OAuth 2.0 Attestation-Based Client Authentication
draft-ietf-oauth-attestation-based-client-auth-02

Network Working Group                                          T. Looker
Internet-Draft                                                     MATTR
Intended status: Informational                                P. Bastian
Expires: 23 October 2024                                   21 April 2024

           OAuth 2.0 Attestation-Based Client Authentication
           draft-ietf-oauth-attestation-based-client-auth-02

Abstract

   This specification defines a new method of client authentication for
   OAuth 2.0 [RFC6749] by extending the approach defined in [RFC7521].
   This new method enables client deployments that are traditionally
   viewed as public clients to be able to authenticate with the
   authorization server through an attestation based authentication
   scheme.

About This Document

   This note is to be removed before publishing as an RFC.

   The latest revision of this draft can be found at
   https://vcstuff.github.io/draft-ietf-oauth-attestation-based-client-
   auth/draft-ietf-oauth-attestation-based-client-auth.html.  Status
   information for this document may be found at
   https://datatracker.ietf.org/doc/draft-ietf-oauth-attestation-based-
   client-auth/.

Status of This Memo

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on 23 October 2024.

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Copyright Notice

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   document authors.  All rights reserved.

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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Conventions and Definitions . . . . . . . . . . . . . . . . .   4
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Client Authentication . . . . . . . . . . . . . . . . . . . .   5
     4.1.  JWT Format and Processing Requirements  . . . . . . . . .   6
       4.1.1.  Client Attestation JWT  . . . . . . . . . . . . . . .   6
       4.1.2.  Client Attestation PoP JWT  . . . . . . . . . . . . .   7
   5.  Implementation Considerations . . . . . . . . . . . . . . . .   9
     5.1.  Reuse of a Client Attestation JWT . . . . . . . . . . . .   9
     5.2.  Refresh token binding . . . . . . . . . . . . . . . . . .   9
     5.3.  Rotation of Client Instance Key . . . . . . . . . . . . .   9
   6.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  10
     6.1.  Client Instance Tracking Across Authorization Servers . .  10
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
     7.1.  Replay Attack Detection . . . . . . . . . . . . . . . . .  10
   8.  Appendix A IANA Considerations  . . . . . . . . . . . . . . .  11
     8.1.  Sub-Namespace Registration of urn:ietf:params:oauth:client-
           assertion-type:jwt-client-attestation . . . . . . . . . .  11
     8.2.  Registration of attest_jwt_client_auth Token Endpoint
           Authentication Method . . . . . . . . . . . . . . . . . .  11
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  11
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  12
   Appendix A.  Additional Examples  . . . . . . . . . . . . . . . .  12
     A.1.  Wallet Instance Attestation . . . . . . . . . . . . . . .  13
   Appendix B.  Document History . . . . . . . . . . . . . . . . . .  13
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  14
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  14

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1.  Introduction

   [RFC7521] defines a way for a client to include an assertion in a
   token request to an authorization server for the purposes of client
   authentication.  This specification uses this framework to define a
   new assertion type that provides a way for a client instance to
   authenticate itself with the authorization server through an
   assertion that is bound to a public key (for proof of possession).
   This assertion is designated with the name of Client Attestation in
   this draft.

   The following diagram depicts the overall architecture and protocol
   flow.

                       (3)
                    +-------+
                    |       |
                    |      \ /
                +---------------+
                |               |
                |    Client     |
                |    Backend    |
                |               |
                +---------------+
                   / \      |
               (2)  |       |  (4)
                    |      \ /
                +---------------+           +---------------+
         +----->|               |           |               |
     (1) |      |    Client     |    (6)    | Authorization |
         |      |   Instance    |<--------->|    Server     |
         +------|               |           |               |
                +---------------+           +---------------+
                   / \      |
                    |       |
                    +-------+
                       (5)

   The following steps describe this OAuth flow:

   (1) The Client Instance generates a key (Client Instance Key) and
   optional further attestations (that are out of scope) to prove its
   authenticity to the Client Backend.

   (2) The Client Instance sends this data to the Client Backend in
   request for a Client Attestation JWT.

