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Versions: (draft-jones-oauth-token-binding) 00 01 02 03

OAuth Working Group                                             M. Jones
Internet-Draft                                                 Microsoft
Intended status: Standards Track                              J. Bradley
Expires: September 14, 2017                                  B. Campbell
                                                           Ping Identity
                                                              W. Denniss
                                                                  Google
                                                          March 13, 2017


                        OAuth 2.0 Token Binding
                   draft-ietf-oauth-token-binding-02

Abstract

   This specification enables OAuth 2.0 implementations to apply Token
   Binding to Access Tokens, Authorization Codes, and Refresh Tokens.
   This cryptographically binds these tokens to a client's Token Binding
   key pair, possession of which is proven on the TLS connections over
   which the tokens 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 September 14, 2017.

Copyright Notice

   Copyright (c) 2017 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



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   publication of this document.  Please review these documents
   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  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Requirements Notation and Conventions . . . . . . . . . .   3
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Token Binding for Refresh Tokens  . . . . . . . . . . . . . .   3
     2.1.  Example Token Binding for Refresh Tokens  . . . . . . . .   4
   3.  Token Binding for Access Tokens . . . . . . . . . . . . . . .   6
     3.1.  Access Tokens Issued from the Authorization Endpoint  . .   7
       3.1.1.  Example Access Token Issued from the Authorization
               Endpoint  . . . . . . . . . . . . . . . . . . . . . .   8
     3.2.  Access Tokens Issued from the Token Endpoint  . . . . . .   9
       3.2.1.  Example Access Token Issued from the Token Endpoint .   9
     3.3.  Protected Resource Token Binding Validation . . . . . . .  11
       3.3.1.  Example Protected Resource Request  . . . . . . . . .  11
     3.4.  Representing Token Binding in JWT Access Tokens . . . . .  11
   4.  Token Binding for Authorization Codes . . . . . . . . . . . .  12
     4.1.  Native Application Clients  . . . . . . . . . . . . . . .  12
       4.1.1.  Code Challenge  . . . . . . . . . . . . . . . . . . .  13
         4.1.1.1.  Example Code Challenge  . . . . . . . . . . . . .  13
       4.1.2.  Code Verifier . . . . . . . . . . . . . . . . . . . .  14
         4.1.2.1.  Example Code Verifier . . . . . . . . . . . . . .  14
     4.2.  Web Server Clients  . . . . . . . . . . . . . . . . . . .  15
       4.2.1.  Code Challenge  . . . . . . . . . . . . . . . . . . .  15
         4.2.1.1.  Example Code Challenge  . . . . . . . . . . . . .  16
       4.2.2.  Code Verifier . . . . . . . . . . . . . . . . . . . .  16
         4.2.2.1.  Example Code Verifier . . . . . . . . . . . . . .  17
   5.  Phasing in Token Binding and Preventing Downgrade Attacks . .  18
   6.  Token Binding Metadata  . . . . . . . . . . . . . . . . . . .  18
     6.1.  Token Binding Client Metadata . . . . . . . . . . . . . .  18
     6.2.  Token Binding Authorization Server Metadata . . . . . . .  19
     6.3.  Token Binding Protected Resource Metadata . . . . . . . .  19
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  19
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  20
     8.1.  OAuth Dynamic Client Registration Metadata Registration .  20
       8.1.1.  Registry Contents . . . . . . . . . . . . . . . . . .  20
     8.2.  OAuth Authorization Server Metadata Registration  . . . .  20
       8.2.1.  Registry Contents . . . . . . . . . . . . . . . . . .  20
     8.3.  OAuth Protected Resource Metadata Registration  . . . . .  21
       8.3.1.  Registry Contents . . . . . . . . . . . . . . . . . .  21
     8.4.  PKCE Code Challenge Method Registration . . . . . . . . .  21



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       8.4.1.  Registry Contents . . . . . . . . . . . . . . . . . .  21
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  21
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  21
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  23
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .  24
   Appendix B.  Open Issues  . . . . . . . . . . . . . . . . . . . .  24
   Appendix C.  Document History . . . . . . . . . . . . . . . . . .  24
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  25

1.  Introduction

   This specification enables OAuth 2.0 [RFC6749] implementations to
   apply Token Binding (TLS Extension for Token Binding Protocol
   Negotiation [I-D.ietf-tokbind-negotiation], The Token Binding
   Protocol Version 1.0 [I-D.ietf-tokbind-protocol] and Token Binding
   over HTTP [I-D.ietf-tokbind-https]) to Access Tokens, Authorization
   Codes, and Refresh Tokens.  This cryptographically binds these tokens
   to a client's Token Binding key pair, possession of which is proven
   on the TLS connections over which the tokens are intended to be used.
   This use of Token Binding protects these tokens from man-in-the-
   middle and token export and replay attacks.

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 Server", "Client",
   "Protected Resource", "Refresh Token", and "Token Endpoint" defined
   by OAuth 2.0 [RFC6749], the terms "Claim" 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 client's Token Binding key pair, possession of
   which is proven on the TLS connections between the client and the
   token endpoint.  This case is straightforward because the refresh



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   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 4 (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.

