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Versions: 00 01 draft-ietf-oauth-pop-key-distribution

Network Working Group                                         J. Bradley
Internet-Draft                                             Ping Identity
Intended status: Standards Track                                 P. Hunt
Expires: October 24, 2014                             Oracle Corporation
                                                                M. Jones
                                                               Microsoft
                                                           H. Tschofenig
                                                             ARM Limited
                                                          April 22, 2014


   OAuth 2.0 Proof-of-Possession: Authorization Server to Client Key
                              Distribution
            draft-bradley-oauth-pop-key-distribution-00.txt

Abstract

   RFC 6750 specified the bearer token concept for securing access to
   protected resources.  Bearer tokens need to be protected in transit
   as well as at rest since the security model is based on proof-of-
   possession.

   The OAuth 2.0 Proof-of-Possession security concept extends bearer
   token security and requires the client to demonstrate possession of a
   key when accessing a protected resource.

   This document describes how the client obtains this keying material
   from the authorization server.

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 October 24, 2014.






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Copyright Notice

   Copyright (c) 2014 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
   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
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Audience  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Audience Parameter  . . . . . . . . . . . . . . . . . . .   5
     3.2.  Processing Instructions . . . . . . . . . . . . . . . . .   5
   4.  Symmetric Key Transport . . . . . . . . . . . . . . . . . . .   6
     4.1.  Client-to-AS Request  . . . . . . . . . . . . . . . . . .   6
     4.2.  Client-to-AS Response . . . . . . . . . . . . . . . . . .   7
   5.  Asymmetric Key Transport  . . . . . . . . . . . . . . . . . .   9
     5.1.  Client-to-AS Request  . . . . . . . . . . . . . . . . . .   9
     5.2.  Client-to-AS Response . . . . . . . . . . . . . . . . . .  11
   6.  Token Types and Algorithms  . . . . . . . . . . . . . . . . .  12
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  13
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  15
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  15
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  15
     10.2.  Informative References . . . . . . . . . . . . . . . . .  16
   Appendix A.  Augmented Backus-Naur Form (ABNF) Syntax . . . . . .  17
     A.1.  'aud' Syntax  . . . . . . . . . . . . . . . . . . . . . .  17
     A.2.  'key' Syntax  . . . . . . . . . . . . . . . . . . . . . .  17
     A.3.  'alg' Syntax  . . . . . . . . . . . . . . . . . . . . . .  17
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  17

1.  Introduction

   The work on additional security mechanisms beyond OAuth 2.0 bearer
   tokens [11] is motivated in [17], which also outlines use cases,
   requirements and an architecture.  This document defines the ability
   for the client indicate support for this functionality and to obtain
   keying material from the authorization server.  As an outcome of the



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   exchange between the client and the authorization server is an access
   token that is bound to keying material.  Clients that access
   protected resources then need to demonstrate knowledge of the secret
   key that is bound to the access token.

   To best describe the scope of this specification, the OAuth 2.0
   protocol exchange sequence is shown in Figure 1.  The extension
   defined in this document piggybacks on the message exchange marked
   with (C) and (D).

        +--------+                               +---------------+
        |        |--(A)- Authorization Request ->|   Resource    |
        |        |                               |     Owner     |
        |        |<-(B)-- Authorization Grant ---|               |
        |        |                               +---------------+
        |        |
        |        |                               +---------------+
        |        |--(C)-- Authorization Grant -->| Authorization |
        | Client |                               |     Server    |
        |        |<-(D)----- Access Token -------|               |
        |        |                               +---------------+
        |        |
        |        |                               +---------------+
        |        |--(E)----- Access Token ------>|    Resource   |
        |        |                               |     Server    |
        |        |<-(F)--- Protected Resource ---|               |
        +--------+                               +---------------+

                Figure 1: Abstract OAuth 2.0 Protocol Flow

   OAuth 2.0 [2] offers different ways to obtain an access token, namely
   using authorization grants and using a refresh token.  The core OAuth
   specification defines four authorization grants, see Section 1.3 of
   [2], and [14] adds an assertion-based authorization grant to that
   list.  The token endpoint, which is described in Section 3.2 of [2],
   is used with every authorization grant except for the implicit grant
   type.  In the implicit grant type the access token is issued
   directly.

