Proof-of-Possession Key Semantics for JSON Web Tokens (JWTs)
draft-ietf-oauth-proof-of-possession-10

OAuth Working Group                                             M. Jones
Internet-Draft                                                 Microsoft
Intended status: Standards Track                              J. Bradley
Expires: June 18, 2016                                     Ping Identity
                                                           H. Tschofenig
                                                             ARM Limited
                                                       December 16, 2015

      Proof-of-Possession Key Semantics for JSON Web Tokens (JWTs)
                draft-ietf-oauth-proof-of-possession-10

Abstract

   This specification defines how to declare in a JSON Web Token (JWT)
   that the presenter of the JWT possesses a particular proof-of-
   possession key and that the recipient can cryptographically confirm
   proof-of-possession of the key by the presenter.  Being able to prove
   possession of a key is also sometimes described as the presenter
   being a holder-of-key.

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 June 18, 2016.

Copyright Notice

   Copyright (c) 2015 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

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

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Notational Conventions . . . . . . . . . . . . . . . . . .  5
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  Representations for Proof-of-Possession Keys . . . . . . . . .  6
     3.1.  Confirmation Claim . . . . . . . . . . . . . . . . . . . .  6
     3.2.  Representation of an Asymmetric Proof-of-Possession Key  .  7
     3.3.  Representation of an Encrypted Symmetric
           Proof-of-Possession Key  . . . . . . . . . . . . . . . . .  8
     3.4.  Representation of a Key ID for a Proof-of-Possession
           Key  . . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     3.5.  Representation of a URL for a Proof-of-Possession Key  . .  9
     3.6.  Specifics Intentionally Not Specified  . . . . . . . . . . 10
   4.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10
   5.  Privacy Considerations . . . . . . . . . . . . . . . . . . . . 11
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 11
     6.1.  JSON Web Token Claims Registration . . . . . . . . . . . . 12
       6.1.1.  Registry Contents  . . . . . . . . . . . . . . . . . . 12
     6.2.  JWT Confirmation Methods Registry  . . . . . . . . . . . . 12
       6.2.1.  Registration Template  . . . . . . . . . . . . . . . . 12
       6.2.2.  Initial Registry Contents  . . . . . . . . . . . . . . 13
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
     7.1.  Normative References . . . . . . . . . . . . . . . . . . . 13
     7.2.  Informative References . . . . . . . . . . . . . . . . . . 14
   Appendix A.  Acknowledgements  . . . . . . . . . . . . . . . . . . 15
   Appendix B.  Document History  . . . . . . . . . . . . . . . . . . 15
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 17

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1.  Introduction

   This specification defines how a JSON Web Token [JWT] can declare
   that the presenter of the JWT possesses a particular proof-of-
   possession key and that the recipient can cryptographically confirm
   proof-of-possession of the key by the presenter.  Proof-of-possession
   of a key is also sometimes described as the presenter being a holder-
   of-key.  The [I-D.ietf-oauth-pop-architecture] specification
   describes key confirmation, among other confirmation mechanisms.
   This specification defines how to communicate key confirmation key
   information in JWTs.

   Envision the following two use cases.  The first use case employs a
   symmetric proof-of-possession key and the second use case employs an
   asymmetric proof-of-possession key.

     +--------------+
     |              |                         +--------------+
     |              |--(3) Presentation of -->|              |
     |              |      JWT w/ Encrypted   |              |
     |  Presenter   |      PoP Key            |              |
     |              |                         |              |
     |              |<-(4) Communication ---->|              |
     |              |      Authenticated by   |              |
     +--------------+      PoP Key            |              |
       ^          ^                           |              |
       |          |                           |              |
      (1) Sym.   (2) JWT w/                   |  Recipient   |
       |  PoP     |  Encrypted                |              |
       |  Key     |  PoP Key                  |              |
       v          |                           |              |
     +--------------+                         |              |
     |              |                         |              |
     |              |                         |              |
     |              |<-(0) Key Exchange for ->|              |
     |   Issuer     |      Key Encryption Key |              |
     |              |                         |              |
     |              |                         |              |
     |              |                         +--------------+
     +--------------+

            Figure 1: Proof-of-Possession with a Symmetric Key

   In the case illustrated in Figure 1, either the presenter generates a
   symmetric key and privately sends it to the issuer (1) or the issuer
   generates a symmetric key and privately sends it to the presenter
   (1).  The issuer generates a JWT with an encrypted copy of this
   symmetric key in the confirmation claim.  This symmetric key is