   How the authorization server remembers the association between the
   refresh token and the Token Binding ID is an 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,
   provided it is integrity-protected, 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.

2.1.  Example Token Binding for Refresh Tokens

   This section provides an example of what the interactions around a
   Token Bound refresh token might look like, along with some details of
   the involved processing.  Token Binding of refresh tokens is most
   useful for native application clients so the example has protocol
   elements typical of a native client flow.  Extra line breaks in all
   examples are for display purposes only.

   A native application client makes the following access token request
   with an authorization code using a TLS connection where Token Binding
   has been negotiated.  A PKCE "code_verifier" is included because use
   the of PKCE is considered best practice for native application
   clients [I-D.ietf-oauth-native-apps].  The base64url-encoded
   representation of the exported keying material (EKM) from that TLS
   connection is "p6ZuSwfl6pIe8es5KyeV76T4swZmQp0_awd27jHfrbo", which is
   needed to validate the Token Binding Message.




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    POST /as/token.oauth2 HTTP/1.1
    Host: server.example.com
    Content-Type: application/x-www-form-urlencoded
    Sec-Token-Binding: AIkAAgBBQGto7hHRR0Y5nkOWqc9KNfwW95dEFmSI_tCZ_Cbl
      7LWlt6Xjp3DbjiDJavGFiKP2HV_2JSE42VzmKOVVV8m7eqAAQOKiDK1Oi0z6v4X5B
      P7uc0pFestVZ42TTOdJmoHpji06Qq3jsCiCRSJx9ck2fWJYx8tLVXRZPATB3x6c24
      aY0ZEAAA

    grant_type=authorization_code&code=4bwcZesc7Xacc330ltc66Wxk8EAfP9j2
      &code_verifier=2x6_ylS390-8V7jaT9wj.8qP9nKmYCf.V-rD9O4r_1
      &client_id=example-native-client-id

                    Figure 1: Initial Request with Code

   A refresh token is issued in response to the prior request.  Although
   it looks like a typical response to the client, the authorization
   server has bound the refresh token to the Provided Token Binding ID
   from the encoded Token Binding message in the "Sec-Token-Binding"
   header of the request.  In this example, that binding is done by
   saving the Token Binding ID alongside other information about the
   refresh token in some server side persistent storage.  The base64url-
   encoded representation of that Token Binding ID is "AgBBQGto7hHRR0Y5n
   kOWqc9KNfwW95dEFmSI_tCZ_Cbl7LWlt6Xjp3DbjiDJavGFiKP2HV_2JSE42VzmKOVVV8
   m7eqA".

    HTTP/1.1 200 OK
    Content-Type: application/json
    Cache-Control: no-cache, no-store

    {
     "access_token":"EdRs7qMrLb167Z9fV2dcwoLTC",
     "refresh_token":"ACClZEIQTjW9arT9GOJGGd7QNwqOMmUYfsJTiv8his4",
     "token_type":"Bearer",
     "expires_in":3600
    }

                       Figure 2: Successful Response

   When the access token expires, the client requests a new one with a
   refresh request to the token endpoint.  In this example, the request
   is made on a new TLS connection so the EKM (base64url-encoded: "va-
   84Ukw4Zqfd7uWOtFrAJda96WwgbdaPDX2knoOiAE") and signature in the Token
   Binding Message are different than in the initial request.








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    POST /as/token.oauth2 HTTP/1.1
    Host: server.example.com
    Content-Type: application/x-www-form-urlencoded
    Sec-Token-Binding: AIkAAgBBQGto7hHRR0Y5nkOWqc9KNfwW95dEFmSI_tCZ_Cbl
      7LWlt6Xjp3DbjiDJavGFiKP2HV_2JSE42VzmKOVVV8m7eqAAQCpGbaG_YRf27qOra
      L0UT4fsKKjL6PukuOT00qzamoAXxOq7m_id7O3mLpnb_sM7kwSxLi7iNHzzDgCAkP
      t3lHwAAA

   refresh_token=ACClZEIQTjW9arT9GOJGGd7QNwqOMmUYfsJTiv8his4
     &grant_type=refresh_token&client_id=example-native-client-id

                         Figure 3: Refresh Request

   However, because the Token Binding ID is long-lived and may span
   multiple TLS sessions and connections, it is the same as in the
   initial request.  That Token Binding ID is what the refresh token is
   bound to, so the authorization server is able to verify it and issue
   a new access token.

    HTTP/1.1 200 OK
    Content-Type: application/json
    Cache-Control: no-cache, no-store

    {
     "access_token":"bwcESCwC4yOCQ8iPsgcn117k7",
     "token_type":"Bearer",
     "expires_in":3600
    }

                       Figure 4: Successful Response

3.  Token Binding for Access Tokens

   Token Binding for access tokens cryptographically binds the access
   token to the client's Token Binding key pair, possession of which is
   proven on the TLS connections between the client and the protected
   resource.  Token Binding is applied to access tokens in a similar
   manner to that described in Section 4 (Federation Use Cases) of Token
   Binding over HTTP [I-D.ietf-tokbind-https].  It 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



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   protected resource in the access token case; therefore, information
   about the Token Binding ID for the TLS connection between the client
   and the protected resource 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 the
   Referred Token Binding ID, just as in the ID Token case.  The only
   difference is that the client needs to explicitly communicate the
   Token Binding ID of the TLS connection between the client and the
   protected resource to the Token Binding implementation so that it is
   sent as the Referred Token Binding ID in the request to the
   authorization server.  This functionality provided by Token Binding
   implementations is described in Section 5 (Implementation
   Considerations) of Token Binding over HTTP [I-D.ietf-tokbind-https].