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   (3) The Client Backend validates the Client Instance Key and optional
   further data.  It generates a signed Client Attestation JWT that is
   cryptographically bound to the Client Instance Key generated by the
   Client.  Therefore, the attestation is bound to this particular
   Client Instance.

   (4) The Client Backend responds to the Client Instance by sending the
   Client Attestation JWT.

   (5) The Client Instance generates a Proof of Possession (PoP) with
   the Client Instance Key.

   (6) The Client Instance sends both the Client Attestation JWT and the
   Client Attestation PoP JWT to the authorization server, e.g. within a
   token request.  The authorization server validates the Client
   Attestation and thus authenticates the Client Instance.

   Note that the protocol for steps (2) and (4) and how the Client
   Instance authenticates to the Client Backend is out of scope of this
   specification.  Note also that this specification can be utilized
   without the client having a backend server at all; in this case, each
   client instance will perform the functions described as being done by
   the backend for itself.

   This specification defines the format of the Client Attestation that
   a Client Instance uses to authenticate in its interactions with an
   authorization server, which is comprised of two key parts:

   1.  A Client Attestation JWT - typically produced by the client
       backend.

   2.  A Client Attestation Proof of Possession (PoP) - produced by the
       client instance.

2.  Conventions and Definitions

   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.

3.  Terminology

   Client Attestation JWT:  A JSON Web Token (JWT) generated by the
      client backend which is bound to a key managed by a client
      instance which can then be used by the instance for client
      authentication.

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   Client Attestation Proof of Possession (PoP) JWT:  A Proof of
      Possession generated by the client instance using the key that the
      Client Attestation JWT is bound to.

   Client Instance Key:  A cryptographic, asymmetric key generated by
      the client instance and proven to the client backend.  The public
      key is contained in the Client Attestation JWT and is used to sign
      the Client Attestation Proof of Possession.

4.  Client Authentication

   To perform client authentication using this scheme, the client
   instance uses the following parameter values and encodings.

   The value of the "client_assertion_type" parameter (as defined in
   [RFC7521]) set to "urn:ietf:params:oauth:client-assertion-type:jwt-
   client-attestation".

   The value of the "client_assertion" parameter (as defined in
   [RFC7521]) set to a value containing two JWTs, separated by a '~'
   character.  It MUST NOT contain more or less than precisely two JWTs
   separated by the '~' character.  The first JWT MUST be the client
   attestation JWT defined in Section 4.1.1, the second JWT MUST be the
   client attestation PoP defined in Section 4.1.2.

   The following example demonstrates client authentication using this
   scheme during the presentation of an authorization code grant in an
   access token request (with extra line breaks for display purposes
   only):

POST /token HTTP/1.1
Host: as.example.com
Content-Type: application/x-www-form-urlencoded

grant_type=authorization_code&
code=n0esc3NRze7LTCu7iYzS6a5acc3f0ogp4&
client_assertion_type=urn%3Aietf%3Aparams%3Aoauth%3A
client-assertion-type%3Ajwt-client-attestation&
client_assertion=eyJhbGciOiJSUzI1NiIsImtpZCI6IjIyIn0.
eyJpc3Mi[...omitted for brevity...].
cC4hiUPo[...omitted for brevity...]~eyJzI1NiIsImtphbGciOimtpZCI6IjIyIn0.
IjIyIn0[...omitted for brevity...].
iOiJSUzI1[...omitted for brevity...]

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4.1.  JWT Format and Processing Requirements

   In order to authenticate the client using this scheme, the
   authorization server MUST validate BOTH the JWTs present in the
   "client_assertion" parameter according to the criteria below.

   It is RECOMMENDED that the authorization server validate the Client
   Attestation JWT prior to validating the Client Attestation PoP.

4.1.1.  Client Attestation JWT

   The following rules apply to validating the client attestation JWT.
   Application of additional restrictions and policy are at the
   discretion of the authorization server.

   1.  The JWT MUST contain an "iss" (issuer) claim that contains a
       unique identifier for the entity that issued the JWT.  In the
       absence of an application profile specifying otherwise, compliant
       applications MUST compare issuer values using the Simple String
       Comparison method defined in Section 6.2.1 of [RFC3986].

