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Versions: (draft-jones-oauth-token-binding) 00 01

OAuth Working Group                                             M. Jones
Internet-Draft                                                 Microsoft
Intended status: Standards Track                              J. Bradley
Expires: March 11, 2017                                      B. Campbell
                                                           Ping Identity
                                                       September 7, 2016


                        OAuth 2.0 Token Binding
                   draft-ietf-oauth-token-binding-00

Abstract

   This specification enables OAuth 2.0 implementations to apply Token
   Binding to Access Tokens and Refresh Tokens.  This cryptographically
   binds these tokens to the TLS connections over which they are
   intended to be used.  This use of Token Binding protects these tokens
   from man-in-the-middle and token export and replay attacks.

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 http://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 March 11, 2017.

Copyright Notice

   Copyright (c) 2016 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
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
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   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of



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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Notation and Conventions . . . . . . . . . .   3
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Token Binding for Refresh Tokens  . . . . . . . . . . . . . .   3
   3.  Token Binding for Access Tokens . . . . . . . . . . . . . . .   4
     3.1.  Initial Access Tokens . . . . . . . . . . . . . . . . . .   5
     3.2.  Refreshed Access Tokens . . . . . . . . . . . . . . . . .   5
     3.3.  Resource Server Token Binding Validation  . . . . . . . .   5
     3.4.  Representing Token Binding in JWT Access Tokens . . . . .   5
   4.  Phasing in Token Binding and Preventing Downgrade Attacks . .   6
   5.  Token Binding Metadata  . . . . . . . . . . . . . . . . . . .   7
     5.1.  Token Binding Client Metadata . . . . . . . . . . . . . .   7
     5.2.  Token Binding Authorization Server Metadata . . . . . . .   7
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
     7.1.  OAuth Parameters Registration . . . . . . . . . . . . . .   8
       7.1.1.  Registry Contents . . . . . . . . . . . . . . . . . .   8
     7.2.  OAuth Dynamic Client Registration Metadata Registration .   8
       7.2.1.  Registry Contents . . . . . . . . . . . . . . . . . .   8
     7.3.  OAuth Authorization Server Discovery Metadata
           Registration  . . . . . . . . . . . . . . . . . . . . . .   8
       7.3.1.  Registry Contents . . . . . . . . . . . . . . . . . .   9
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  10
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .  10
   Appendix B.  Open Issues  . . . . . . . . . . . . . . . . . . . .  11
   Appendix C.  Document History . . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   This specification enables OAuth 2.0 [RFC6749] implementations to
   apply Token Binding The Token Binding Protocol Version 1.0
   [I-D.ietf-tokbind-protocol] Token Binding over HTTP
   [I-D.ietf-tokbind-https] to Access Tokens and Refresh Tokens.  This
   cryptographically binds these tokens to the TLS connections over
   which they are intended to be used.  This use of Token Binding
   protects these tokens from man-in-the-middle and token export and
   replay attacks.






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1.1.  Requirements Notation and Conventions

   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 RFC
   2119 [RFC2119].

1.2.  Terminology

   This specification uses the terms "Access Token", "Authorization
   Code", "Authorization Endpoint", "Authorization Grant",
   "Authorization Server", "Client", "Client Authentication", "Client
   Identifier", "Client Secret", "Grant Type", "Protected Resource",
   "Redirection URI", "Refresh Token", "Resource Owner", "Resource
   Server", "Response Type", and "Token Endpoint" defined by OAuth 2.0
   [RFC6749], the terms "Claim", "Claim Name", "Claim Value", and "JSON
   Web Token (JWT)" defined by JSON Web Token (JWT) [JWT], the term
   "User Agent" defined by RFC 7230 [RFC7230], and the terms "Provided",
   "Referred", "Token Binding" and "Token Binding ID" defined by Token
   Binding over HTTP [I-D.ietf-tokbind-https].

2.  Token Binding for Refresh Tokens

   Token Binding of refresh tokens is a straightforward first-party
   scenario, applying term "first-party" as used in Token Binding over
   HTTP [I-D.ietf-tokbind-https].  It cryptographically binds the
   refresh token to the TLS connection between the client and the token
   endpoint.  This case is straightforward because the refresh token is
   both retrieved by the client from the token endpoint and sent by the
   client to the token endpoint.  Unlike the federated scenarios
   described in Section 3 (Federation Use Cases) of Token Binding over
   HTTP [I-D.ietf-tokbind-https] and the access token case described in
   the next section, only a single TLS connection is involved in the
   refresh token case.

