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Versions: 00 01 02 03 04

Web Authorization Protocol                                       D. Fett
Internet-Draft                                                   yes.com
Intended status: Standards Track                              J. Bradley
Expires: September 28, 2019                                       Yubico
                                                             B. Campbell
                                                           Ping Identity
                                                          T. Lodderstedt
                                                                 yes.com
                                                                M. Jones
                                                               Microsoft
                                                          March 27, 2019


OAuth 2.0 Demonstration of Proof-of-Possession at the Application Layer
                        draft-fett-oauth-dpop-00

Abstract

   This document defines a sender-constraint mechanism for OAuth 2.0
   access tokens and refresh tokens utilizing an application-level
   proof-of-possession mechanism based on public/private key pairs.

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 September 28, 2019.

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   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



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   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  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Conventions and Terminology . . . . . . . . . . . . . . .   3
   2.  Main Objective  . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Concept . . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Token Request (Binding Tokens to a Public Key)  . . . . . . .   5
   5.  Resource Access (Proof of Possession for Access Tokens) . . .   7
   6.  Refresh Token Usage (Proof of Possession for Refresh Tokens)    8
   7.  Public Key Confirmation . . . . . . . . . . . . . . . . . . .   8
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   9
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
     9.1.  JWT Confirmation Methods Registration . . . . . . . . . .   9
     9.2.  OAuth Parameters Registry . . . . . . . . . . . . . . . .  10
     9.3.  JSON Web Signature and Encryption Type Values
           Registration  . . . . . . . . . . . . . . . . . . . . . .  10
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  10
     10.1.  Token Replay at the same authorization server  . . . . .  11
     10.2.  Token Replay at the same resource server endpoint  . . .  11
     10.3.  Signed JWT Swapping  . . . . . . . . . . . . . . . . . .  11
     10.4.  Comparison to mTLS and OAuth Token Binding . . . . . . .  11
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  11
     11.2.  Informative References . . . . . . . . . . . . . . . . .  11
     11.3.  URIs . . . . . . . . . . . . . . . . . . . . . . . . . .  12
   Appendix A.  Document History . . . . . . . . . . . . . . . . . .  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13

1.  Introduction

   [I-D.ietf-oauth-mtls] describes methods to bind (sender-constrain)
   access tokens using mutual Transport Layer Security (TLS)
   authentication with X.509 certificates.

   [I-D.ietf-oauth-token-binding] provides mechanisms to sender-
   constrain access tokens using HTTP token binding.

   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.




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   This document defines an application-level sender-constraint
   mechanism for OAuth 2.0 access tokens and refresh tokens that can be
   applied when neither mTLS nor OAuth Token Binding are utilized.  It
   achieves proof-of-possession using a public/private key pair.

1.1.  Conventions and Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   This specification uses the terms "access token", "refresh token",
   "authorization server", "resource server", "authorization endpoint",
   "authorization request", "authorization response", "token endpoint",
   "grant type", "access token request", "access token response", and
   "client" defined by The OAuth 2.0 Authorization Framework [RFC6749].

2.  Main Objective

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

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

   Secondary objectives are discussed in Section 10.

3.  Concept


















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   +--------+                               +---------------+
   |        |                               |               |
   |        |--(A)- Authorization Request ->|   Resource    |
   |        |                               |     Owner     |
   |        |<-(B)-- Authorization Grant ---|               |
   |        |                               +---------------+
   |        |
   |        |                               +---------------+
   |        |--(C)-- Token Request -------->|               |
   | Client |        (DPop-Binding)         | Authorization |
   |        |                               |     Server    |
   |        |<-(D)-- PoP Access Token ------|               |
   |        |                               +---------------+
   |        |        PoP Refresh Token for public clients
   |        |
   |        |                               +---------------+
   |        |--(E)-- PoP Access Token ----->|               |
   |        |        (DPoP-Proof)           |    Resource   |
   |        |                               |     Server    |
   |        |<-(F)--- Protected Resource ---|               |
   |        |                               +---------------+
   |        |
   |        | public client refresh token usage:
   |        |                               +---------------+
   |        |--(G)-- PoP Refresh Token ---->|               |
   |        |        (DPoP-Proof)           | Authorization |
   |        |                               |     Server    |
   |        |<-(H)-- PoP Access Token ------|               |
   |        |                               +---------------+
   |        |
   +--------+

   Figure 1: Basic DPoP Flow

   The new elements introduced by this specification are shown in
   Figure 1:

   o  In the Token Request (C), the client proves the possession of a
      private key belonging to some public key by using the private key
      to sign the authorization code.  The matching public key is sent
      in the same request.

   o  The AS binds (sender-constrains) the access token to the public
      key claimed by the client; 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
      "bearer+dpop".  If a refresh token is issued to the client, it is
      sender-constrained in the same way if the client is a public



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      client and thus is not able to authenticate requests to the token
      endpoint.

   o  If the client wants to use the access token (E) or the (public)
      client wants to use a refresh token, the client has to prove
      possession of the private key by signing a message containing the
      respective token, the endpoint URL, and the request method.  This
      signature is provided as a signed JWT.

   o  In the case of the refresh token, the AS can immediately check
      that the JWT was signed using the matching private key claimed in
      request (C).

   o  In the case of the access token, the resource server needs to
      receive information about which public key to check against.  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 (request not
      shown).