   Note that to obtain this Token Binding ID, the client may need to
   establish a TLS connection between itself and the protected resource
   prior to making the request to the authorization server so that the
   Provided Token Binding ID for the TLS connection to the protected
   resource can be obtained.  How 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 protected
   resource, use the Token Binding ID for that key, and then later use
   that key when the TLS connection to the protected resource is
   established.

3.1.  Access Tokens Issued from the Authorization Endpoint

   For access tokens returned directly from the authorization endpoint,
   such as with the implicit grant defined in Section 4.2 of OAuth 2.0
   [RFC6749], the Token Binding ID of the client's TLS channel to the
   protected resource is sent with the authorization request as the
   Referred Token Binding ID in the "Sec-Token-Binding" header, and is
   used to Token Bind the access token.

   Upon receiving the Referred Token Binding ID in an authorization
   request, the authorization server associates (Token Binds) the ID
   with the access token in a way that can be accessed by the protected
   resource.  Such methods include embedding the Referred Token Binding
   ID (or a cryptographic hash of it) in the issued access token itself,
   possibly using the syntax described at Section 3.4, or through token
   introspection [RFC7662].  The method for associating the referred
   token binding ID with the access token is determined by the
   authorization server and the protected resource, and is beyond the
   scope for this specification.




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3.1.1.  Example Access Token Issued from the Authorization Endpoint

   This section provides an example of what the interactions around a
   Token Bound access token issued from the authorization endpoint might
   look like, along with some details of the involved processing.  Extra
   line breaks in all examples are for display purposes only.

   The client directs the user-agent to make the following HTTP request
   to the authorization endpoint.  It is a typical authorization request
   that, because Token Binding was negotiated on the underlying TLS
   connection and the user-agent was signaled to reveal the Referred
   Token Binding, also includes the "Sec-Token-Binding" header with a
   Token Binding Message that contains both a Provided and Referred
   Token Binding.  The base64url-encoded EKM from the TLS connection
   over which the request was made is
   "jI5UAyjs5XCPISUGQIwgcSrOiVIWq4fhLVIFTQ4nLxc".

    GET /as/authorization.oauth2?response_type=token
      &client_id=example-client-id&state=rM8pZxG1c3gKy6rEbsD8s
      &redirect_uri=https%3A%2F%2Fclient%2Eexample%2Eorg%2Fcb HTTP/1.1
    Host: server.example.com
    Sec-Token-Binding: ARIAAgBBQIEE8mSMtDy2dj9EEBdXaQT9W3Rq1NS-jW8ebPoF
      6FyL0jIfATVE55zlircgOTZmEg1xeIrC3DsGegwjs4bhw14AQGKDlAXFFMyQkZegC
      wlbTlqX3F9HTt-lJxFU_pi16ezka7qVRCpSF0BQLfSqlsxMbYfSSCJX1BDtrIL7PX
      j__fUAAAECAEFA1BNUnP3te5WrwlEwiejEz0OpesmC5PElWc7kZ5nlLSqQTj1ciIp
      5vQ30LLUCyM_a2BYTUPKtd5EdS-PalT4t6ABADgeizRa5NkTMuX4zOdC-R4cLNWVV
      O8lLu2Psko-UJLR_XAH4Q0H7-m0_nQR1zBN78nYMKPvHsz8L3zWKRVyXEgAA

                      Figure 5: Authorization Request

   The authorization server issues an access token and delivers it to
   the client by redirecting the user-agent with the following HTTP
   response:

    HTTP/1.1 302 Found
    Location: https://client.example.org/cb#state=rM8pZxG1c3gKy6rEbsD8s
      &expires_in=3600&token_type=Bearer
      &access_token=eyJhbGciOiJFUzI[...omitted for brevity...]8xy5W5sQ

                      Figure 6: Authorization Request

   The access token is bound to the Referred Token Binding ID from the
   authorization request, which when represented as a JWT, as described
   in Section 3.4, contains the SHA-256 hash of the Token Binding ID as
   the value of the "tbh" (token binding hash) member of the "cnf"
   (confirmation) claim.  The confirmation claim portion of the JWT
   Claims Set is shown in the following figure.




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    {
      ...other claims omitted for brevity...
      "cnf":{
         "tbh": "vowQESa_MgbGJwIXaFm_BTN2QDPwh8PhuBm-EtUAqxc"
      }
    }

                       Figure 7: Confirmation Claim

3.2.  Access Tokens Issued from the Token Endpoint

   For access tokens returned from the token endpoint, the Token Binding
   ID of the client's TLS channel to the protected resource is sent as
   the Referred Token Binding ID in the "Sec-Token-Binding" header, and
   is used to Token Bind the access token.  This applies to all the
   grant types from OAuth 2.0 [RFC6749] using the token endpoint,
   including, but not limited to the refresh and authorization code
   token requests, as well as some extension grants, such as JWT
   assertion authorization grants [RFC7523].