   This document extends the functionality of the token endpoint, i.e.,
   the protocol exchange between the client and the authorization
   server, to allow keying material to be bound to an access token.  Two
   types of keying material can be bound to an access token, namely
   symmetric keys and asymmetric keys.  Conveying symmetric keys from
   the authorization server to the client is described in Section 4 and
   the procedure for dealing with asymmetric keys is described in
   Section 5.




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   QUESTION: This document focuses on binding the keys to access tokens
   only.  Keys can, however, also be bound to refresh tokens and to the
   authorization code, as described in [15].  Should the scope of this
   document be extended?

2.  Terminology

   The key words 'MUST', 'MUST NOT', 'REQUIRED', 'SHALL', 'SHALL NOT',
   'SHOULD', 'SHOULD NOT', 'RECOMMENDED', 'MAY', and 'OPTIONAL' in this
   specification are to be interpreted as described in [1].

   Session Key:

      The term session key refers to fresh and unique keying material
      established between the client and the resource server.  This
      session key has a lifetime that corresponds to the lifetime of the
      access token, is generated by the authorization server and bound
      to the access token.


   This document uses the following abbreviations:

   JWA:  JSON Web Algorithms (JWA) [7]

   JWT:  JSON Web Token (JWT) [9]

   JWS:  JSON Web Signature (JWS) [6]

   JWK:  JSON Web Key (JWK) [5]

   JWE:  JSON Web Encryption (JWE) [8]

3.  Audience

   When an authorization server creates an access token, according to
   the PoP security architecture [17], it may need to know which
   resource server will process it.  This information is necessary when
   the authorization server applies integrity protection to the JWT
   using a symmetric key and has to selected a corresponding key based
   on the resource server that has to verify the signature.  The
   authorization server also requires this audience information if it
   embeds a session key inside the access token and if this symmetric
   key is encrypted with a symmetric key.

   This section defines a new header that is used by the client to
   indicate what protected resource at which resource server it wants to
   access.  This information may subsequently also communicated by the




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   authorization server securely to the resource server, for example
   within the audience field of the access token.

   QUESTION: With the use of public key cryptography the client might
   request a PoP token for use with multiple resource servers.  The
   audience parameter could carry an array of values.  Is this
   desirable?

3.1.  Audience Parameter

   The client constructs the access token request to the token endpoint
   by adding the 'aud' parameter using the "application/x-www-form-
   urlencoded" format with a character encoding of UTF-8 in the HTTP
   request entity-body.

   The URI included in the aud parameter MUST be an absolute URI as
   defined by Section 4.3 of [3].  It MAY include an "application/x-www-
   form-urlencoded" formatted query component (Section 3.4 of [3] ).
   The URI MUST NOT include a fragment component.

   The ABNF syntax for the 'aud' element is defined in Appendix A.

3.2.  Processing Instructions

      Step (0): As an initial step the client typically determines the
      resource server it wants to interact with, for example, as part of
      a discovery procedure.

      Step (1): The client starts the OAuth 2.0 protocol interaction
      based on the selected grant type.

      Step (2): When the client interacts with the token endpoint to
      obtain an access token it MUST populate the newly defined
      'audience' parameter with the information obtained in step (0).

      Step (2): The authorization server who obtains the request from
      the client needs to parse it to determine whether the provided
      audience value matches any of the resource servers it has a
      relationship with.  If the authorization server fails to parse the
      provided value it MUST reject the request using an error response
      with the error code "invalid_request".  If the authorization
      server does not consider the resource server acceptable it MUST
      return an error response with the error code "access_denied".  In
      both cases additional error information may be provided via the
      error_description, and the error_uri parameters.  If the request
      has, however, been verified successfully then the authorization
      server MUST include the audience claim into the access token with
      the value copied from the audience field provided by the client.



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      In case the access token is encoded using the JSON Web Token
      format [9] the "aud" claim MUST be used.  The access token MUST be
      protected against modification by either using a digital signature
      or a keyed message digest.  The authorization server returns the
      access token to the client, as specified in [2].

   Subsequent steps for the interaction between the client and the
   resource server are beyond the scope of this document.

4.  Symmetric Key Transport

4.1.  Client-to-AS Request

   In case a symmetric key shall be bound to an access token the
   following procedure is applicable.  In the request message from the
   OAuth client to the OAuth authorization server the following
   parameters MAY be included:

   token_type:  OPTIONAL.  See Section 6 for more details.

   alg:  OPTIONAL.  See Section 6 for more details.

