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

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

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

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
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   This Internet-Draft will expire on 2 November 2020.

Copyright Notice

   Copyright (c) 2020 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
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   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Conventions and Terminology . . . . . . . . . . . . . . .   3
   2.  Main Objective  . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Concept . . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  DPoP Proof JWTs . . . . . . . . . . . . . . . . . . . . . . .   6
     4.1.  DPoP Proof JWT Syntax . . . . . . . . . . . . . . . . . .   6
     4.2.  Checking DPoP Proofs  . . . . . . . . . . . . . . . . . .   7
   5.  Token Request (Binding Tokens to a Public Key)  . . . . . . .   8
   6.  Resource Access (Proof of Possession for Access Tokens) . . .   9
   7.  Public Key Confirmation . . . . . . . . . . . . . . . . . . .  11
   8.  Authorization Server Metadata . . . . . . . . . . . . . . . .  12
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
     9.1.  DPoP Proof Replay . . . . . . . . . . . . . . . . . . . .  13
     9.2.  Signed JWT Swapping . . . . . . . . . . . . . . . . . . .  13
     9.3.  Signature Algorithms  . . . . . . . . . . . . . . . . . .  13
     9.4.  Message Integrity . . . . . . . . . . . . . . . . . . . .  13
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
     10.1.  OAuth Access Token Type Registration . . . . . . . . . .  14
     10.2.  HTTP Authentication Scheme Registration  . . . . . . . .  14
     10.3.  Media Type Registration  . . . . . . . . . . . . . . . .  14
     10.4.  JWT Confirmation Methods Registration  . . . . . . . . .  15
     10.5.  JSON Web Token Claims Registration . . . . . . . . . . .  15
     10.6.  HTTP Message Header Field Names Registration . . . . . .  15
     10.7.  Authorization Server Metadata Registration . . . . . . .  16
   11. Normative References  . . . . . . . . . . . . . . . . . . . .  16
   12. Informative References  . . . . . . . . . . . . . . . . . . .  17
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .  19
   Appendix B.  Document History . . . . . . . . . . . . . . . . . .  19
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  21

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1.  Introduction

   This document outlines a relatively simple 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 the "DPoP" header in an HTTP request.
   Using that header, an authorization server is able to bind issued
   tokens to the public part of the client's key pair.  Recipients of
   such tokens are then able to verify the binding of the token to the
   key pair that the client has demonstrated that it holds via the
   "DPoP" header, thereby providing some assurance that the client
   presenting the token also possesses the private key.  In other words,
   the legitimate presenter of the token is constrained to be the sender
   that holds and can prove possession of the private part of the key
   pair.

   The mechanism described herein can be used in cases where potentially
   stronger 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.

   DPoP can be used with public clients to sender-constrain access
   tokens and refresh tokens.  With confidential clients, DPoP can be
   used in conjunction with any client authentication method to sender-
   constrain access tokens.

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].

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2.  Main Objective

   Under the attacker model defined in [I-D.ietf-oauth-security-topics],
   the mechanism defined by this specification aims to prevent token
   replay at a different endpoint.

   More precisely, if an adversary is able to get hold of an access
   token or refresh token because it set up a counterfeit authorization
   server or resource server, the adversary is not able to replay the
   respective token at another authorization or resource server.

   Secondary objectives are discussed in Section 9.

3.  Concept

   The main data structure introduced by this specification is a DPoP
   proof JWT, described in detail below, 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.

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

                         Figure 1: Basic DPoP Flow

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

   *  (A) In the Token Request, the client sends an authorization code
      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.

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   *  (B) The AS 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.  This is signaled to the client by using
      the "token_type" value "DPoP".

   *  If a refresh token is issued to a public client, it is bound to
      the public key of the DPoP proof in a similar way.  Note that for
      confidential clients, refresh tokens are required by [RFC6749] to
      bound to the "client_id" and associated authentication
      credentials, which is a sender-constraining mechanism that is more
      flexible than binding to a particular public key.