   Upon receiving the Referred Token Binding ID in a token request, the
   authorization server associates (Token Binds) the ID with the access
   token in a way that can be accessed by the protected resource.  Such
   methods include embedding the Referred Token Binding ID (or a
   cryptographic hash of it) in the issued access token itself, possibly
   using the syntax described at Section 3.4, or through token
   introspection [RFC7662].  The method for associating the referred
   token binding ID with the access token is determined by the
   authorization server and the protected resource, and is beyond the
   scope for this specification.

   Note that if the request results in a new refresh token being
   generated, it can be Token bound using the Provided Token Binding ID,
   per Section 2.

3.2.1.  Example Access Token Issued from the Token Endpoint

   This section provides an example of what the interactions around a
   Token Bound access token issued from the token endpoint might look
   like, along with some details of the involved processing.  Extra line
   breaks in all examples are for display purposes only.

   The client makes an access token request to the token endpoint and
   includes the "Sec-Token-Binding" header with a Token Binding Message
   that contains both Provided and Referred Token Binding IDs.  The
   Provided Token Binding ID is used to validate the token binding of
   the refresh token in the request (and to Token Bind a new refresh
   token, if one is issued), and the Referred Token Binding ID is used



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   to Token Bind the access token that is generated.  The base64url-
   encoded EKM from the TLS connection over which the access token
   request was made is "4jTc5e1QpocqPTZ5l6jsb6pRP18IFKdwwPvasYjn1-E".

    POST /as/token.oauth2 HTTP/1.1
    Host: server.example.com
    Content-Type: application/x-www-form-urlencoded
    Sec-Token-Binding: ARIAAgBBQJFXJir2w4gbJ7grBx9uTYWIrs9V50-PW4ZijegQ
      0LUM-_bGnGT6DizxUK-m5n3dQUIkeH7ybn6wb1C5dGyV_IAAQDDFToFrHt41Zppq7
      u_SEMF_E-KimAB-HewWl2MvZzAQ9QKoWiJCLFiCkjgtr1RrA2-jaJvoB8o51DTGXQ
      ydWYkAAAECAEFAuC1GlYU83rqTGHEau1oqvNwy0fDsdXzIyT_4x1FcldsMxjFkJac
      IBJFGuYcccvnCak_duFi3QKFENuwxql-H9ABAMcU7IjJOUA4IyE6YoEcfz9BMPQqw
      M5M6hw4RZNQd58fsTCCslQE_NmNCl9JXy4NkdkEZBxqvZGPr0y8QZ_bmAwAA

    refresh_token=gZR_ZI8EAhLgWR-gWxBimbgZRZi_8EAhLgWRgWxBimbf
     &grant_type=refresh_token&client_id=example-client-id

                      Figure 8: Access Token Request

   The authorization server issues an access token bound to the Referred
   Token Binding ID and delivers it in a response the client.

    HTTP/1.1 200 OK
    Content-Type: application/json
    Cache-Control: no-cache, no-store

    {
     "access_token":"eyJhbGciOiJFUzI1NiIsImtp[...omitted...]1cs29j5c3",
     "token_type":"Bearer",
     "expires_in":3600
    }

                      Figure 9: Authorization Request

   The access token is bound to the Referred Token Binding ID of the
   access token request, which when represented as a JWT, as described
   in Section 3.4, contains the SHA-256 hash of the Token Binding ID as
   the value of the "tbh" (token binding hash) member of the "cnf"
   (confirmation) claim.  The confirmation claim portion of the JWT
   Claims Set of the access token is shown in the following figure.











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    {
      ...other claims omitted for brevity...
      "cnf":{
         "tbh": "7NRBu9iDdJlYCTOqyeYuLxXv0blEA-yTpmGIrAwKAws"
      }
    }

                       Figure 10: Confirmation Claim

3.3.  Protected Resource Token Binding Validation

   Upon receiving a token bound access token, the protected resource
   validates the binding by comparing the Provided Token Binding ID to
   the Token Binding ID for the access token.  Alternatively,
   cryptographic hashes of these Token Binding ID values can be
   compared.  If the values do not match, the resource access attempt
   MUST be rejected with an error.

3.3.1.  Example Protected Resource Request

   For example, a protected resource request using the access token from
   Section 3.2.1 would look something like the following.  The
   base64url-encoded EKM from the TLS connection over which the request
   was made is "7LsNP3BT1aHHdXdk6meEWjtSkiPVLb7YS6iHp-JXmuE".  The
   protected resource validates the binding by comparing the Provided
   Token Binding ID from the "Sec-Token-Binding" header to the token
   binding hash confirmation of the access token.  Extra line breaks in
   the example are for display purposes only.