   Token Binding a refresh token requires that the authorization server
   do two things.  First, when refresh token is sent to the client, the
   authorization server needs to remember the Provided Token Binding ID
   and remember its association with the issued refresh token.  Second,
   when a token request containing a refresh token is received at the
   token endpoint, the authorization server needs to verify that the
   Provided Token Binding ID for the request matches the remembered
   Token Binding ID associated with the refresh token.  If the Token
   Binding IDs do not match, the authorization server should return an
   error in response to the request.

   The means by which the authorization server remembers the association
   between the refresh token and the Token Binding ID is an



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   implementation detail that beyond the scope of this specification.
   Some authorization servers will choose to store the Token Binding ID
   (or a cryptographic hash of it, such a SHA-256 hash [SHS]) in the
   refresh token itself, thus reducing the amount of state to be kept by
   the server.  Other authorization servers will add the Token Binding
   ID value (or a hash of it) to an internal data structure also
   containing other information about the refresh token, such as grant
   type information.  These choices make no difference to the client,
   since the refresh token is opaque to it.

3.  Token Binding for Access Tokens

   Token Binding for access tokens cryptographically binds the access
   token to the TLS connection between the client and the resource
   server.  Token Binding is applied to access tokens in a similar
   manner to that described in Section 3 (Federation Use Cases) of Token
   Binding over HTTP [I-D.ietf-tokbind-https].  It is also builds upon
   the mechanisms for Token Binding of ID Tokens defined in OpenID
   Connect Token Bound Authentication 1.0 [OpenID.TokenBinding].

   In the OpenID Connect [OpenID.Core] use case, HTTP redirects are used
   to pass information between the identity provider and the relying
   party; this HTTP redirect makes the Token Binding ID of the relying
   party available to the identity provider as the Referred Token
   Binding ID, information about which is then added to the ID Token.
   No such redirect occurs between the authorization server and the
   resource server in the access token case; therefore, information
   about the Token Binding ID for the TLS connection between the client
   and the resource server needs to be explicitly communicated by the
   client to the authorization server to achieve Token Binding of the
   access token.  This information is passed to the authorization server
   using this request parameter:

   resource_tbh
      Base64url encoding of the SHA-256 hash [SHS] of the Token Binding
      ID for the TLS connection between the client and the resource
      server.

   Note that to obtain this Token Binding ID, the client needs to
   establish a TLS connection between itself and the resource server
   prior to making the authorization request so that the Provided Token
   Binding ID for the TLS connection to the resource server can be
   obtained.  The means by which the client retrieves this Token Binding
   ID from the underlying Token Binding API is implementation and
   operating system specific.  An alternative, if supported, is for the
   client to generate a Token Binding key to use for the resource
   server, use the Token Binding ID for that key, and then later use




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   that key when the TLS connection to the resource server is
   established.

   The authorization server MUST ignore the "resource_tbh" parameter if
   it does not support Token Binding for the access token.

3.1.  Initial Access Tokens

   Upon receiving the hash of the Token Binding ID in an authorization
   request containing the "resource_tbh" (resource token binding hash)
   authorization request parameter, the authorization server then
   records it in the issued access token.  Alternatively, in some
   implementations, the resource's Token Binding ID hash might be
   communicated to the resource server by other means, such as by
   introspecting [RFC7662] the access token.

3.2.  Refreshed Access Tokens

   Access tokens obtained from refresh requests can also be token bound.
   In this case, the hash of the Token Binding ID of the TLS connection
   between the client and the resource server is sent to the
   authorization server at the token endpoint using the "resource_tbh"
   (resource token binding hash) token request parameter; its syntax is
   exactly the same as the corresponding authorization request
   parameter.  The authorization server then records it in the issued
   access token or communicates it to the resource server by other
   means, just as in the previous case.

3.3.  Resource Server Token Binding Validation

   Upon receiving a token bound access token, the resource server
   validates the binding by computing a SHA-256 hash of the Provided
   Token Binding ID and comparing it to the token binding hash value for
   the access token.  If these values do not match, the resource access
   attempt MUST be rejected with an error.

3.4.  Representing Token Binding in JWT Access Tokens

   If the access token is represented as a JWT, the token binding
   information SHOULD be represented in the same way that it is in token
   bound OpenID Connect ID Tokens [OpenID.TokenBinding].  That
   specification defines the new JWT Confirmation Method RFC 7800
   [RFC7800] member "tbh" (token binding hash) to represent the SHA-256
   hash of a Token Binding ID in an ID Token.  The value of the "tbh"
   member is the base64url encoding of the SHA-256 hash of the Token
   Binding ID.