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   encrypted with a key known only to the issuer and the recipient,
   which was previously established in step (0).  The entire JWT is
   integrity protected by the issuer.  The JWT is then (2) sent to the
   presenter.  Now, the presenter is in possession of the symmetric key
   as well as the JWT (which includes the confirmation claim).  When the
   presenter (3) presents the JWT to the recipient, it also needs to
   demonstrate possession of the symmetric key; the presenter, for
   example, (4) uses the symmetric key in a challenge/response protocol
   with the recipient.  The recipient is then able to verify that it is
   interacting with the genuine presenter by decrypting the key in the
   confirmation claim of the JWT.  By doing this, the recipient obtains
   the symmetric key, which it then uses to verify cryptographically
   protected messages exchanged with the presenter (4).  This symmetric
   key mechanism described above is conceptually similar to the use of
   Kerberos tickets.

     +--------------+
     |              |                         +--------------+
     |              |--(3) Presentation of -->|              |
     |              |      JWT w/ Public      |              |
     |  Presenter   |      PoP Key            |              |
     |              |                         |              |
     |              |<-(4) Communication ---->|              |
     |              |      Authenticated by   |              |
     +--------------+      PoP Key            |              |
       |          ^                           |              |
       |          |                           |              |
      (1) Public (2) JWT w/                   |  Recipient   |
       |  PoP     |  Public                   |              |
       |  Key     |  PoP Key                  |              |
       v          |                           |              |
     +--------------+                         |              |
     |              |                         |              |
     |              |                         |              |
     |              |                         |              |
     |    Issuer    |                         |              |
     |              |                         |              |
     |              |                         |              |
     |              |                         +--------------+
     +--------------+

           Figure 2: Proof-of-Possession with an Asymmetric Key

   In the case illustrated in Figure 2, the presenter generates a
   public/private key pair and (1) sends the public key to the issuer,
   which creates a JWT that contains the public key (or an identifier
   for it) in the confirmation claim.  The entire JWT is integrity
   protected using a digital signature to protect it against

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   modifications.  The JWT is then (2) sent to the presenter.  When the
   presenter (3) presents the JWT to the recipient, it also needs to
   demonstrate possession of the private key.  The presenter, for
   example, (4) uses the private key in a TLS exchange with the
   recipient or (4) signs a nonce with the private key.  The recipient
   is able to verify that it is interacting with the genuine presenter
   by extracting the public key from the confirmation claim of the JWT
   (after verifying the digital signature of the JWT) and utilizing it
   with the private key in the TLS exchange or by checking the nonce
   signature.

   In both cases, the JWT may contain other claims that are needed by
   the application.

1.1.  Notational 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].

   Unless otherwise noted, all the protocol parameter names and values
   are case sensitive.

2.  Terminology

   This specification uses terms defined in the JSON Web Token [JWT],
   JSON Web Key [JWK], and JSON Web Encryption [JWE] specifications.

   These terms are defined by this specification:

   Issuer
      Party that creates the JWT and binds the proof-of-possession key
      to it.

   Presenter
      Party that proves possession of a private key (for asymmetric key
      cryptography) or secret key (for symmetric key cryptography) to a
      recipient.

   Recipient
      Party that receives the JWT containing the proof-of-possession key
      information from the presenter.

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3.  Representations for Proof-of-Possession Keys

   By including a "cnf" (confirmation) claim in a JWT, the issuer of the
   JWT declares that the presenter possesses a particular key, and that
   the recipient can cryptographically confirm that the presenter has
   possession of that key.  The value of the "cnf" claim is a JSON
   object and the members of that object identify the proof-of-
   possession key.

   The presenter can be identified in one of several ways by the JWT,
   depending upon the application requirements.  If the JWT contains a
   "sub" (subject) claim [JWT], the presenter is normally the subject
   identified by the JWT.  (In some applications, the subject identifier
   will be relative to the issuer identified by the "iss" (issuer) claim
   [JWT].)  If the JWT contains no "sub" (subject) claim, the presenter
   is normally the issuer identified by the JWT using the "iss" (issuer)
   claim.  The case in which the presenter is the subject of the JWT is
   analogous to SAML 2.0 [OASIS.saml-core-2.0-os] SubjectConfirmation
   usage.  At least one of the "sub" and "iss" claims MUST be present in
   the JWT.  Some use cases may require that both be present.

