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Versions: (draft-jones-oauth-proof-of-possession) 00 01 02 03 04 05 06 07 08 09 10 11 RFC 7800

OAuth Working Group                                             M. Jones
Internet-Draft                                                 Microsoft
Intended status: Standards Track                              J. Bradley
Expires: January 7, 2016                                   Ping Identity
                                                           H. Tschofenig
                                                             ARM Limited
                                                            July 6, 2015


      Proof-of-Possession Key Semantics for JSON Web Tokens (JWTs)
                draft-ietf-oauth-proof-of-possession-03

Abstract

   This specification defines how to express a declaration in a JSON Web
   Token (JWT) that the presenter of the JWT possesses a particular key
   and that the recipient can cryptographically confirm proof-of-
   possession of the key by the presenter.  This property 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 January 7, 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
   to this document.  Code Components extracted from this document must



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   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 . . . . . . . . . . . . . . . . . .  4
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Representations for Proof-of-Possession Keys . . . . . . . . .  4
     3.1.  Representation for an Asymmetric Proof-of-Possession
           Key  . . . . . . . . . . . . . . . . . . . . . . . . . . .  5
     3.2.  Representation for an Encrypted Symmetric
           Proof-of-Possession Key  . . . . . . . . . . . . . . . . .  5
     3.3.  Representation of a Key ID for a Proof-of-Possession
           Key  . . . . . . . . . . . . . . . . . . . . . . . . . . .  6
     3.4.  Confirmation . . . . . . . . . . . . . . . . . . . . . . .  7
     3.5.  Specifics Intentionally Not Specified  . . . . . . . . . .  7
   4.  Security Considerations  . . . . . . . . . . . . . . . . . . .  8
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  8
     5.1.  JSON Web Token Claims Registration . . . . . . . . . . . .  9
       5.1.1.  Registry Contents  . . . . . . . . . . . . . . . . . . 10
     5.2.  JWT Confirmation Methods Registry  . . . . . . . . . . . . 10
       5.2.1.  Registration Template  . . . . . . . . . . . . . . . . 10
       5.2.2.  Initial Registry Contents  . . . . . . . . . . . . . . 10
   6.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
     6.1.  Normative References . . . . . . . . . . . . . . . . . . . 11
     6.2.  Informative References . . . . . . . . . . . . . . . . . . 11
   Appendix A.  Acknowledgements  . . . . . . . . . . . . . . . . . . 12
   Appendix B.  Document History  . . . . . . . . . . . . . . . . . . 12
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13



















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1.  Introduction

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

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

   An issuer generates a JWT and places an encrypted symmetric key
   inside the newly introduced confirmation claim.  This symmetric key
   is encrypted with a key known only to the issuer and the recipient.
   The entire JWT is then integrity protected by the issuer.  The JWT is
   then sent to the presenter.  Since the presenter is unable to obtain
   the encrypted symmetric key from the JWT itself, the issuer conveys
   that symmetric key separately to the presenter.  Now, the presenter
   is in possession of the symmetric key as well as the JWT (which
   includes the confirmation claim member).  When the presenter needs to
   present the JWT to the recipient, it also needs to demonstrate
   possession of the symmetric key; the presenter, for example, 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 JWK contained inside 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.  This symmetric key
   mechanism described above is conceptually similar to the use of
   Kerberos tickets.

   In the second case, consider a presenter that generates a public/
   private key pair.  It then sends the public key to an issuer, which
   creates a JWT and places a public key (or an identifier for it)
   inside the newly introduced confirmation claim.  The entire JWT is
   integrity protected using a digital signature to protect it against
   modifications.  The JWT is then sent to the presenter.  When the
   presenter needs to present the JWT to the recipient, it also needs to
   demonstrate possession of the private key.  The presenter, for
   example, uses the private key in a TLS exchange with the recipient or
   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 checking the nonce signature.

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



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   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)
   [JWT], JSON Web Key (JWK) [JWK], and JSON Web Encryption (JWE) [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.  The presenter may be the issuer or a party distinct
      from the issuer.

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


3.  Representations for Proof-of-Possession Keys

   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 a
   JSON object containing a "jwk" (JSON Web Key) member, a "jwe" (JSON
   Web Encryption) member, or a "kid" (key ID) member identifying the
   key.