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   The following example demonstrates the JWT Claims Set of an access
   token containing the base64url encoding of the SHA-256 hash of a
   Token Binding ID as the value of the "tbh" (token binding hash)
   element in the "cnf" (confirmation) claim:

     {
      "iss": "https://server.example.com",
      "aud": "https://resource.example.com",
      "iat": 1467324320,
      "exp": 1467324920,
      "cnf":{
        "tbh": "n0jI3trBK6_Gp2qiLOf48ZEZTjpBnhm-QOyzJxhBeAk"
       }
     }

4.  Phasing in Token Binding and Preventing Downgrade Attacks

   Many OAuth implementations will be deployed in situations in which
   not all participants support Token Binding.  Any of combination of
   the client, the authorization server, the resource server, and the
   User Agent may not yet support Token Binding, in which case it will
   not work end-to-end.

   It is a context-dependent deployment choice whether to allow
   interactions to proceed in which Token Binding is not supported or
   whether to treat Token Binding failures at any step as fatal errors.
   Particularly in dynamic deployment environments in which End Users
   have choices of clients, authorization servers, resource servers,
   and/or User Agents, it is RECOMMENDED that authorizations using one
   or more components that do not implement Token Binding be allowed to
   successfully proceed.  This enables different components to be
   upgraded to supporting Token Binding at different times, providing a
   smooth transition path for phasing in Token Binding.  However, when
   Token Binding has been performed, any Token Binding key mismatches
   MUST be treated as fatal errors.

   If all the participants in an authorization interaction support Token
   Binding and yet one or more of them does not use it, this is likely
   evidence of a downgrade attack.  In this case, the authorization
   SHOULD be aborted with an error.  For instance, if the resource
   server knows that the authorization server and the User Agent both
   support Token Binding and yet the access token received does not
   contain Token Binding information, this is almost certainly a sign of
   an attack.

   The authorization server and client can determine whether the other
   supports Token Binding using the metadata values defined in the next
   section.  They can determine whether the User Agent supports Token



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   Binding by whether it negotiated Token Binding for the TLS
   connection.  At present, there is no defined mechanism for
   determining whether the resource server supports Token Binding or
   not.  However, it always safe to proceed as if it does; at worst, the
   resource server simply won't verify the Token Binding.

5.  Token Binding Metadata

5.1.  Token Binding Client Metadata

   Clients supporting Token Binding that also support the OAuth 2.0
   Dynamic Client Registration Protocol [RFC7591] use these metadata
   values to register their support for Token Binding of Access Tokens
   and Refresh Tokens:

   client_access_token_token_binding_supported
      OPTIONAL.  Boolean value specifying whether the Client supports
      Token Binding of Access Tokens.  If omitted, the default value is
      "false".

   client_refresh_token_token_binding_supported
      OPTIONAL.  Boolean value specifying whether the Client supports
      Token Binding of Refresh Tokens.  If omitted, the default value is
      "false".

5.2.  Token Binding Authorization Server Metadata

   Authorization Servers supporting Token Binding that also support
   OAuth 2.0 Authorization Server Metadata [OAuth.AuthorizationMetadata]
   use these metadata values to register their support for Token Binding
   of Access Tokens and Refresh Tokens:

   as_access_token_token_binding_supported
      OPTIONAL.  Boolean value specifying whether the Authorization
      Server supports Token Binding of Access Tokens.  If omitted, the
      default value is "false".

   as_refresh_token_token_binding_supported
      OPTIONAL.  Boolean value specifying whether the Authorization
      Server supports Token Binding of Refresh Tokens.  If omitted, the
      default value is "false".

6.  Security Considerations

   If a refresh request is received by the authorization server
   containing a "resource_tbh" (resource token binding hash) value
   requesting a token bound access token and the refresh token in the
   request is not itself token bound, then it is not clear that token



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   binding the access token adds significant value.  This situation
   should be considered an open issue for discussion by the working
   group.

7.  IANA Considerations

7.1.  OAuth Parameters Registration

   This specification registers the following parameter in the IANA
   "OAuth Parameters" registry [IANA.OAuth.Parameters] established by
   RFC 6749 [RFC6749]:

7.1.1.  Registry Contents

   o  Parameter name: "resource_tbh"
   o  Parameter usage location: Authorization Request, Token Request
   o  Change controller: IESG
   o  Specification document(s): Section 3 of this document
   o  Related information: None

7.2.  OAuth Dynamic Client Registration Metadata Registration

   This specification registers the following client metadata
   definitions in the IANA "OAuth Dynamic Client Registration Metadata"
   registry [IANA.OAuth.Parameters] established by [RFC7591]:

7.2.1.  Registry Contents

   o  Client Metadata Name:
      "client_access_token_token_binding_supported"
   o  Client Metadata Description: Boolean value specifying whether the
      Client supports Token Binding of Access Tokens
   o  Change Controller: IESG
   o  Specification Document(s): Section 5.1 of [[ this specification ]]

   o  Client Metadata Name:
      "client_refresh_token_token_binding_supported"
   o  Client Metadata Description: Boolean value specifying whether the
      Client supports Token Binding of Refresh Tokens
   o  Change Controller: IESG
   o  Specification Document(s): Section 5.1 of [[ this specification ]]