   Another means used by some applications to identify the presenter is
   an explicit claim, such as the "azp" (authorized party) claim defined
   by OpenID Connect [OpenID.Core].  Ultimately, the means of
   identifying the presenter is application-specific, as is the means of
   confirming possession of the key that is communicated.

3.1.  Confirmation Claim

   The "cnf" (confirmation) claim is used in the JWT to contain members
   used to identify the proof-of-possession key.  Other members of the
   "cnf" object may be defined because a proof-of-possession key may not
   be the only means of confirming the authenticity of the token.  This
   is analogous to the SAML 2.0 [OASIS.saml-core-2.0-os]
   SubjectConfirmation element, in which a number of different subject
   confirmation methods can be included, including proof-of-possession
   key information.

   The set of confirmation members that a JWT must contain to be
   considered valid is context dependent and is outside the scope of
   this specification.  Specific applications of JWTs will require
   implementations to understand and process some confirmation members
   in particular ways.  However, in the absence of such requirements,
   all confirmation members that are not understood by implementations
   MUST be ignored.

   This specification establishes the IANA "JWT Confirmation Methods"
   registry for these members in Section 6.2 and registers the members

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   defined by this specification.  Other specifications can register
   other members used for confirmation, including other members for
   conveying proof-of-possession keys, possibly using different key
   representations.

   The "cnf" claim value MUST represent only a single proof-of-
   possession key; thus, at most one of the "jwk", "jwe", and "jku"
   confirmation values defined below may be present.  Note that if an
   application needs to represent multiple proof-of-possession keys in
   the same JWT, one way for it to achieve this is to use other claim
   names, in addition to "cnf", to hold the additional proof-of-
   possession key information.  These claims could use the same syntax
   and semantics as the "cnf" claim.  Those claims would be defined by
   applications or other specifications and could be registered in the
   IANA "JSON Web Token Claims" registry [IANA.JWT.Claims].

3.2.  Representation of an Asymmetric Proof-of-Possession Key

   When the key held by the presenter is an asymmetric private key, the
   "jwk" member is a JSON Web Key [JWK] representing the corresponding
   asymmetric public key.  The following example demonstrates such a
   declaration in the JWT Claims Set of a JWT:

     {
      "iss": "https://server.example.com",
      "aud": "https://client.example.org",
      "exp": 1361398824,
      "cnf":{
        "jwk":{
          "kty": "EC",
          "use": "sig",
          "crv": "P-256",
          "x": "18wHLeIgW9wVN6VD1Txgpqy2LszYkMf6J8njVAibvhM",
          "y": "-V4dS4UaLMgP_4fY4j8ir7cl1TXlFdAgcx55o7TkcSA"
         }
       }
     }

   The JWK MUST contain the required key members for a JWK of that key
   type and MAY contain other JWK members, including the "kid" (key ID)
   member.

   The "jwk" member MAY also be used for a JWK representing a symmetric
   key, provided that the JWT is encrypted so that the key is not
   revealed to unintended parties.  If the JWT is not encrypted, the
   symmetric key MUST be encrypted as described below.

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3.3.  Representation of an Encrypted Symmetric Proof-of-Possession Key

   When the key held by the presenter is a symmetric key, the "jwe"
   member is an encrypted JSON Web Key [JWK] encrypted to a key known to
   the recipient using the JWE Compact Serialization containing the
   symmetric key.  The rules for encrypting a JWK are found in Section 7
   of the JSON Web Key [JWK] specification.

   The following example illustrates a symmetric key that could
   subsequently be encrypted for use in the "jwe" member:

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

   The UTF-8 [RFC3629] encoding of this JWK is used as the JWE Plaintext
   when encrypting the key.

   The following example is a JWE Header that could be used when
   encrypting this key:

     {
      "alg": "RSA-OAEP",
      "enc": "A128CBC-HS256"
     }

   The following example JWT Claims Set of a JWT illustrates the use of
   an encrypted symmetric key as the "jwe" member value:

     {
      "iss": "https://server.example.com",
      "sub": "24400320",
      "aud": "s6BhdRkqt3",
      "nonce": "n-0S6_WzA2Mj",
      "exp": 1311281970,
      "iat": 1311280970,
      "cnf":{
        "jwe":
          "eyJhbGciOiJSU0EtT0FFUCIsImVuYyI6IkExMjhDQkMtSFMyNTYifQ.
          (remainder of JWE omitted for brevity)"
        }
     }

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3.4.  Representation of a Key ID for a Proof-of-Possession Key

   The proof-of-possession key can also be identified by the use of a
   Key ID instead of communicating the actual key, provided the
   recipient is able to obtain the identified key using the Key ID.  In
   this case, the issuer of a JWT declares that the presenter possesses
   a particular key and that the recipient can cryptographically confirm
   proof-of-possession of the key by the presenter by including a "cnf"
   (confirmation) claim in the JWT whose value is a JSON object, with
   the JSON object containing a "kid" (key ID) member identifying the
   key.