   2.  The JWT MUST contain a "sub" (subject) claim with a value
       corresponding to the "client_id" of the OAuth client.

   3.  The JWT MUST contain an "exp" (expiration time) claim that limits
       the time window during which the JWT can be used.  The
       authorization server MUST reject any JWT with an expiration time
       that has passed, subject to allowable clock skew between systems.

   4.  The JWT MUST contain an "cnf" claim conforming [RFC7800] that
       conveys the key to be used for producing the client attestation
       pop for client authentication with an authorization server.  The
       key MUST be expressed using the "jwk" representation.

   5.  The JWT MAY contain an "nbf" (not before) claim that identifies
       the time before which the token MUST NOT be accepted for
       processing.

   6.  The JWT MAY contain an "iat" (issued at) claim that identifies
       the time at which the JWT was issued.

   7.  The JWT MAY contain other claims.

   8.  The JWT MUST be digitally signed using an asymmetric
       cryptographic algorithm.  The authorization server MUST reject
       the JWT if it is using a Message Authentication Code (MAC) based
       algorithm.  The authorization server MUST reject JWTs with an
       invalid signature.

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   9.  The authorization server MUST reject a JWT that is not valid in
       all other respects per "JSON Web Token (JWT)" [RFC7519].

   The following example is the decoded header and payload of a JWT
   meeting the processing rules as defined above.

   {
     "alg": "ES256",
     "kid": "11"
   }
   .
   {
     "iss": "https://client.example.com",
     "sub": "https://client.example.com",
     "nbf":1300815780,
     "exp":1300819380,
     "cnf": {
       "jwk": {
         "kty": "EC",
         "use": "sig",
         "crv": "P-256",
         "x": "18wHLeIgW9wVN6VD1Txgpqy2LszYkMf6J8njVAibvhM",
         "y": "-V4dS4UaLMgP_4fY4j8ir7cl1TXlFdAgcx55o7TkcSA"
       }
     }
   }

4.1.2.  Client Attestation PoP JWT

   The following rules apply to validating the Client Attestation JWT.
   Application of additional restrictions and policy are at the
   discretion of the Authorization Server.

   1.   The JWT MUST contain an "iss" (issuer) claim with a value
        corresponding to the "client_id" of the OAuth client.

   2.   The JWT MUST contain an "exp" (expiration time) claim that
        limits the time window during which the JWT can be used.  The
        authorization server MUST reject any JWT with an expiration time
        that has passed, subject to allowable clock skew between
        systems.  Note that the authorization server may reject JWTs
        with an "exp" claim value that is unreasonably far in the
        future.

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   3.   The JWT MUST contain a "jti" (JWT ID) claim that provides a
        unique identifier for the token.  The authorization server MAY
        ensure that JWTs are not replayed by maintaining the set of used
        "jti" values for the length of time for which the JWT would be
        considered valid based on the applicable "exp" instant.

   4.   The JWT MUST contain an "aud" (audience) claim containing a
        value that identifies the authorization server as an intended
        audience.  The [RFC8414] issuer identifier URL of the
        authorization server MUST be used as a value for an "aud"
        element to identify the authorization server as the intended
        audience of the JWT.

   5.   The JWT MAY contain an "nonce" claim containing a String value
        that is provided by the authorization server to associate the
        Client Attestation PoP JWT with a particular transaction and
        prevent replay attacks.

   6.   The JWT MAY contain an "nbf" (not before) claim that identifies
        the time before which the token MUST NOT be accepted for
        processing.

   7.   The JWT MAY contain an "iat" (issued at) claim that identifies
        the time at which the JWT was issued.  Note that the
        authorization server may reject JWTs with an "iat" claim value
        that is unreasonably far in the past.

   8.   The JWT MAY contain other claims.

   9.   The JWT MUST be digitally signed using an asymmetric
        cryptographic algorithm.  The authorization server MUST reject
        the JWT if it is using a Message Authentication Code (MAC) based
        algorithm.  The authorization server MUST reject JWTs with an
        invalid signature.

   10.  The public key used to verify the JWT MUST be the key located in
        the "cnf" claim of the corresponding client attestation JWT.

   11.  The authorization server MUST reject a JWT that is not valid in
        all other respects per "JSON Web Token (JWT)" [RFC7519].