   These two new parameters are optional in the case where the
   authorization server has prior knowledge of the capabilities of the
   client otherwise these two parameters are required.

   For example, the client makes the following HTTP request using TLS
   (extra line breaks are 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=pop
        &alg=HS256


                Example Request to the Authorization Server








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4.2.  Client-to-AS Response

   If the access token request is valid and authorized, the
   authorization server issues an access token and optionally a refresh
   token.  If the request client authentication failed or is invalid,
   the authorization server returns an error response as described in
   Section 5.2 of [2].

   The authorization server MUST include an access token and a 'key'
   element in a successful response.  The 'key' parameter either
   contains a plain JWK structure or a JWK encrypted with a JWE.  The
   difference between the two approaches is the following:

   Plain JWK:  If the JWK container is placed in the 'key' element then
      the security of the overall PoP architecture relies on Transport
      Layer Security protection between the authorization server and the
      client.  Figure 2 illustrates an example response using a plain
      JWK for key transport from the authorization server to the client.

   JWK protected by JWE:  If the JWK container is protected by a JWE
      then additional security protection at the application layer is
      provided between the authorization server and the client beyond
      the use of TLS.  This approach is a reasonable choice, for
      example, when a trusted security module is available on the client
      device and the confidentiality protection is offered directly to
      this trusted element.

   Note that there are potentially two JSON-encoded structures in the
   response, namely the access token that may utilize the JWT format and
   the actual key transport mechanism itself.





















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

     {
       "access_token":"SlAV32hkKG ...
         ... (remainder of JWT omitted for brevity)",
       "token_type":"pop",
       "expires_in":3600,
       "refresh_token":"8xLOxBtZp8",
       "key":"eyJhbGciOiJSU0ExXzUi ...
         ... (remainder of JWK/JWE omitted for brevity)"
     }


   Figure 2: Example: Response from the Authorization Server (Symmetric
                                 Variant)

   The content of the key parameter, which is a JWK in our example, is
   shown in Figure 3.


     {
      "kty":"oct",
      "alg":"HS256",
      "k":"ZoRSOrFzN_FzUA5XKMYoVHyzff5oRJxl-IXRtztJ6uE"
     }


           Figure 3: Example: Key Transport to Client via a JWK

   The content of the 'access_token' in JWT format contains the 'cnf'
   element, as shown in Figure 4.  The digital signature or the keyed
   message digest offering integrity protection is not shown in this
   example but must be present in a real deployment to mitigate a number
   of security threats.

   The JWK in the key element of the response from the authorization
   server shown in Figure 2 contains the same session key as the JWK
   inside the access token shown in Figure 4 but is, in this example,
   protected by TLS and transmitted from the authorization server to the
   client (for processing by the client).









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      {
         "iss": "https://server.example.com",
         "sub": "24400320",
         "aud": "s6BhdRkqt3",
         "nonce": "n-0S6_WzA2Mj",
         "exp": 1311281970,
         "iat": 1311280970,
         "cnf":{
           "jwk":
             "JDLUhTMjU2IiwiY3R5Ijoi ...
              ... (remainder of JWE omitted for brevity)"
           }
      }


               Figure 4: Example: Access Token in JWT Format

   Note: The JWK content inside the access token is meant for
   consumption by the resource server.  It MUST be confidentiality
   protected using a JWE.

   This document does not impose requirements on the encoding of the
   access token.  The examples used in this document make use of the JWT
   structure since this is the only standardized format if access tokens
   are conveyed per value between the authorization server, the client,
   and the resource server.

   If the access token is only a handle to the access token itself then
   a look-up by the resource server is needed, as described in the token
   introspection specification [18].

5.  Asymmetric Key Transport

5.1.  Client-to-AS Request

   In case an asymmetric key shall be bound to an access token then the
   following procedure is applicable.  In the request message from the
   OAuth client to the OAuth authorization server the following
   parameters MAY include the following parameters:

   token_type:  OPTIONAL.  See Section 6 for more details.

   alg:  OPTIONAL.  See Section 6 for more details.

   key:  OPTIONAL.  This field contains information about the public key
         the client would like to bind to the access token in the JSON
         Web Key format.  If the client does not provide a public key
         then the authorization server MUST create an ephemeral key pair



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         (considering the profile information provided by the client) or
         alternative respond with an error message.