   The presenter can be identified in one of several ways by the JWT,
   depending upon the application requirements.  If the JWT contains a



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   "sub" (subject) claim, 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.)  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.  Representation for 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) [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",
      "nbf": "1360189224",
      "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.

3.2.  Representation for 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) [JWK] encrypted to a key
   known to the recipient using the JWE Compact Serialization containing



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   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 would be 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": "RSA1_5",
      "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":
          "eyJhbGciOiJSU0ExXzUiLCJlbmMiOiJBMTI4Q0JDLUhTMjU2In0.
          (remainder of JWE omitted for brevity)"
        }
     }

3.3.  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"



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   (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",
      "nbf": "1360189224",
      "cnf":{
        "kid": "dfd1aa97-6d8d-4575-a0fe-34b96de2bfad"
       }
     }

3.4.  Confirmation

   The "cnf" (confirmation) claim is used in the JWT to contain the
   "jwk", "jwe", or "kid" member 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.  When a recipient receives a "cnf" claim with a
   member that it does not understand, it MUST ignore that member.

   This specification establishes the IANA "JWT Confirmation Methods"
   registry for these members in Section 5.2 and registers the "jwk",
   "jwe", and "kid" members within the registry.  Other specifications
   can register other members used for confirmation, including members
   for conveying other proof-of-possession keys, possibly using
   different key representations.

3.5.  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.




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   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 an example specification that uses the mechanisms defined in this
   document, see [I-D.ietf-oauth-pop-architecture].


4.  Security Considerations

   All of the normal security issues, especially in relationship to
   comparing URIs and dealing with unrecognized values, that are
   discussed in JWT [JWT] also apply here.

   In addition, proof-of-possession introduces its own unique security
   issues.  Possessing the key is only valuable if it is kept secret.
   Appropriate means must be used to ensure that unintended parties do
   not learn the private key or symmetric key value.

   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
   challenged 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
   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 may not understand the newly introduced "cnf" claim and
   may consequently treat it as a bearer token.  While this is a
   legitimate concern, it is outside the scope of this specification,
   since demonstration the possession of the key associated with the
   "cnf" claim is not covered by this specification.  For more details,
   please consult [I-D.ietf-oauth-pop-architecture].


5.  IANA Considerations

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




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   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").

   Within the review period, the Designated Experts will either approve
   or deny the registration request, communicating this decision to the
   review list and IANA.  Denials should include an explanation and, if
   applicable, suggestions as to how to make the request successful.
   Registration requests that are undetermined for a period longer than
   21 days can be brought to the IESG's attention (using the
   iesg@ietf.org mailing list) for resolution.

   Criteria that should be applied by the Designated Experts includes
   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.

   IANA must only accept registry updates from the Designated Experts
   and should direct all requests for registration to the review mailing
   list.

   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.

5.1.  JSON Web Token Claims Registration

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







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5.1.1.  Registry Contents

   o  Claim Name: "cnf"
   o  Claim Description: Confirmation
   o  Change Controller: IESG
   o  Specification Document(s): Section 3.4 of this document

5.2.  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.

5.2.1.  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.

   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.

5.2.2.  Initial Registry Contents

   o  Confirmation Method Value: "jwk"
   o  Confirmation Method Description: JSON Web Key Representing Public
      Key
   o  Change Controller: IESG





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   o  Specification Document(s): Section 3.1 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 3.2 of [[ this document ]]

   o  Confirmation Method Value: "kid"
   o  Confirmation Method Description: Key Identifier
   o  Change Controller: IESG
   o  Specification Document(s): Section 3.3 of [[ this document ]]


6.  References

6.1.  Normative References

   [JWE]      Jones, M. and J. Hildebrand, "JSON Web Encryption (JWE)",
              RFC 7516, May 2015,
              <http://www.rfc-editor.org/info/rfc7516>.

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

   [JWT]      Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
              (JWT)", RFC 7519, 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, March 1997.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, November 2003.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.

6.2.  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-01 (work
              in progress), March 2015.

   [OASIS.saml-core-2.0-os]
              Cantor, S., Kemp, J., Philpott, R., and E. Maler,



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              "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, James Manger, 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 ]]

   -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.

   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.



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   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/


   Hannes Tschofenig
   ARM Limited
   Austria

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














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