   The mechanism presented herein is not a client authentication method.
   In fact, a primary use case are public clients (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.

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

   To bind an tokens to a public key in the token request, the client
   MUST provide a public key and prove the possession of the
   corresponding private key.  The following HTTPS request illustrates
   the protocol for this (with extra line breaks for display purposes
   only):

   POST /token HTTP/1.1
   Host: server.example.com
   Authorization: Basic czZCaGRSa3F0MzpnWDFmQmF0M2JW
   Content-Type: application/x-www-form-urlencoded;charset=UTF-8

   grant_type=authorization_code
   &code=SplxlOBeZQQYbYS6WxSbIA
   &redirect_uri=https%3A%2F%2Fclient%2Eexample%2Ecom%2Fcb
   &token_type=bearer+dpop
   &dpop_binding=eyJhbGciOiJSU0ExXzUi ...
   (remainder of JWK omitted for brevity)

   Figure 2: Token Request for a DPoP bound token.




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   The parameter "dpop_binding" MUST contain a JWT signed using the
   asymmetric key chosen by the client.  The header of the JWT contains
   the following fields:

   o  "typ": with value "dpop_binding+jwt" (REQUIRED).

   o  "jwk": The public key chosen by the client, in JWK format
      (REQUIRED).

   The body of the JWT contains the following fields:

   o  "http_method": The HTTP method used for the request (REQUIRED).

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

   o  "exp": Expiration time of the JWT (REQUIRED).  See Security
      Considerations [1].

   o  "jti": Unique, freshly chosen identifier for this JWT (REQUIRED).
      SHOULD be used by the AS for replay detection and prevention.  See
      Security Considerations [2].

   An example JWT is shown in Figure 3.

   {
       "typ": "dpop_binding+jwt",
       "alg": "ES512",
       "jwk": {
            "kty" : "EC",
            "kid" : "11",
            "crv" : "P-256",
            "x" : "usWxHK2PmfnHKwXPS54m0kTcGJ90UiglWiGahtagnv8",
            "y" : "3BttVivg+lSreASjpkttcsz+1rb7btKLv8EX4"
        }

   }.{
       "jti": "HK2PmfnHKwXP",
       "http_method": "get",
       "http_uri": "https://resource-server.example.com?path=something",
       "exp": "..."
   }

   Figure 3: Example JWT for "dpop_binding" parameter.

   If the authorization server receives a "dpop_binding" parameter in a
   token request, the authorization server MUST check that:




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   o  the parameter value is a well-formed JWT,

   o  all required claims are contained in the JWT,

   o  the algorithm in the header of the JWT is supported by the
      application and deemed secure,

   o  the JWT is signed using the public key contained in the header of
      the JWT,

   o  the "typ" field in the header has the correct value,

   o  the "http_method" and "http_uri" claims match the respective
      values for the HTTP request in which the parameter was received,

   o  the token has not expired, and

   o  if replay protection is desired, that a JWT with the same "jti"
      value has not been received previously.

   If these checks are successful, the authorization server MUST
   associate the access token with the public key.  It then sets
   "token_type" to "bearer+dpop" in the token response.

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

   To make use of an access token that is token bound to a public key
   using DPoP, a client MUST prove the possession of the corresponding
   private key.  More precisely, the client MUST create a JWT and sign
   it using the previously chosen private key.

   The JWT has the same format as above, except:

   o  The header MUST contain a "typ" claim with the value
      "dpop_proof+jwt".

   o  The header SHOULD not contain a "jwk" field.

   The signed JWT MUST then be sent in the "dpop_proof" request
   parameter.

   If a resource server detects that an access token that is to be used
   for resource access is bound to a public key using DPoP (via the
   methods described in Section 7) it MUST check that:

   o  a parameter "dpop_proof" was received in the HTTP request,

   o  the parameter's value is a well-formed JWT,



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   o  all required claims are contained in the JWT,

   o  the algorithm in the header of the JWT is supported by the
      application and deemed secure,

   o  the JWT is signed using the public key to which the access token
      was bound,

   o  the "typ" field in the header has the correct value,

   o  the "http_method" and "http_uri" claims match the respective
      values for the HTTP request in which the parameter was received,

   o  the token has not expired, and

   o  if replay protection is desired, that a JWT with the same "jti"
      value has not been received previously.

   If any of these checks fails, the resource server MUST NOT grant
   access to the resource.

6.  Refresh Token Usage (Proof of Possession for Refresh Tokens)

   At the token endpoint, public clients MUST provide a proof of
   possession in the same way as for access tokens.

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 "dpop+jwk" of the "cnf" claim, as shown in Figure 4.
