   The token_type and the alg parameters are optional in the case where
   the authorization server has prior knowledge of the capabilities of
   the client otherwise these two parameters are required.

   For example, the client makes the following HTTP request using TLS
   (extra line breaks are for display purposes only) shown in Figure 5.


        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=pop
        &alg=RS256
        &key=eyJhbGciOiJSU0ExXzUi ...
         ... (remainder of JWK omitted for brevity)

   Figure 5: Example Request to the Authorization Server (Asymmetric Key
                                 Variant)

   The content of the key parameter contains the RSA public key the
   client would like to associate with the access token, as shown in
   Figure 6.


      {"kty":"RSA",
             "n": "0vx7agoebGcQSuuPiLJXZptN9nndrQmbXEps2aiAFbWhM78LhWx
        4cbbfAAtVT86zwu1RK7aPFFxuhDR1L6tSoc_BJECPebWKRXjBZCiFV4n3oknjhMs
        tn64tZ_2W-5JsGY4Hc5n9yBXArwl93lqt7_RN5w6Cf0h4QyQ5v-65YGjQR0_FDW2
        QvzqY368QQMicAtaSqzs8KJZgnYb9c7d0zgdAZHzu6qMQvRL5hajrn1n91CbOpbI
        SD08qNLyrdkt-bFTWhAI4vMQFh6WeZu0fM4lFd2NcRwr3XPksINHaQ-G_xBniIqb
        w0Ls1jF44-csFCur-kEgU8awapJzKnqDKgw",
             "e":"AQAB",
             "alg":"RS256",
           "kid":"client@example.com"}

       Figure 6: Client Providing Public Key to Authorization Server







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5.2.  Client-to-AS Response

   If the access token request is valid and authorized, the
   authorization server issues an access token and optionally a refresh
   token.  If the request client authentication failed or is invalid,
   the authorization server returns an error response as described in
   Section 5.2 of [2].

   The authorization server also places information about the public key
   used by the client into the access token to create the binding
   between the two.  The new token type "public_key" is placed into the
   'token_type' parameter.

   An example of a successful response is shown in Figure 7.


        HTTP/1.1 200 OK
        Content-Type: application/json;charset=UTF-8
        Cache-Control: no-store
        Pragma: no-cache

        {
          "access_token":"2YotnFZFE....jr1zCsicMWpAA",
          "token_type":"pop",
          "alg":"RS256",
          "expires_in":3600,
          "refresh_token":"tGzv3JOkF0XG5Qx2TlKWIA"
        }

   Figure 7: Example: Response from the Authorization Server (Asymmetric
                                 Variant)

   The content of the 'access_token' field contains an encoded JWT with
   the following structure, as shown in Figure 8.  The digital signature
   or the keyed message digest offering integrity protection is not
   shown (but must be present).















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       {
         "iss":"xas.example.com",
         "aud":"http://auth.example.com",
         "exp":"1361398824",
         "nbf":"1360189224",
         "cnf":{
           "jwk":{"kty":"RSA",
             "n": "0vx7agoebGcQSuuPiLJXZptN9nndrQmbXEps2aiAFbWhM78LhWx
        4cbbfAAtVT86zwu1RK7aPFFxuhDR1L6tSoc_BJECPebWKRXjBZCiFV4n3oknjhMs
        tn64tZ_2W-5JsGY4Hc5n9yBXArwl93lqt7_RN5w6Cf0h4QyQ5v-65YGjQR0_FDW2
        QvzqY368QQMicAtaSqzs8KJZgnYb9c7d0zgdAZHzu6qMQvRL5hajrn1n91CbOpbI
        SD08qNLyrdkt-bFTWhAI4vMQFh6WeZu0fM4lFd2NcRwr3XPksINHaQ-G_xBniIqb
        w0Ls1jF44-csFCur-kEgU8awapJzKnqDKgw",
             "e":"AQAB",
             "alg":"RS256",
           "kid":"client@example.com"}
          }
        }

      Figure 8: Example: Access Token Structure (Asymmetric Variant)

   Note: In this example there is no need for the authorization server
   to convey further keying material to the client.

6.  Token Types and Algorithms

   To allow clients to indicate support for specific token types and
   respective algorithms they need to interact with authorization
   servers.  They can either provide this information out-of-band, for
   example, via pre-configuration or up-front via the dynamic client
   registration protocol [16].