    GET /api/stuff HTTP/1.1
    Host: resource.example.org
    Content-Type: application/x-www-form-urlencoded
    Sec-Token-Binding: AIkAAgBBQLgtRpWFPN66kxhxGrtaKrzcMtHw7HV8yMk_-MdR
      XJXbDMYxZCWnCASRRrmHHHL5wmpP3bhYt0ChRDbsMapfh_QAQN1He3Ftj4Wa_S_fz
      ZVns4saLfj6aBoMSQW6rLs19IIvHze7LrGjKyCfPTKXjajebxp-TLPFZCc0JTqTY5
      _0MBAAAA

                   Figure 11: Protected Resource Request

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"




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   member is the base64url encoding of the SHA-256 hash of the Token
   Binding ID.

   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.org",
      "sub": "brian@example.com"
      "iat": 1467324320,
      "exp": 1467324920,
      "cnf":{
        "tbh": "7NRBu9iDdJlYCTOqyeYuLxXv0blEA-yTpmGIrAwKAws"
       }
     }

         Figure 12: JWT with Token Binding Hash Confirmation Claim

4.  Token Binding for Authorization Codes

   There are two variations for Token Binding of an authorization code.
   One is appropriate for native application clients and the other for
   web server clients.  The nature of where the various components
   reside for the different client types demands different methods of
   Token Binding the authorization code so that it is bound to a Token
   Binding key on the end user's device.  This ensures that a lost or
   stolen authorization code cannot be successfully utilized from a
   different device.  For native application clients, the code is bound
   to a Token Binding key pair that the native client itself possesses.
   For web server clients, the code is bound to a Token Binding key pair
   on the end user's browser.  Both variations utilize the extensible
   framework of Proof Key for Code Exchange (PKCE) [RFC7636], which
   enables the client to show possession of a certain key when
   exchanging the authorization code for tokens.  The following
   subsections individually describe each of the two PKCE methods
   respectively.

4.1.  Native Application Clients

   This section describes a PKCE method suitable for native application
   clients that cryptographically binds the authorization code to a
   Token Binding key pair on the client, which the client proves
   possession of on the TLS connection during the access token request
   containing the authorization code.  The authorization code is bound
   to the Token Binding ID that the native application client uses to



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   resolve the authorization code at the token endpoint.  This binding
   ensures that the client that made the authorization request is the
   same client that is presenting the authorization code.

4.1.1.  Code Challenge

   As defined in Proof Key for Code Exchange [RFC7636], the client sends
   the code challenge as part of the OAuth 2.0 authorization request
   with the two additional parameters: "code_challenge" and
   "code_challenge_method".

   For this Token Binding method of PKCE, "TB-S256" is used as the value
   of the "code_challenge_method" parameter.

   The value of the "code_challenge" parameter is the base64url encoding
   (per Section 5 of [RFC4648] with all trailing padding ('=')
   characters omitted and without the inclusion of any line breaks or
   whitespace) of the SHA-256 hash of the Provided Token Binding ID that
   the client will use when calling the authorization server's token
   endpoint.  Note that, prior to making the authorization request, the
   client may need to establish a TLS connection between itself and the
   authorization server's token endpoint in order to establish the
   appropriate Token Binding ID.

   When the authorization server issues the authorization code in the
   authorization response, it associates the code challenge and method
   values with the authorization code so they can be verified later when
   the authorization code is presented in the access token request.

4.1.1.1.  Example Code Challenge

   For example, a native application client sends an authorization
   request by sending the user's browser to the authorization endpoint.
   The resulting HTTP request looks something like the following (with
   extra line breaks for display purposes only).

    GET /as/authorization.oauth2?response_type=code
      &client_id=example-native-client-id&state=oUC2jyYtzRCrMyWrVnGj
      &code_challenge=rBlgOyMY4teiuJMDgOwkrpsAjPyI07D2WsEM-dnq6eE
      &code_challenge_method=TB-S256 HTTP/1.1
    Host: server.example.com

                     Figure 13: Authorization Request








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4.1.2.  Code Verifier

   Upon receipt of the authorization code, the client sends the access
   token request to the token endpoint.  The Token Binding Protocol
   [I-D.ietf-tokbind-protocol] is negotiated on the TLS connection
   between the client and the authorization server and the "Sec-Token-
   Binding" header, as defined in Token Binding over HTTP
   [I-D.ietf-tokbind-https], is included in the access token request.
   The authorization server extracts the Provided Token Binding ID from
   the header value, hashes it with SHA-256, and compares it to the
   "code_challenge" value previously associated with the authorization
   code.  If the values match, the token endpoint continues processing
   as normal (as defined by OAuth 2.0 [RFC6749]).  If the values do not
   match, an error response indicating "invalid_grant" MUST be returned.

   The "Sec-Token-Binding" header contains sufficient information for
   verification of the authorization code and its association to the
   original authorization request.  However, PKCE [RFC7636] requires
   that a "code_verifier" parameter be sent with the access token
   request, so the static value "provided_tb" is used to meet that
   requirement and indicate that the Provided Token Binding ID is used
   for the verification.

4.1.2.1.  Example Code Verifier

   An example access token request, correlating to the authorization
   request in the previous example, to the token endpoint over a TLS
   connection for which Token Binding has been negotiated would look
   like the following (with extra line breaks for display purposes
   only).  The base64url-encoded EKM from the TLS connection over which
   the request was made is
   "pNVKtPuQFvylNYn000QowWrQKoeMkeX9H32hVuU71Bs".

