OAuth 2.0 Demonstration of Proof-of-Possession at the Application Layer (DPoP)

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Table of Contents

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.¶

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

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.¶
• (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.¶

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.¶

{
  "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
}

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-

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.¶

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

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.¶

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

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

In such challenges:¶

• The scheme name is DPoP.¶
• The authentication parameter realm MAY be included to indicate the scope of protection in the manner described in [RFC7235], Section 2.2.¶
• A scope authentication parameter MAY be included as defined in [RFC6750], Section 3.¶
• An error parameter ([RFC6750], Section 3) SHOULD be included to indicate the reason why the request was declined, if the request included an access token but failed authorization. Parameter values are described in Section 3.1 of [RFC6750].¶
• An error_description parameter ([RFC6750], Section 3) MAY be included along with the error parameter to provide developers a human-readable explanation that is not meant to be displayed to end-users.¶
• An algs parameter SHOULD be included to signal to the client the JWS algorithms that are acceptable for the DPoP proof JWT. The value of the parameter is a space-delimited list of JWS alg (Algorithm) header values ([RFC7515], Section 4.1.1).¶
• Additional authentication parameters MAY be used and unknown parameters MUST be ignored by recipients¶

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

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

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"

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.¶

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

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.¶

10. IANA Considerations

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¶
• Attempt to more formally define the DPoP Authorization header scheme¶
• Define the 401/WWW-Authenticate challenge¶
• Added invalid_dpop_proof error code for DPoP errors in token request¶
• Fixed up and added to the IANA section¶
• Added dpop_signing_alg_values_supported authorization server metadata¶
• Moved the Acknowledgements into an Appendix and added a bunch of names (best effort)¶

-00 [[ Working Group Draft ]]¶

• Working group draft¶

-04¶

• Update OAuth MTLS reference to RFC 8705¶
• Use the newish RFC v3 XML and HTML format¶

-03¶

• rework the text around uniqueness requirements on the jti claim in the DPoP proof JWT¶
• make tokens a bit smaller by using htm, htu, and jkt rather than http_method, http_uri, and jkt#S256 respectively¶
• more explicit recommendation to use mTLS if that is available¶
• added David Waite as co-author¶
• editorial updates¶

-02¶

• added normalization rules for URIs¶
• removed distinction between proof and binding¶
• "jwk" header again used instead of "cnf" claim in DPoP proof¶
• renamed "Bearer-DPoP" token type to "DPoP"¶
• removed ability for key rotation¶
• added security considerations on request integrity¶
• explicit advice on extending DPoP proofs to sign other parts of the HTTP messages¶
• only use the jkt#S256 in ATs¶
• iat instead of exp in DPoP proof JWTs¶
• updated guidance on token_type evaluation¶

-01¶

• fixed inconsistencies¶
• moved binding and proof messages to headers instead of parameters¶
• extracted and unified definition of DPoP JWTs¶
• improved description¶

-00¶

• first draft¶

Authors' Addresses