc-editor.org/info/rfc8707>.

   [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>.

   [W3C.CSP]  West, M., "Content Security Policy Level 3", World Wide
              Web Consortium Working Draft WD-CSP3-20181015, 15 October
              2018, <https://www.w3.org/TR/2018/WD-CSP3-20181015/>.

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

Appendix A.  Acknowledgements

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






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   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 ]]

   -04

   *  Added the option for a server-provided nonce in the DPoP proof.

   *  Registered the invalid_dpop_proof and use_dpop_nonce error codes.

   *  Removed fictitious uses of realm from the examples, as they added
      no value.

   *  State that if the introspection response has a token_type, it has
      to be DPoP.

   *  Mention that RFC7235 allows multiple authentication schemes in
      WWW-Authenticate with a 401.

   *  Editorial fixes.

   -03

   *  Add an access token hash (ath) claim to the DPoP proof when used
      in conjunction with the presentation of an access token for
      protected resource access

   *  add Untrusted Code in the Client Context section to security
      considerations

   *  Editorial updates and fixes

   -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






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   *  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




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   *  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




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   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






















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