   The following example demonstrates such a declaration in the JWT
   Claims Set of a JWT:

     {
      "iss": "https://server.example.com",
      "aud": "https://client.example.org",
      "exp": 1361398824,
      "cnf":{
        "kid": "dfd1aa97-6d8d-4575-a0fe-34b96de2bfad"
       }
     }

   The content of the "kid" value is application specific.  For
   instance, some applications may choose to use a JWK Thumbprint
   [JWK.Thumbprint] value as the "kid" value.

3.5.  Representation of a URL for a Proof-of-Possession Key

   The proof-of-possession key can be passed by reference instead of
   being passed by value.  This is done using the "jku" (JWK Set URL)
   member.  Its value is a URI [RFC3986] that refers to a resource for a
   set of JSON-encoded public keys represented as a JWK Set [JWK], one
   of which is the proof-of-possession key.  If there are multiple keys
   in the referenced JWK Set document, a "kid" member MUST also be
   included, with the referenced key's JWK also containing the same
   "kid" value.

   The protocol used to acquire the resource MUST provide integrity
   protection.  An HTTP GET request to retrieve the JWK Set MUST use
   Transport Layer Security (TLS) [RFC5246] and the identity of the
   server MUST be validated, as per Section 6 of RFC 6125 [RFC6125].

   The following example demonstrates such a declaration in the JWT
   Claims Set of a JWT:

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     {
      "iss": "https://server.example.com",
      "sub": "17760704",
      "aud": "https://client.example.org",
      "exp": 1440804813,
      "cnf":{
        "jku": "https://keys.example.net/pop-keys.json",
        "kid": "2015-08-28"
       }
     }

3.6.  Specifics Intentionally Not Specified

   Proof-of-possession is typically demonstrated by having the presenter
   sign a value determined by the recipient using the key possessed by
   the presenter.  This value is sometimes called a "nonce" or a
   "challenge".

   The means of communicating the nonce and the nature of its contents
   are intentionally not described in this specification, as different
   protocols will communicate this information in different ways.
   Likewise, the means of communicating the signed nonce is also not
   specified, as this is also protocol-specific.

   Note that another means of proving possession of the key when it is a
   symmetric key is to encrypt the key to the recipient.  The means of
   obtaining a key for the recipient is likewise protocol-specific.

   For examples using the mechanisms defined in this specification, see
   [I-D.ietf-oauth-pop-architecture].

4.  Security Considerations

   All of the security considerations that are discussed in [JWT] also
   apply here.  In addition, proof-of-possession introduces its own
   unique security issues.  Possessing a key is only valuable if it is
   kept secret.  Appropriate means must be used to ensure that
   unintended parties do not learn private key or symmetric key values.

   Proof-of-possession via encrypted symmetric secrets is subject to
   replay attacks.  This attack can be avoided when a signed nonce or
   challenge is used, since the recipient can use a distinct nonce or
   challenge for each interaction.

   Similarly to other information included in a JWT, it is necessary to
   apply data origin authentication and integrity protection (via a
   keyed message digest or a digital signature).  Data origin

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   authentication ensures that the recipient of the JWT learns about the
   entity that created the JWT, since this will be important for any
   policy decisions.  Integrity protection prevents an adversary from
   changing any elements conveyed within the JWT payload.  Special care
   has to be applied when carrying symmetric keys inside the JWT, since
   those not only require integrity protection, but also confidentiality
   protection.

   A recipient might not understand the "cnf" claim.  Applications that
   require the proof-of-possession keys communicated with it to be
   understood and processed must ensure that the parts of this
   specification that they use are implemented.

   Applications utilizing proof-of-possession should also utilize
   audience restriction, as described in Section 4.1.3 of [JWT], as it
   provides different protections.  Proof-of-possession can be used by
   recipients to reject messages from unauthorized senders.  Audience
   restriction can be used by recipients to reject messages intended for
   different recipients.

5.  Privacy Considerations

   A proof-of-possession key can be used as a correlation handle if the
   same key is used with multiple parties.  Thus, for privacy reasons,
   it is recommended that different proof-of-possession keys be used
   when interacting with different parties.