   The value in the 'alg' parameter together with value from the
   'token_type' parameter allow the client to indicate the supported
   algorithms for a given token type.  The token type refers to the
   specification used by the client to interact with the resource server
   to demonstrate possession of the key.  The alg parameter provides
   further information about the algorithm, such as whether a symmetric
   or an asymmetric crypto-system is used.  Hence, a client supporting a
   specific token type also knows how to populate the values to the alg
   parameter.

   The value for the token type MUST be taken from the 'OAuth Access
   Token Types' registry created by [2].

   This document does not register a new value for the Access Token
   Types registry nor does it define values to be used for the alg
   parameter.  Instead this is the responsibility of specifications



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   defining the mechanism for clients interacting with resource servers,
   such as [19].

   The values in the alg parameter are case-sensitive.  If the client
   supports more than one algorithm then each individual value MUST be
   separated by a space.

7.  Security Considerations

   [17] describes the architecture for the OAuth 2.0 proof-of-possession
   security architecture, including use cases, threats, and
   requirements.  This requirements describes one solution component of
   that architecture, namely the mechanism for the client to interact
   with the authorization server to either obtain a symmetric key from
   the authorization server, to obtain an asymmetric key pair, or to
   offer a public key to the authorization.  In any case, these keys are
   then bound to the access token by the authorization server.

   To summarize the main security recommendations: A large range of
   threats can be mitigated by protecting the contents of the access
   token by using a digital signature or a keyed message digest.
   Consequently, the token integrity protection MUST be applied to
   prevent the token from being modified, particularly since it contains
   a reference to the symmetric key or the asymmetric key.  If the
   access token contains the symmetric key (see Section 2.2 of [10] for
   a description about how symmetric keys can be securely conveyed
   within the access token) this symmetric key MUST be encrypted by the
   authorization server with a long-term key shared with the resource
   server.

   To deal with token redirect, it is important for the authorization
   server to include the identity of the intended recipient (the
   audience), typically a single resource server (or a list of resource
   servers), in the token.  Using a single shared secret with multiple
   authorization server to simplify key management is NOT RECOMMENDED
   since the benefit from using the proof-of-possession concept is
   significantly reduced.

   Token replay is also not possible since an eavesdropper will also
   have to obtain the corresponding private key or shared secret that is
   bound to the access token.  Nevertheless, it is good practice to
   limit the lifetime of the access token and therefore the lifetime of
   associated key.

   The authorization server MUST offer confidentiality protection for
   any interactions with the client.  This step is extremely important
   since the client will obtain the session key from the authorization
   server for use with a specific access token.  Not using



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   confidentiality protection exposes this secret (and the access token)
   to an eavesdropper thereby making the OAuth 2.0 proof-of-possession
   security model completely insecure.  OAuth 2.0 [2] relies on TLS to
   offer confidentiality protection and additional protection can be
   applied using the JSON Web Key (JWK) [5] offered security mechanism,
   which would add an additional layer of protection on top of TLS for
   cases where the keying material is conveyed, for example, to a
   hardware security module.  Which version(s) of TLS ought to be
   implemented will vary over time, and depend on the widespread
   deployment and known security vulnerabilities at the time of
   implementation.  At the time of this writing, TLS version 1.2 [4] is
   the most recent version.  The client MUST validate the TLS
   certificate chain when making requests to protected resources,
   including checking the validity of the certificate.

   Similarly to the security recommendations for the bearer token
   specification [11] developers MUST ensure that the ephemeral
   credentials (i.e., the private key or the session key) is not leaked
   to third parties.  An adversary in possession of the ephemeral
   credentials bound to the access token will be able to impersonate the
   client.  Be aware that this is a real risk with many smart phone app
   and Web development environments.

   Clients can at any time request a new proof-of-possession capable
   access token.  Using a refresh token to regularly request new access
   tokens that are bound to fresh and unique keys is important.  Keeping
   the lifetime of the access token short allows the authorization
   server to use shorter key sizes, which translate to a performance
   benefit for the client and for the resource server.  Shorter keys
   also lead to shorter messages (particularly with asymmetric keying
   material).

   When authorization servers bind symmetric keys to access tokens then
   they MUST scope these access tokens to a specific resource server.

8.  IANA Considerations

   This specification registers the following parameters in the OAuth
   Parameters Registry established by [2].