   *  (C) If the client wants to use the access token, it has to prove
      possession of the private key by, again, adding a header to the
      request that carries the DPoP proof.  The resource server needs to
      receive information about the public key to which the access token
      is bound.  This information is either encoded directly into the
      access token (for JWT structured access tokens), or provided at
      the token introspection endpoint of the authorization server (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.

   *  When a refresh token that is sender-constrained using DPoP is used
      by the client, the client has to provide a DPoP proof just as in
      the case of a resource access.  The new access token will be bound
      to the same public key.

   The 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) 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 9 for details.

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4.  DPoP Proof JWTs

   DPoP introduces concept of a DPoP proof JWT, which is used for
   binding public keys and proving knowledge about private keys.  The
   DPoP proof JWT is sent with an HTTP request using the "DPoP" header
   field.

4.1.  DPoP Proof JWT Syntax

   A DPoP proof is a JWT ([RFC7519]) that is signed (using JWS,
   [RFC7515]) using 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 [RFC7515] (REQUIRED)

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

   *  "jti": Unique identifier for the DPoP proof JWT (REQUIRED).  The
      value MUST be assigned such that there is a negligible probability
      that the same value will be assigned to any other DPoP proof used
      in the same context during the time window of validity.  Such
      uniqueness can be accomplished by encoding (base64url or any other
      suitable encoding) at least 96 bits of pseudorandom data or by
      using a version 4 UUID string according to [RFC4122].  The "jti"
      SHOULD be used by the server for replay detection and prevention,
      see Section 9.1.

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

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

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

   Figure 2 shows the JSON header and payload of a DPoP proof JWT.

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   {
     "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 2: Example JWT content for "DPoP" proof header

   Note: To keep DPoP simple to implement, only the HTTP method and URI
   are signed in DPoP proofs.  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 9.4).

4.2.  Checking DPoP Proofs

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

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

   2.  all required claims are contained in the JWT,

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

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

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

   6.  the "htm" claim matches the HTTP method value of the HTTP request
       in which the JWT was received (case-insensitive),

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

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

   9.  that, within a reasonable consideration of accuracy and resource
       utilization, a JWT with the same "jti" value has not been
       received previously (see Section 9.1).

   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.

5.  Token Request (Binding Tokens to a Public Key)

   To bind a token to a public key in the token request, the client MUST
   provide a valid DPoP proof JWT in a "DPoP" header.  The HTTPS request
   shown in Figure 3 illustrates the protocol for this (with extra line
   breaks for display purposes only).

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

        Figure 3: Token Request for a DPoP sender-constrained token

   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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   The authorization server, after checking the validity of the DPoP
   proof, associates the access token issued at the token endpoint with
   the public key.  It then sets "token_type" to "DPoP" in the token
   response, which signals to the client that the access token was bound
   to its DPoP key and can used as described in Section 6.

   If a refresh token is issued to a public client at the token endpoint
   and a valid DPoP proof is presented, the refresh token MUST be bound
   to the public key contained in the header of the DPoP proof JWT.

   When a DPoP-bound refresh token is used at the token endpoint by a
   public client, the AS MUST ensure that the DPoP proof contains the
   same public key as the one the refresh token is bound to.  The access
   token issued MUST be bound to the public key contained in the DPoP
   proof.

6.  Resource Access (Proof of Possession for Access Tokens)

   To make use of an access token that is bound to a public key using
   DPoP, a client MUST prove the possession of the corresponding private
   key by providing a DPoP proof in the "DPoP" request header.