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   {
       "iss": "https://server.example.com",
       "sub": "something@example.com",
       "exp": 1493726400,
       "nbf": 1493722800,
       "cnf":{
           "dpop+jwk": {
               "kty" : "EC",
               "kid" : "11",
               "crv" : "P-256",
               "x" : "usWxHK2PmfnHKwXPS54m0kTcGJ90UiglWiGahtagnv8",
               "y" : "3BttVivg+lSreASjpkttcsz+1rb7btKLv8EX4"
           }
       }
   }

   Figure 4: Example access token with "cnf" claim.

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

8.  Acknowledgements

   This document resulted from discussions at the 4th OAuth Security
   Workshop in Stuttgart, Germany.  We thank the organizers of this
   workshop (Ralf Kuesters, Guido Schmitz).

9.  IANA Considerations

9.1.  JWT Confirmation Methods Registration

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

   o  Confirmation Method Value: "dpop+jwk"

   o  Confirmation Method Description: JWK encoded public key for dpop
      proof token

   o  Change Controller: IESG

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








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9.2.  OAuth Parameters Registry

   This specification registers the following parameters in the IANA
   "OAuth Parameters" registry defined in OAuth 2.0 [RFC6749].

   o  Parameter name: dpop_binding

   o  Parameter usage location: token request

   o  Change controller: IESG

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

   o  Parameter name: dpop_proof

   o  Parameter usage location: token request

   o  Change controller: IESG

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

9.3.  JSON Web Signature and Encryption Type Values Registration

   This specification registers the "dpop+jwt" type value in the IANA
   JSON Web Signature and Encryption Type Values registry [RFC7515]:

   o  "typ" Header Parameter Value: "dpop_proof+jwt"

   o  Abbreviation for MIME Type: None

   o  Change Controller: IETF

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

   o  "typ" Header Parameter Value: "dpop_binding+jwt"

   o  Abbreviation for MIME Type: None

   o  Change Controller: IETF

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

10.  Security Considerations

   The Prevention of Token Replay at a Different Endpoint [3] is
   achieved through the binding of the DPoP JWT to a certain URI and
   HTTP method.




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10.1.  Token Replay at the same authorization server

   If an adversary is able to get hold of an DPoP-Binding JWT, it might
   replay it at the authorization server's token endpoint with the same
   or different payload.  The issued access token is useless as long as
   the adversary does not get hold of a valid DPoP-Binding JWT for the
   corresponding resource server.

10.2.  Token Replay at the same resource server endpoint

   If an adversary is able to get hold of a DPoP-Proof JWT, the
   adversary could replay that token later at the same endpoint (the
   HTTP endpoint and method are enforced via the respective claims in
   the JWTs).  To prevent this, clients MUST limit the lifetime of the
   JWTs, preferably to a brief period.  Furthermore, the "jti" claim in
   each JWT MUST contain a unique (incrementing or randomly chosen)
   value, as proposed in [RFC7253].  Resource servers SHOULD store
   values at least for the lifetime of the respective JWT and decline
   HTTP requests by clients if a "jti" value has been seen before.

10.3.  Signed JWT Swapping

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

10.4.  Comparison to mTLS and OAuth Token Binding

   o  mTLS stronger against intercepted connections

11.  References

11.1.  Normative References

   [RFC7253]  Krovetz, T. and P. Rogaway, "The OCB Authenticated-
              Encryption Algorithm", RFC 7253, DOI 10.17487/RFC7253, May
              2014, <https://www.rfc-editor.org/info/rfc7253>.

   [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>.

11.2.  Informative References








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   [I-D.ietf-oauth-mtls]
              Campbell, B., Bradley, J., Sakimura, N., and T.
              Lodderstedt, "OAuth 2.0 Mutual TLS Client Authentication
              and Certificate-Bound Access Tokens", draft-ietf-oauth-
              mtls-13 (work in progress), February 2019.

   [I-D.ietf-oauth-security-topics]
              Lodderstedt, T., Bradley, J., Labunets, A., and D. Fett,
              "OAuth 2.0 Security Best Current Practice", draft-ietf-
              oauth-security-topics-12 (work in progress), March 2019.

   [I-D.ietf-oauth-token-binding]
              Jones, M., Campbell, B., Bradley, J., and W. Denniss,
              "OAuth 2.0 Token Binding", draft-ietf-oauth-token-
              binding-08 (work in progress), October 2018.

   [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>.

   [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>.

   [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>.

   [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>.

   [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>.

11.3.  URIs

   [1] #Security

   [2] #Security

   [3] #Objective_Replay_Different_Endpoint






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Appendix A.  Document History

   [[ To be removed from the final specification ]]

   -00

   o  first draft

Authors' Addresses

   Daniel Fett
   yes.com

   Email: mail@danielfett.de


   John Bradley
   Yubico

   Email: ve7jtb@ve7jtb.com


   Brian Campbell
   Ping Identity

   Email: bcampbell@pingidentity.com


   Torsten Lodderstedt
   yes.com

   Email: torsten@lodderstedt.net


   Michael Jones
   Microsoft

   Email: mbj@microsoft.com













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