6.  IANA Considerations

   The following registration procedure is used for all the registries
   established by this specification.

   Values are registered on a Specification Required [RFC5226] basis
   after a three-week review period on the oauth-pop-reg-review@ietf.org
   mailing list, on the advice of one or more Designated Experts.
   However, to allow for the allocation of values prior to publication,
   the Designated Experts may approve registration once they are
   satisfied that such a specification will be published. [[ Note to the
   RFC Editor: The name of the mailing list should be determined in
   consultation with the IESG and IANA.  Suggested name:
   oauth-pop-reg-review@ietf.org. ]]

   Registration requests sent to the mailing list for review should use
   an appropriate subject (e.g., "Request to register JWT Confirmation
   Method: example").  Registration requests that are undetermined for a
   period longer than 21 days can be brought to the IESG's attention

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   (using the iesg@ietf.org mailing list) for resolution.

   Criteria that should be applied by the Designated Experts include
   determining whether the proposed registration duplicates existing
   functionality, determining whether it is likely to be of general
   applicability or whether it is useful only for a single application,
   and whether the registration makes sense.

   It is suggested that multiple Designated Experts be appointed who are
   able to represent the perspectives of different applications using
   this specification, in order to enable broadly-informed review of
   registration decisions.  In cases where a registration decision could
   be perceived as creating a conflict of interest for a particular
   Expert, that Expert should defer to the judgment of the other
   Experts.

6.1.  JSON Web Token Claims Registration

   This specification registers the "cnf" claim in the IANA "JSON Web
   Token Claims" registry [IANA.JWT.Claims] established by [JWT].

6.1.1.  Registry Contents

   o  Claim Name: "cnf"
   o  Claim Description: Confirmation
   o  Change Controller: IESG
   o  Specification Document(s): Section 3.1 of [[ this document ]]

6.2.  JWT Confirmation Methods Registry

   This specification establishes the IANA "JWT Confirmation Methods"
   registry for JWT "cnf" member values.  The registry records the
   confirmation method member and a reference to the specification that
   defines it.

6.2.1.  Registration Template

   Confirmation Method Value:
      The name requested (e.g., "kid").  Because a core goal of this
      specification is for the resulting representations to be compact,
      it is RECOMMENDED that the name be short -- not to exceed 8
      characters without a compelling reason to do so.  This name is
      case-sensitive.  Names may not match other registered names in a
      case-insensitive manner unless the Designated Experts state that
      there is a compelling reason to allow an exception.

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   Confirmation Method Description:
      Brief description of the confirmation method (e.g., "Key
      Identifier").

   Change Controller:
      For Standards Track RFCs, list the "IESG".  For others, give the
      name of the responsible party.  Other details (e.g., postal
      address, email address, home page URI) may also be included.

   Specification Document(s):
      Reference to the document or documents that specify the parameter,
      preferably including URIs that can be used to retrieve copies of
      the documents.  An indication of the relevant sections may also be
      included but is not required.

6.2.2.  Initial Registry Contents

   o  Confirmation Method Value: "jwk"
   o  Confirmation Method Description: JSON Web Key Representing Public
      Key
   o  Change Controller: IESG
   o  Specification Document(s): Section 3.2 of [[ this document ]]

   o  Confirmation Method Value: "jwe"
   o  Confirmation Method Description: Encrypted JSON Web Key
   o  Change Controller: IESG
   o  Specification Document(s): Section 3.3 of [[ this document ]]

   o  Confirmation Method Value: "kid"
   o  Confirmation Method Description: Key Identifier
   o  Change Controller: IESG
   o  Specification Document(s): Section 3.4 of [[ this document ]]

   o  Confirmation Method Value: "jku"
   o  Confirmation Method Description: JWK Set URL
   o  Change Controller: IESG
   o  Specification Document(s): Section 3.5 of [[ this document ]]

7.  References

7.1.  Normative References

   [IANA.JWT.Claims]
              IANA, "JSON Web Token Claims",
              <http://www.iana.org/assignments/jwt>.

   [JWE]      Jones, M. and J. Hildebrand, "JSON Web Encryption (JWE)",

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              RFC 7516, DOI 10.17487/RFC7156, May 2015,
              <http://www.rfc-editor.org/info/rfc7516>.

   [JWK]      Jones, M., "JSON Web Key (JWK)", RFC 7517, DOI 10.17487/
              RFC7157, May 2015,
              <http://www.rfc-editor.org/info/rfc7517>.