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

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

    credentials = "DPoP" 1*SP token68

                  Figure 4: DPoP Authorization Scheme ABNF

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

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

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   GET /protectedresource HTTP/1.1
   Host: resource.example.org
   Authorization: DPoP eyJhbGciOiJFUzI1NiIsImtpZCI6IkJlQUxrYiJ9.eyJzdWI
    iOiJzb21lb25lQGV4YW1wbGUuY29tIiwiaXNzIjoiaHR0cHM6Ly9zZXJ2ZXIuZXhhbX
    BsZS5jb20iLCJhdWQiOiJodHRwczovL3Jlc291cmNlLmV4YW1wbGUub3JnIiwibmJmI
    joxNTYyMjYyNjExLCJleHAiOjE1NjIyNjYyMTYsImNuZiI6eyJqa3QiOiIwWmNPQ09S
    Wk5ZeS1EV3BxcTMwalp5SkdIVE4wZDJIZ2xCVjN1aWd1QTRJIn19.vsFiVqHCyIkBYu
    50c69bmPJsj8qYlsXfuC6nZcLl8YYRNOhqMuRXu6oSZHe2dGZY0ODNaGg1cg-kVigzY
    hF1MQ
   DPoP: eyJ0eXAiOiJkcG9wK2p3dCIsImFsZyI6IkVTMjU2IiwiandrIjp7Imt0eSI6Ik
    VDIiwieCI6Imw4dEZyaHgtMzR0VjNoUklDUkRZOXpDa0RscEJoRjQyVVFVZldWQVdCR
    nMiLCJ5IjoiOVZFNGpmX09rX282NHpiVFRsY3VOSmFqSG10NnY5VERWclUwQ2R2R1JE
    QSIsImNydiI6IlAtMjU2In19.eyJqdGkiOiJlMWozVl9iS2ljOC1MQUVCIiwiaHRtIj
    oiR0VUIiwiaHR1IjoiaHR0cHM6Ly9yZXNvdXJjZS5leGFtcGxlLm9yZy9wcm90ZWN0Z
    WRyZXNvdXJjZSIsImlhdCI6MTU2MjI2MjYxOH0.lNhmpAX1WwmpBvwhok4E74kWCiGB
    NdavjLAeevGy32H3dbF0Jbri69Nm2ukkwb-uyUI4AUg1JSskfWIyo4UCbQ

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

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

   In such challenges:

   *  The scheme name is "DPoP".

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

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

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

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

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

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

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

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

                                  Figure 6

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

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

                                  Figure 7

7.  Public Key Confirmation

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

   Access tokens that are represented as JSON Web Tokens (JWT) [RFC7519]
   MUST contain information about the DPoP public key (in JWK format) in
   the member "jkt" of the "cnf" claim, as shown in Figure 8.

   The value in "jkt" MUST be the base64url encoding [RFC7515] of the
   JWK SHA-256 Thumbprint (according to [RFC7638]) of the public key to
   which the access token is bound.

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

            Figure 8: Example access token body with "cnf" claim

   When access token introspection is used, the same "cnf" claim as
   above MUST be contained in the introspection response.

   Resource servers MUST ensure that the fingerprint of the public key
   in the DPoP proof JWT equals the value in the "jkt" claim in the
   access token or introspection response.

8.  Authorization Server Metadata

   This document introduces the following new authorization server
   metadata [RFC8414] parameter to signal the JWS "alg" values the
   authorization server supports for DPoP proof JWTs:

   "dpop_signing_alg_values_supported"  OPTIONAL.  JSON array containing
      a list of the JWS "alg" values supported by the authorization
      server for DPoP proof JWTs

9.  Security Considerations

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

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9.1.  DPoP Proof Replay

   If an adversary is able to get hold of a DPoP proof JWT, the
   adversary could replay that token at the same endpoint (the HTTP
   endpoint and method are enforced via the respective claims in the
   JWTs).  To prevent this, servers MUST only accept DPoP proofs for a
   limited time window after their "iat" time, preferably only for a
   relatively brief period.  Servers SHOULD store 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 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 near future (e.g., up to a few
   seconds in the future).

9.2.  Signed JWT Swapping

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

9.3.  Signature Algorithms

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

9.4.  Message Integrity

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

   This is an intentional design decision 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 on (parts of) requests are out of the scope of
   this specification, signatures or information to be signed can be
   added into DPoP proofs.