   The following example is the decoded header and payload of a JWT
   meeting the processing rules as defined above.

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   {
     "alg": "ES256"
   }
   .
   {
     "iss": "https://client.example.com",
     "aud": "https://as.example.com",
     "nbf":1300815780,
     "exp":1300819380,
     "jti": "d25d00ab-552b-46fc-ae19-98f440f25064"
   }

5.  Implementation Considerations

5.1.  Reuse of a Client Attestation JWT

   Implementers should be aware that the design of this authentication
   mechanism deliberately allows for a client instance to re-use a
   single Client Attestation JWT in multiple interactions/requests with
   an authorization server, whilst producing a fresh Client Attestation
   PoP JWT.  Client deployments should consider this when determining
   the validity period for issued Client Attestation JWTs as this
   ultimately controls how long a client instance can re-use a single
   Client Attestation JWT.

5.2.  Refresh token binding

   Authorization servers issuing a refresh token in response to a token
   request using the "urn:ietf:params:oauth:client-assertion-type:jwt-
   client-attestation" client authentication method MUST bind the
   refresh token to the client instance, and NOT just the client as
   specified in section 6 [RFC6749].  To prove this binding, the client
   instance MUST authenticate itself to the authorization server when
   refreshing an access token using the "urn:ietf:params:oauth:client-
   assertion-type:jwt-client-attestation" authentication method.  The
   client MUST also use the same key that was present in the "cnf" claim
   of the client attestation that was used for client authentication
   when the refresh token was issued.

5.3.  Rotation of Client Instance Key

   This specification does not provide a mechanism to rotate the Client
   Instance Key in the Client Attestation JWT's "cnf" claim.  If the
   Client Instance needs to use a new Client Instance Key for any
   reason, then it MUST request a new Client Attestation JWT from its
   Client Backend.

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6.  Privacy Considerations

6.1.  Client Instance Tracking Across Authorization Servers

   Implementers should be aware that using the same client attestation
   across multiple authorization servers could result in correlation of
   the end user using the client instance through claim values
   (including the public key in the cnf claim).  Client deployments are
   therefore RECOMMENDED to use different client attestations across
   different authorization servers.

7.  Security Considerations

   The guidance provided by [RFC7519] and [RFC8725] applies.

7.1.  Replay Attack Detection

   The following mechanisms exist within this client authentication
   method in order to allow an authorization server to detect replay
   attacks for presented client attestation PoPs:

   *  The client uses "jti" (JWT ID) claims for the Client Attestation
      PoP JWT and the authorization server maintains a list of used
      (seen) "jti" values for the time of which the JWT would be
      considered valid based on the applicable "exp" claim.  If any
      Client Attestation PoP JWT would be replayed, the authorization
      server would recognize the "jti" and respond with an
      authentication error.

   *  The authorization server provides a nonce for the particular
      transaction and the client uses it for the "nonce" claim in the
      Client Attestation PoP JWT.  The authorization server validates
      that the nonce matches for the transaction.  This approach may
      require an additional roundtrip in the protocol.  The
      authorization server MUST ensure that the nonce provides
      sufficient entropy.

   *  The authorization server may expect the usage of a nonce in the
      Client Attestation PoP JWT, but instead of providing the nonce
      explicitly, the client may implicitly reuse an existing artefact,
      e.g. the authorization code.  The authorization server MUST ensure
      that the nonce provides sufficient entropy.

   The approach using a nonce explicitly provided by the authorization
   server gives stronger replay attack detection guarantees, however
   support by the authorization server is OPTIONAL to simplify mandatory
   implementation requirements.  The "jti" method is mandatory and hence
   acts as a default fallback.

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8.  Appendix A IANA Considerations

8.1.  Sub-Namespace Registration of urn:ietf:params:oauth:client-
      assertion-type:jwt-client-attestation

   This section registers the value "client-assertion-type:jwt-client-
   attestation" in the IANA "OAuth URI" registry established by "An IETF
   URN Sub-Namespace for OAuth" [RFC6755].