   [JWT]      Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
              (JWT)", RFC 7519, DOI 10.17487/RFC7159, May 2015,
              <http://www.rfc-editor.org/info/rfc7519>.

   [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>.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, DOI 10.17487/RFC3629,
              November 2003, <http://www.rfc-editor.org/info/rfc3629>.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <http://www.rfc-editor.org/info/rfc3986>.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              DOI 10.17487/RFC5226, May 2008,
              <http://www.rfc-editor.org/info/rfc5226>.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, DOI 10.17487/
              RFC5246, August 2008,
              <http://www.rfc-editor.org/info/rfc5246>.

   [RFC6125]  Saint-Andre, P. and J. Hodges, "Representation and
              Verification of Domain-Based Application Service Identity
              within Internet Public Key Infrastructure Using X.509
              (PKIX) Certificates in the Context of Transport Layer
              Security (TLS)", RFC 6125, DOI 10.17487/RFC6125,
              March 2011, <http://www.rfc-editor.org/info/rfc6125>.

7.2.  Informative References

   [I-D.ietf-oauth-pop-architecture]
              Hunt, P., Richer, J., Mills, W., Mishra, P., and H.
              Tschofenig, "OAuth 2.0 Proof-of-Possession (PoP) Security
              Architecture", draft-ietf-oauth-pop-architecture-05 (work

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              in progress), October 2015.

   [JWK.Thumbprint]
              Jones, M. and N. Sakimura, "JSON Web Key (JWK)
              Thumbprint", RFC 7638, DOI 10.17487/RFC7638,
              September 2015, <http://www.rfc-editor.org/info/rfc7638>.

   [OASIS.saml-core-2.0-os]
              Cantor, S., Kemp, J., Philpott, R., and E. Maler,
              "Assertions and Protocol for the OASIS Security Assertion
              Markup Language (SAML) V2.0", OASIS Standard saml-core-
              2.0-os, March 2005.

   [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>.

Appendix A.  Acknowledgements

   The authors wish to thank Brian Campbell, Kepeng Li, James Manger,
   Kathleen Moriarty, Justin Richer, and Nat Sakimura for their reviews
   of the specification.

Appendix B.  Document History

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

   -10

   o  Addressed review comments by Barry Leiba.

   -09

   o  Removed erroneous quotation marks around numeric "exp" claim
      values in examples.

   -08

   o  Added security consideration about also utilizing audience
      restriction.

   -07

   o  Addressed review comments by Hannes Tschofenig, Kathleen Moriarty,
      and Justin Richer.  Changes were:

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   o  Clarified that symmetric proof-of-possession keys can be generated
      by either the presenter or the issuer.

   o  Clarified that confirmation members that are not understood must
      be ignored unless otherwise specified by the application.

   -06

   o  Added diagrams to the introduction.

   -05

   o  Addressed review comments by Kepeng Li.

   -04

   o  Allowed the use of "jwk" for symmetric keys when the JWT is
      encrypted.

   o  Added the "jku" (JWK Set URL) member.

   o  Added privacy considerations.

   o  Reordered sections so that the "cnf" (confirmation) claim is
      defined before it is used.

   o  Noted that applications can define new claim names, in addition to
      "cnf", to represent additional proof-of-possession keys, using the
      same representation as "cnf".

   o  Applied wording clarifications suggested by Nat Sakimura.

   -03

   o  Separated the "jwk" and "jwe" confirmation members; the former
      represents a public key as a JWK and the latter represents a
      symmetric key as a JWE encrypted JWK.

   o  Changed the title to indicate that a proof-of-possession key is
      being communicated.

   o  Updated language that formerly assumed that the issuer was an
      OAuth 2.0 authorization server.

   o  Described ways that applications can choose to identify the
      presenter, including use of the "iss", "sub", and "azp" claims.

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   o  Harmonized the registry language with that used in JWT [RFC 7519].

   o  Addressed other issues identified during working group last call.

   o  Referenced the JWT and JOSE RFCs.

   -02

   o  Defined the terms Issuer, Presenter, and Recipient and updated
      their usage within the document.

   o  Added a description of a use case using an asymmetric proof-of-
      possession key to the introduction.

   o  Added the "kid" (key ID) confirmation method.

   o  These changes address the open issues identified in the previous
      draft.

   -01

   o  Updated references.

   -00

   o  Created the initial working group draft from
      draft-jones-oauth-proof-of-possession-02.

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/

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   Hannes Tschofenig
   ARM Limited
   Austria

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

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