    POST /as/token.oauth2 HTTP/1.1
    Host: server.example.com
    Content-Type: application/x-www-form-urlencoded
    Sec-Token-Binding: AIkAAgBBQEOO9GRFP-LM0hoWw6-2i318BsuuUum5AL8bt1sz
      lr1EFfp5DMXMNW3O8WjcIXr2DKJnI4xnuGsE6GywQd9RbD0AQJDb3xyo9PBxj8M6Y
      jLt-6OaxgDkyoBoTkyrnNbLc8tJQ0JtXomKzBbj5qPtHDduXc6xz_lzvNpxSPxi42
      8m7wkAAA

    grant_type=authorization_code&code=mJAReTWKX7zI3oHUNd4o3PeNqNqxKGp6
      &code_verifier=provided_tb&client_id=example-native-client-id

                     Figure 14: Authorization Request






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4.2.  Web Server Clients

   This section describes a PKCE method suitable for web server clients,
   which cryptographically binds the authorization code to a Token
   Binding key pair on the browser.  The authorization code is bound to
   the Token Binding ID that the browser uses to deliver the
   authorization code to a web server client, which is sent to the
   authorization server as the Referred Token Binding ID during the
   authorization request.  The web server client conveys the Token
   Binding ID to the authorization server when making the access token
   request containing the authorization code.  This binding ensures that
   the authorization code cannot successfully be played or replayed to
   the web server client from a different browser than the one that made
   the authorization request.

4.2.1.  Code Challenge

   As defined in Proof Key for Code Exchange [RFC7636], the client sends
   the code challenge as part of the OAuth 2.0 Authorization Request
   with the two additional parameters: "code_challenge" and
   "code_challenge_method".

   The client must send the authorization request through the browser
   such that the Token Binding ID established between the browser and
   itself is revealed to the authorization server's authorization
   endpoint as the Referred Token Binding ID.  Typically, this is done
   with an HTTP redirection response and the "Include-Referred-Token-
   Binding-ID" header, as defined in Section 5.3 of Token Binding over
   HTTP [I-D.ietf-tokbind-https].

   For this Token Binding method of PKCE, "referred_tb" is used for the
   value of the "code_challenge_method" parameter.

   The value of the "code_challenge" parameter is "referred_tb".  The
   static value for the required PKCE parameter indicates that the
   authorization code is to be bound to the Referred Token Binding ID
   from the Token Binding Message sent in the "Sec-Token-Binding" header
   of the authorization request.

   When the authorization server issues the authorization code in the
   authorization response, it associates the Token Binding ID (or hash
   thereof) and code challenge method with the authorization code so
   they can be verified later when the authorization code is presented
   in the access token request.







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4.2.1.1.  Example Code Challenge

   For example, the web server client sends the authorization request by
   redirecting the browser to the authorization endpoint.  That HTTP
   redirection response looks like the following (with extra line breaks
   for display purposes only).

    HTTP/1.1 302 Found
    Location: https://server.example.com?response_type=code
      &client_id=example-web-client-id&state=P4FUFqYzs1ij3ffsYCP34d3
      &redirect_uri=https%3A%2F%2Fclient%2Eexample%2Eorg%2Fcb
      &code_challenge=referred_tb&code_challenge_method=referred_tb
    Include-Referred-Token-Binding-ID: true

                      Figure 15: Redirect the Browser

   The redirect includes the "Include-Referred-Token-Binding-ID"
   response header field that signals to the user-agent that it should
   reveal, to the authorization server, the Token Binding ID used on the
   connection to the web server client.  The resulting HTTP request to
   the authorization server looks something like the following (with
   extra line breaks for display purposes only).  The base64url-encoded
   EKM from the TLS connection over which the request was made is
   "7gOdRzMhPeO-1YwZGmnVHyReN5vd2CxcsRBN69Ue4cI".

    GET /as/authorization.oauth2?response_type=code
      &client_id=example-web-client-id&state=dryo8YFpWacbUPjhBf4Nvt51
      &redirect_uri=https%3A%2F%2Fclient%2Eexample%2Eorg%2Fcb
      &code_challenge=referred_tb
      &code_challenge_method=referred_tb HTTP/1.1
    Host: server.example.com
    Sec-Token-Binding: ARIAAgBBQB-XOPf5ePlf7ikATiAFEGOS503lPmRfkyymzdWw
      HCxl0njjxC3D0E_OVfBNqrIQxzIfkF7tWby2ZfyaE6XpwTsAQBYqhFX78vMOgDX_F
      d_b2dlHyHlMmkIz8iMVBY_reM98OUaJFz5IB7PG9nZ11j58LoG5QhmQoI9NXYktKZ
      RXxrYAAAECAEFAdUFTnfQADkn1uDbQnvJEk6oQs38L92gv-KO-qlYadLoDIKe2h53
      hSiKwIP98iRj_unedkNkAMyg9e2mY4Gp7WwBAeDUOwaSXNz1e6gKohwN4SAZ5eNyx
      45Mh8VI4woL1BipLoqrJRoK6dxFkWgHRMuBROcLGUj5PiOoxybQH_Tom3gAA