   Parameter name:  alg

   Parameter usage location:  token request, token response,
      authorization response

   Change controller:  IETF

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



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   Related information:  None

   Parameter name:  key

   Parameter usage location:  token request, token response,
      authorization response

   Change controller:  IETF

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

   Related information:  None

   Parameter name:  aud

   Parameter usage location:  token request

   Change controller:  IETF

   Specification document(s):  [[This document.]

   Related information:  None

9.  Acknowledgements

   Add your name here.

10.  References

10.1.  Normative References

   [1]        Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [2]        Hardt, D., "The OAuth 2.0 Authorization Framework", RFC
              6749, October 2012.

   [3]        Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66, RFC
              3986, January 2005.

   [4]        Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

   [5]        Jones, M., "JSON Web Key (JWK)", draft-ietf-jose-json-web-
              key-25 (work in progress), March 2014.





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   [6]        Jones, M., Bradley, J., and N. Sakimura, "JSON Web
              Signature (JWS)", draft-ietf-jose-json-web-signature-25
              (work in progress), March 2014.

   [7]        Jones, M., "JSON Web Algorithms (JWA)", draft-ietf-jose-
              json-web-algorithms-25 (work in progress), March 2014.

   [8]        Jones, M. and J. Hildebrand, "JSON Web Encryption (JWE)",
              draft-ietf-jose-json-web-encryption-25 (work in progress),
              March 2014.

   [9]        Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
              (JWT)", draft-ietf-oauth-json-web-token-19 (work in
              progress), March 2014.

   [10]       Jones, M., Bradley, J., and H. Tschofenig, "Proof-Of-
              Possession Semantics for JSON Web Tokens (JWTs)", draft-
              jones-oauth-proof-of-possession-00 (work in progress),
              April 2014.

10.2.  Informative References

   [11]       Jones, M. and D. Hardt, "The OAuth 2.0 Authorization
              Framework: Bearer Token Usage", RFC 6750, October 2012.

   [12]       Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66, RFC
              3986, January 2005.

   [13]       Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.

   [14]       Campbell, B., Mortimore, C., Jones, M., and Y. Goland,
              "Assertion Framework for OAuth 2.0 Client Authentication
              and Authorization Grants", draft-ietf-oauth-assertions-15
              (work in progress), March 2014.

   [15]       Sakimura, N., Bradley, J., and N. Agarwal, "OAuth
              Symmetric Proof of Posession for Code Extension", draft-
              sakimura-oauth-tcse-03 (work in progress), April 2014.

   [16]       Richer, J., Jones, M., Bradley, J., Machulak, M., and P.
              Hunt, "OAuth 2.0 Dynamic Client Registration Core
              Protocol", draft-ietf-oauth-dyn-reg-16 (work in progress),
              February 2014.






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   [17]       Hunt, P., Richer, J., Mills, W., Mishra, P., and H.
              Tschofenig, "OAuth 2.0 Proof-of-Possession (PoP) Security
              Architecture", draft-hunt-oauth-pop-architecture-00 (work
              in progress), April 2014.

   [18]       Richer, J., "OAuth Token Introspection", draft-richer-
              oauth-introspection-04 (work in progress), May 2013.

   [19]       Richer, J., "A Method for Signing an HTTP Requests for
              OAuth", April 2014.

Appendix A.  Augmented Backus-Naur Form (ABNF) Syntax

   This section provides Augmented Backus-Naur Form (ABNF) syntax
   descriptions for the elements defined in this specification using the
   notation of [13].

A.1.  'aud' Syntax

   The ABNF syntax is defined as follows where by the "URI-reference"
   definition is taken from [3]:

      aud = URI-reference

A.2.  'key' Syntax

   The "key" element is defined in Section 4 and Section 5:

      key = 1*VSCHAR

A.3.  'alg' Syntax

   The "alg" element is defined in Section 6:

      alg = alg-token *( SP alg-token )

      alg-token = 1*NQCHAR

Authors' Addresses

   John Bradley
   Ping Identity

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






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   Phil Hunt
   Oracle Corporation

   Email: phil.hunt@yahoo.com
   URI:   http://www.indepdentid.com


   Michael B. Jones
   Microsoft

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


   Hannes Tschofenig
   ARM Limited
   Austria

   Email: Hannes.Tschofenig@gmx.net
   URI:   http://www.tschofenig.priv.at































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