7.3.  OAuth Authorization Server Discovery Metadata Registration

   This specification registers the following discovery metadata
   definitions in the IANA "OAuth Authorization Server Discovery
   Metadata" registry established by [OAuth.AuthorizationMetadata]:




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7.3.1.  Registry Contents

   o  Discovery Metadata Name: "as_access_token_token_binding_supported"
   o  Discovery Metadata Description: Boolean value specifying whether
      the Authorization Server supports Token Binding of Access Tokens
   o  Change Controller: IESG
   o  Specification Document(s): Section 5.2 of [[ this specification ]]

   o  Discovery Metadata Name:
      "as_refresh_token_token_binding_supported"
   o  Discovery Metadata Description: Boolean value specifying whether
      the Authorization Server supports Token Binding of Refresh Tokens
   o  Change Controller: IESG
   o  Specification Document(s): Section 5.2 of [[ this specification ]]

8.  References

8.1.  Normative References

   [I-D.ietf-tokbind-https]
              Popov, A., Nystrom, M., Balfanz, D., Langley, A., and J.
              Hodges, "Token Binding over HTTP", draft-ietf-tokbind-
              https-06 (work in progress), August 2016.

   [I-D.ietf-tokbind-protocol]
              Popov, A., Nystrom, M., Balfanz, D., Langley, A., and J.
              Hodges, "The Token Binding Protocol Version 1.0", draft-
              ietf-tokbind-protocol-10 (work in progress), September
              2016.

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

   [JWT]      Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
              (JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,
              <http://tools.ietf.org/html/rfc7519>.

   [OpenID.TokenBinding]
              Jones, M., Bradley, J., and B. Campbell, "OpenID Connect
              Token Bound Authentication 1.0", July 2016,
              <http://openid.net/specs/
              openid-connect-token-bound-authentication-1_0.html>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.



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

   [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing",
              RFC 7230, DOI 10.17487/RFC7230, June 2014,
              <http://www.rfc-editor.org/info/rfc7230>.

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

   [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,
              <http://www.rfc-editor.org/info/rfc7800>.

   [SHS]      National Institute of Standards and Technology, "Secure
              Hash Standard (SHS)", FIPS PUB 180-4, March 2012,
              <http://csrc.nist.gov/publications/fips/fips180-4/
              fips-180-4.pdf>.

8.2.  Informative References

   [OAuth.AuthorizationMetadata]
              Jones, M., Sakimura, N., and J. Bradley, "OAuth 2.0
              Authorization Server Metadata", draft-ietf-oauth-
              discovery-02 (work in progress), August 2016,
              <http://tools.ietf.org/html/
              draft-ietf-oauth-discovery-04>.

   [OpenID.Core]
              Sakimura, N., Bradley, J., Jones, M., de Medeiros, B., and
              C. Mortimore, "OpenID Connect Core 1.0", November 2014,
              <http://openid.net/specs/openid-connect-core-1_0.html>.

   [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,
              <http://www.rfc-editor.org/info/rfc7591>.

Appendix A.  Acknowledgements

   The authors would like to thank the following people for their
   contributions to the specification: Dirk Balfanz, William Denniss,
   Andrei Popov, and Nat Sakimura.




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Appendix B.  Open Issues

   o  Some token binding implementations apparently provide APIs that
      enable native applications to provide Referred Token Bindings,
      just as the federation support in the HTTPS Token Binding spec
      does.  Can we count on these APIs being supported on all
      platforms, and if so, does this enable us to somehow do without
      the "resource_tbh" parameter by mandating that the client send
      both a Provided and a Referred Token Binding to the authorization
      server?  If this isn't the case, is "resource_tbh" actually secure
      or does this open a cross-channel validation hole?  This area
      probably needs more attention from both the Token Binding and
      OAuth working groups.
   o  How should we support crypto agility for the hash function?

Appendix C.  Document History

   [[ to be removed by the RFC Editor before publication as an RFC ]]

   -00

   o  Created the initial working group version from draft-jones-oauth-
      token-binding-00.

Authors' Addresses

   Michael B. Jones
   Microsoft

   Email: mbj@microsoft.com
   URI:   http://self-issued.info/


   John Bradley
   Ping Identity

   Email: ve7jtb@ve7jtb.com
   URI:   http://www.thread-safe.com/


   Brian Campbell
   Ping Identity

   Email: brian.d.campbell@gmail.com
   URI:   https://twitter.com/__b_c






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