                     Figure 16: Authorization Request

4.2.2.  Code Verifier

   The web server client receives the authorization code from the
   browser and extracts the Provided Token Binding ID from the "Sec-
   Token-Binding" header of the request.  The client sends the
   base64url-encoded (per Section 5 of [RFC4648] with all trailing
   padding ('=') characters omitted and without the inclusion of any
   line breaks or whitespace) Provided Token Binding ID as the value of



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   the "code_verifier" parameter in the access token request to the
   authorization server's token endpoint.  The authorization server
   compares the value of the "code_verifier" parameter to the Token
   Binding ID value previously associated with the authorization code.
   If the values match, the token endpoint continues processing as
   normal (as defined by OAuth 2.0 [RFC6749]).  If the values do not
   match, an error response indicating "invalid_grant" MUST be returned.

4.2.2.1.  Example Code Verifier

   Continuing the example from the previous section, the authorization
   server sends the code to the web server client by redirecting the
   browser to the client's "redirect_uri", which results in the browser
   making a request like the following (with extra line breaks for
   display purposes only) to the web server client over a TLS channel
   for which Token Binding has been established.  The base64url-encoded
   EKM from the TLS connection over which the request was made is
   "EzW60vyINbsb_tajt8ij3tV6cwy2KH-i8BdEMYXcNn0".

    GET /cb?state=dryo8YFpWacbUPjhBf4Nvt51&code=jwD3oOa5cQvvLc81bwc4CMw
    Host: client.example.org
    Sec-Token-Binding: AIkAAgBBQHVBU530AA5J9bg20J7yRJOqELN_C_doL_ijvqpW
      GnS6AyCntoed4UoisCD_fIkY_7p3nZDZADMoPXtpmOBqe1sAQEwgC9Zpg7QFCDBib
      6GlZki3MhH32KNfLefLJc1vR1xE8l7OMfPLZHP2Woxh6rEtmgBcAABubEbTz7muNl
      Ln8uoAAA

          Figure 17: Authorization Response to Web Server Client

   The web server client takes the Provided Token Binding ID from the
   above request from the browser and sends it, base64url encoded, to
   the authorization server in the "code_verifier" parameter of the
   authorization code grant type request.  Extra line breaks in the
   example request are for display purposes only.

    POST /as/token.oauth2 HTTP/1.1
    Host: server.example.com
    Content-Type: application/x-www-form-urlencoded
    Authorization: Basic b3JnLmV4YW1wbGUuY2xpZW50OmlldGY5OGNoaWNhZ28=

    grant_type=authorization_code&code=jwD3oOa5cQvvLc81bwc4CMw
      &redirect_uri=https%3A%2F%2Fclient%2Eexample%2Eorg%2Fcb
      &client_id=example-web-client-id
      &code_verifier=AgBBQHVBU530AA5J9bg20J7yRJOqELN_C_doL_ijv
      qpWGnS6AyCntoed4UoisCD_fIkY_7p3nZDZADMoPXtpmOBqe1s

                  Figure 18: Exchange Authorization Code





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5.  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 protected resource, 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, protected resources,
   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 protected
   resource 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, client, and protected resource can
   determine whether the others support Token Binding using the metadata
   values defined in the next section.  They can determine whether the
   user agent supports Token Binding by whether it negotiated Token
   Binding for the TLS connection.

6.  Token Binding Metadata

6.1.  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 declare their support for Token Binding of access tokens
   and refresh tokens:

   client_access_token_token_binding_supported





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      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".

6.2.  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 declare 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".

6.3.  Token Binding Protected Resource Metadata

   Protected resources supporting Token Binding that also support the
   OAuth 2.0 Protected Resource Metadata [OAuth.ResourceMetadata] use
   this metadata value to declare their support for Token Binding of
   access tokens:

   resource_access_token_token_binding_supported
      OPTIONAL.  Boolean value specifying whether the protected resource
      supports Token Binding of access tokens.  If omitted, the default
      value is "false".

7.  Security Considerations

   If a refresh request is received by the authorization server
   containing a Referred Token Binding ID and the refresh token in the
   request is not itself token bound, then it is not clear that token
   binding the access token adds significant value.  This situation
   should be considered an open issue for discussion by the working
   group.





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8.  IANA Considerations

8.1.  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]:

8.1.1.  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 6.1 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 6.1 of [[ this specification ]]

8.2.  OAuth Authorization Server Metadata Registration

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

8.2.1.  Registry Contents

   o  Metadata Name: "as_access_token_token_binding_supported"
   o  Metadata Description: Boolean value specifying whether the
      authorization server supports Token Binding of access tokens
   o  Change Controller: IESG
   o  Specification Document(s): Section 6.2 of [[ this specification ]]

   o  Metadata Name: "as_refresh_token_token_binding_supported"
   o  Metadata Description: Boolean value specifying whether the
      authorization server supports Token Binding of refresh tokens
   o  Change Controller: IESG
   o  Specification Document(s): Section 6.2 of [[ this specification ]]








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8.3.  OAuth Protected Resource Metadata Registration

   This specification registers the following client metadata definition
   in the IANA "OAuth Protected Resource Metadata" registry established
   by [OAuth.ResourceMetadata]:

8.3.1.  Registry Contents

   o  Resource Metadata Name:
      "resource_access_token_token_binding_supported"
   o  Resource Metadata Description: Boolean value specifying whether
      the protected resource supports Token Binding of access tokens
   o  Change Controller: IESG
   o  Specification Document(s): Section 6.3 of [[ this specification ]]

8.4.  PKCE Code Challenge Method Registration

   This specification requests registration of the following Code
   Challenge Method Parameter Names in the IANA "PKCE Code Challenge
   Methods" registry [IANA.OAuth.Parameters] established by [RFC7636].