   *  URN: urn:ietf:params:oauth:client-assertion-type:jwt-client-
      attestation

   *  Common Name: OAuth 2.0 Attested Key-Based Client Authentication

   *  Change Controller: IESG

   *  Specification Document: TBC

8.2.  Registration of attest_jwt_client_auth Token Endpoint
      Authentication Method

   This section registers the value "attest_jwt_client_auth" in the IANA
   "OAuth Token Endpoint Authentication Methods" registry established by
   OAuth 2.0 Dynamic Client Registration Protocol [RFC7591].

   *  Token Endpoint Authentication Method Name:
      "attest_jwt_client_auth"

   *  Change Controller: IESG

   *  Specification Document(s): TBC

9.  References

9.1.  Normative References

   [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/rfc/rfc2119>.

   [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/rfc/rfc3986>.

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   [RFC6755]  Campbell, B. and H. Tschofenig, "An IETF URN Sub-Namespace
              for OAuth", RFC 6755, DOI 10.17487/RFC6755, October 2012,
              <https://www.rfc-editor.org/rfc/rfc6755>.

   [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/rfc/rfc7519>.

   [RFC7591]  Richer, J., Ed., Jones, M., Bradley, J., Machulak, M., and
              P. Hunt, "OAuth 2.0 Dynamic Client Registration Protocol",
              RFC 7591, DOI 10.17487/RFC7591, July 2015,
              <https://www.rfc-editor.org/rfc/rfc7591>.

   [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/rfc/rfc7800>.

   [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/rfc/rfc8174>.

   [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/rfc/rfc8414>.

   [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/rfc/rfc8725>.

9.2.  Informative References

   [ARF]      "The European Digital Identity Wallet Architecture and
              Reference Framework", n.d..

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

   [RFC7521]  Campbell, B., Mortimore, C., Jones, M., and Y. Goland,
              "Assertion Framework for OAuth 2.0 Client Authentication
              and Authorization Grants", RFC 7521, DOI 10.17487/RFC7521,
              May 2015, <https://www.rfc-editor.org/rfc/rfc7521>.

Appendix A.  Additional Examples

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A.1.  Wallet Instance Attestation

   This non-normative example shows a client attestations used as an
   wallet instance attestation in the context of eIDAS 2.0 [ARF], e.g.
   to secure a Type-1 configuration credential.  The additional claims
   describe the wallet's device binding und user binding capabilities
   and the achievable level of assurance.

{
        "typ": "wallet-attestation+jwt",
        "alg": "ES256",
        "kid": "1"
}
.
{
        "iss": "https://attestation-service.com",
        "sub": "https://wallet-provider.com",
        "iat": 1541493724,
        "exp": 1516247022,
        "attested_security_context" : "https://eu-trust-list.eu/asc/high",
        "cnf": {
                "jwk" : {
                        "kty": "EC",
                        "crv": "P-256",
                        "x": "TCAER19Zvu3OHF4j4W4vfSVoHIP1ILilDls7vCeGemc",
                        "y": "ZxjiWWbZMQGHVWKVQ4hbSIirsVfuecCE6t4jT9F2HZQ"
                },
                "key_type" : "STRONGBOX",
                "user_authentication" : "SYSTEM_PIN"
        }
}

Appendix B.  Document History

   -02

   *  Add text on rotation of the confirmation key

   -01

   *  Updated eIDAS example in appendix

   *  Removed text around jti claim in client attestation, refined text
      for its usage in the client attestation pop

   *  Refined text around cnf claim in client attestation

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   *  Clarified how to bind refresh tokens to a client instance using
      this client authentication method

   *  Made it more explicit that the client authentication mechanism is
      general purpose making it compatible with extensions like PAR

   *  Updated acknowledgments

   *  Simplified the diagram in the introduction

   *  Updated references

   *  Added some guidance around replay attack detection

   -00

   *  Initial draft

Acknowledgments

   We would like to thank Brian Campbell, Francesco Marino, Guiseppe De
   Marco, Kristina Yasuda, Michael B.  Jones, Takahiko Kawasaki and
   Torsten Lodderstedt for their valuable contributions to this
   specification.

Authors' Addresses

   Tobias Looker
   MATTR
   Email: tobias.looker@mattr.global

   Paul Bastian
   Email: paul.bastian@bdr.de

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