10.  IANA Considerations

10.1.  OAuth Access Token Type Registration

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

   *  Type name: "DPoP"

   *  Additional Token Endpoint Response Parameters: (none)

   *  HTTP Authentication Scheme(s): "DPoP"

   *  Change controller: IESG

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

10.2.  HTTP Authentication Scheme Registration

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

   *  Authentication Scheme Name: "DPoP"

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

10.3.  Media Type Registration

   [[ Is a media type registration at [IANA.MediaTypes] necessary for
   "application/dpop+jwt"?  There is a "+jwt" structured syntax suffix
   registered already at [IANA.MediaType.StructuredSuffixs] by
   Section 7.2 of [RFC8417], which is maybe sufficient?  A fullblown
   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. ]]

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10.4.  JWT Confirmation Methods Registration

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

   *  Confirmation Method Value: "jkt"

   *  Confirmation Method Description: JWK SHA-256 Thumbprint

   *  Change Controller: IESG

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

10.5.  JSON Web Token Claims Registration

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

   HTTP method:

   *  Claim Name: "htm"

   *  Claim Description: The HTTP method of the request

   *  Change Controller: IESG

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

   HTTP URI:

   *  Claim Name: "htu"

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

   *  Change Controller: IESG

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

10.6.  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"

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   *  Applicable protocol: HTTP

   *  Status: standard

   *  Author/change Controller: IETF

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

10.7.  Authorization Server Metadata Registration

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

   *  Metadata Name: "dpop_signing_alg_values_supported"

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

   *  Change Controller: IESG

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

11.  Normative References

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

   [RFC6749]  Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
              RFC 6749, DOI 10.17487/RFC6749, October 2012,
              <https://www.rfc-editor.org/info/rfc6749>.

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

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <https://www.rfc-editor.org/info/rfc3986>.

   [RFC6750]  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>.

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   [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>.

   [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>.

12.  Informative References

   [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>.

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

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

   [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>.

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   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

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

   [RFC8414]  Jones, 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>.

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

   [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>.

   [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>.

   [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>.

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

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

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

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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>.

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

   [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>.

   [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-15, 5 April 2020, <https://tools.ietf.org/html/
              draft-ietf-oauth-security-topics-15>.

   [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>.

Appendix A.  Acknowledgements

   We would like to thank Filip Skokan, Mike Engan, Justin Richer,
   Michael Peck, Vladimir Dzhuvinov, Rob Otto, Dominick Baier, Jim
   Willeke, Annabelle Backman, Bjorn Hjelm, Steinar Noem, Aaron Parecki,
   Neil Madden, Paul Querna, Dick Hardt, Dave Tonge, Jared Jennings,
   Mark Haine 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 ]]

   -01

   *  Editorial updates

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   *  Attempt to more formally define the DPoP Authorization header
      scheme

   *  Define the 401/WWW-Authenticate challenge

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

   *  Fixed up and added to the IANA section

   *  Added "dpop_signing_alg_values_supported" authorization server
      metadata

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

   -00 [[ Working Group Draft ]]

   *  Working group draft

   -04

   *  Update OAuth MTLS reference to RFC 8705

   *  Use the newish RFC v3 XML and HTML format

   -03

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

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

   *  more explicit recommendation to use mTLS if that is available

   *  added David Waite as co-author

   *  editorial updates

   -02

   *  added normalization rules for URIs

   *  removed distinction between proof and binding

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

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

   *  removed ability for key rotation

   *  added security considerations on request integrity

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

   *  only use the jkt#S256 in ATs

   *  iat instead of exp in DPoP proof JWTs

   *  updated guidance on token_type evaluation

   -01

   *  fixed inconsistencies

   *  moved binding and proof messages to headers instead of parameters

   *  extracted and unified definition of DPoP JWTs

   *  improved description

   -00

   *  first draft

Authors' Addresses

   Daniel Fett
   yes.com

   Email: mail@danielfett.de

   Brian Campbell
   Ping Identity

   Email: bcampbell@pingidentity.com

   John Bradley
   Yubico

   Email: ve7jtb@ve7jtb.com

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   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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