8.4.1.  Registry Contents

   o  Code Challenge Method Parameter Name: TB-S256
   o  Change controller: IESG
   o  Specification document(s): Section 4.1.1 of [[ this specification
      ]]

   o  Code Challenge Method Parameter Name: referred_tb
   o  Change controller: IESG
   o  Specification document(s): Section 4.2.1 of [[ this specification
      ]]

9.  References

9.1.  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-08 (work in progress), February 2017.

   [I-D.ietf-tokbind-negotiation]
              Popov, A., Nystrom, M., Balfanz, D., and A. Langley,
              "Transport Layer Security (TLS) Extension for Token
              Binding Protocol Negotiation", draft-ietf-tokbind-
              negotiation-07 (work in progress), February 2017.




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   [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-13 (work in progress), February
              2017.

   [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>.

   [OAuth.AuthorizationMetadata]
              Jones, M., Sakimura, N., and J. Bradley, "OAuth 2.0
              Authorization Server Metadata", draft-ietf-oauth-
              discovery-06 (work in progress), March 2017,
              <http://tools.ietf.org/html/
              draft-ietf-oauth-discovery-06>.

   [OAuth.ResourceMetadata]
              Jones, M. and P. Hunt, "OAuth 2.0 Protected Resource
              Metadata", draft-jones-oauth-resource-metadata-01 (work in
              progress), January 2017, <http://tools.ietf.org/html/
              draft-jones-oauth-resource-metadata-01>.

   [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>.

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
              <http://www.rfc-editor.org/info/rfc4648>.

   [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>.






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   [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>.

   [RFC7636]  Sakimura, N., Ed., Bradley, J., and N. Agarwal, "Proof Key
              for Code Exchange by OAuth Public Clients", RFC 7636,
              DOI 10.17487/RFC7636, September 2015,
              <http://www.rfc-editor.org/info/rfc7636>.

   [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>.

9.2.  Informative References

   [I-D.ietf-oauth-native-apps]
              Denniss, W. and J. Bradley, "OAuth 2.0 for Native Apps",
              draft-ietf-oauth-native-apps-08 (work in progress), March
              2017.

   [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>.

   [RFC7523]  Jones, M., Campbell, B., and C. Mortimore, "JSON Web Token
              (JWT) Profile for OAuth 2.0 Client Authentication and
              Authorization Grants", RFC 7523, DOI 10.17487/RFC7523, May
              2015, <http://www.rfc-editor.org/info/rfc7523>.

   [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>.






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Appendix A.  Acknowledgements

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

Appendix B.  Open Issues

   o  What should we do in the case that a refresh request for a token
      bound access token is received when the refresh token used in the
      request is not token bound?

   o  Should the scope of this document include standardizing or
      recommending how to convey token binding information of an access
      token via RFC 7662 OAuth 2.0 Token Introspection?

   o  Should the scope of this document include standardization or
      guidance on token binding of JWT Client Authentication and/or
      Authorization Grants from RFC 7523?

   o  The Metadata (Section 6) and what can and cannot be reliably
      inferred from it (Section 5) need additional evaluation and work.
      OAuth 2.0 Protected Resource Metadata [OAuth.ResourceMetadata] is
      no longer a going concern, but is currently referenced herein.
      Boolean values do not adequately convey Token Binding support, as
      different components may support different key parameters types.
      And successful negotiation likely doesn't provide the application
      layer info about all the supported key parameters types but rather
      just the one that was negotiated.

Appendix C.  Document History

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

   -02

   o  Added a section on Token Binding for authorization codes with one
      variation for native clients and one for web server clients.

   o  Updated language to reflect that the binding is to the token
      binding key pair and that proof-of-possession of that key is done
      on the TLS connection.

   o  Added a bunch of examples.

   o  Added a few Open Issues so they are tracked in the document.

   o  Updated the Token Binding and OAuth Metadata references.



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   o  Added William Denniss as an author.

   -01

   o  Changed Token Binding for access tokens to use the Referred Token
      Binding ID, now that the Implementation Considerations in the
      Token Binding HTTPS specification make it clear that
      implementations will enable using the Referred Token Binding ID.

   o  Defined Protected Resource Metadata value.

   o  Changed to use the more specific term "protected resource" instead
      of "resource server".

   -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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   William Denniss
   Google
   1600 Amphitheatre Pkwy
   Mountain View, CA  94043
   USA

   Email: wdenniss@google.com
   URI:   http://wdenniss.com/











































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