Web Authorization Protocol                                       D. Fett
Internet-Draft                                                   yes.com
Intended status: Standards Track                             B. Campbell
Expires: 9 October 2021                                    Ping Identity
                                                              J. Bradley
                                                                  Yubico
                                                          T. Lodderstedt
                                                                 yes.com
                                                                M. Jones
                                                               Microsoft
                                                                D. Waite
                                                           Ping Identity
                                                            7 April 2021


  OAuth 2.0 Demonstrating Proof-of-Possession at the Application Layer
                                 (DPoP)
                        draft-ietf-oauth-dpop-03

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 9 October 2021.

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 . . . . . . . . . . . . . . .   3
   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 . . . . . . . . . . . . . . . . . .  10
     5.1.  Authorization Server Metadata . . . . . . . . . . . . . .  13
   6.  Public Key Confirmation . . . . . . . . . . . . . . . . . . .  13
     6.1.  JWK Thumbprint Confirmation Method  . . . . . . . . . . .  14
     6.2.  JWK Thumbprint Confirmation Method in Token
           Introspection . . . . . . . . . . . . . . . . . . . . . .  14
   7.  Protected Resource Access . . . . . . . . . . . . . . . . . .  16
     7.1.  The DPoP Authorization Request Header Scheme  . . . . . .  16
     7.2.  The Bearer Authorization Request Header Scheme  . . . . .  19
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  19
     8.1.  DPoP Proof Replay . . . . . . . . . . . . . . . . . . . .  19
     8.2.  Untrusted Code in the Client Context  . . . . . . . . . .  20
     8.3.  Signed JWT Swapping . . . . . . . . . . . . . . . . . . .  21
     8.4.  Signature Algorithms  . . . . . . . . . . . . . . . . . .  21
     8.5.  Message Integrity . . . . . . . . . . . . . . . . . . . .  21
     8.6.  Access Token and Public Key Binding . . . . . . . . . . .  22
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  22
     9.1.  OAuth Access Token Type Registration  . . . . . . . . . .  22
     9.2.  HTTP Authentication Scheme Registration . . . . . . . . .  22
     9.3.  Media Type Registration . . . . . . . . . . . . . . . . .  23
     9.4.  JWT Confirmation Methods Registration . . . . . . . . . .  23
     9.5.  JSON Web Token Claims Registration  . . . . . . . . . . .  23
     9.6.  HTTP Message Header Field Names Registration  . . . . . .  24
     9.7.  Authorization Server Metadata Registration  . . . . . . .  24
   10. Normative References  . . . . . . . . . . . . . . . . . . . .  25
   11. Informative References  . . . . . . . . . . . . . . . . . . .  25
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .  29
   Appendix B.  Document History . . . . . . . . . . . . . . . . . .  29
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  31



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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
   demonstrate proof-of-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, 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 to sender-constrain access tokens regardless of the
   client authentication method employed.  Furthermore, DPoP can also be
   used to 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.








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   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
   demonstrate 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 disclose 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 play
   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 (even despite extensions such as [RFC8707])
   for many deployments.  Sender-constraining access tokens is 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



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   independent from 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.
   The impact of such 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 don't 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 8.

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.  Roughly
   speaking, a DPoP proof is a signature over a timestamp and some data
   of the HTTP request to which it is attached.















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   +--------+                                          +---------------+
   |        |--(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 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
      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.



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   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.  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.

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










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   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 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).




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   *  "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 also contains 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.

   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 8.5).







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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 per Section 4.2 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.  the JWT signature verifies with 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 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 8.1).

   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.

   If presented with an access token to a protected resource, the server
   MUST ensure that the value of the "ath" claim equals the hash of the
   access token that has been presented along side the DPoP proof.

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



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   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.

   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" in
   the access token response signals to the client that the access token
   was bound to its DPoP key and can 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"
   }




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                      Figure 5: Access Token Response

   The example response in Figure 5 included 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, which is 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

   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's 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.




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   A client expecting a DPoP-bound access token MAY discard the
   response, if a "Bearer" token type is received.

   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.

   "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.





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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.

   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.






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   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.

   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.

   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











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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 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 11: DPoP Authorization 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.







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   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

   {
     "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 authorization, 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".




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   *  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 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 realm="WallyWorld", 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







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7.2.  The Bearer Authorization Request Header 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.

   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.  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, 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 8.1 and Section 8.5).  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.

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 (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



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   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).

8.2.  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.

   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.






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8.3.  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.

8.4.  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.

8.5.  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].

   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.

















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8.6.  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.

9.  IANA Considerations

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 ]]

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]:



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   *  Authentication Scheme Name: "DPoP"

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

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.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 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. ]]

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 6 of this specification ]]

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




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   *  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 ]]

   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 ]]

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 ]]

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"




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   *  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 ]]

10.  Normative References

   [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>.

   [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>.

   [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>.

   [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>.

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

11.  Informative References





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   [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>.

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

   [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>.

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

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

   [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>.

   [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>.

   [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>.

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

   [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>.






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   [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>.

   [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-16, 5 October 2020, <https://tools.ietf.org/html/
              draft-ietf-oauth-security-topics-16>.

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

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

   [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>.

   [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>.

   [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>.

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

   [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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   [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>.

   [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>.

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

   [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-11, 22 January 2021,
              <https://tools.ietf.org/html/draft-ietf-oauth-access-
              token-jwt-11>.

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

   [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>.

   [I-D.ietf-oauth-token-binding]
              Jones, M., 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://tools.ietf.org/html/draft-ietf-oauth-token-
              binding-08>.

   [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>.











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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, Steinar
   Noem, Neil Madden, Rob Otto, Aaron Parecki, Michael Peck, Paul
   Querna, Justin Richer, Filip Skokan, 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.

   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 ]]

   -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

   *  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




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   -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




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   *  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






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   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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