OAuth Working Group                                             M. Jones
Internet-Draft                                                 Microsoft
Intended status: Standards Track                              J. Bradley
Expires: January 22, August 1, 2015                                    Ping Identity
                                                           H. Tschofenig
                                                             ARM Limited
                                                           July 21, 2014
                                                        January 28, 2015

        Proof-Of-Possession Semantics for JSON Web Tokens (JWTs)


   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 this Memo

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . .   2 .  3
     1.1.  Notational Conventions . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Proof-Of-Possession Representation . . . . . . . . . . . . .   3 .  4
     3.1.  Proof-of-Possession of an Asymmetric Key . . . . . . . . .  4
     3.2.  Proof-of-Possession of a Symmetric Key . . . . . . . . .   4 .  5
     3.3.  Confirmation . . . . . . . . . . . . . . . . . . . . . .   5 .  6
     3.4.  Specifics Intentionally Not Specified  . . . . . . . . . .  6
   4.  Security Considerations  . . . . . . . . . . . . . . . . . . .  6
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  7
     5.1.  JSON Web Token Claims Registration . . . . . . . . . . . .  8
       5.1.1.  Registry Contents  . . . . . . . . . . . . . . . . . .  8
     5.2.  JWT Confirmation Methods Registry  . . . . . . . . . . . .  8
       5.2.1.  Registration Template  . . . . . . . . . . . . . . . .   8  9
       5.2.2.  Initial Registry Contents  . . . . . . . . . . . . . .  9
   6.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     6.1.  Normative References . . . . . . . . . . . . . . . . . . .  9
     6.2.  Informative References . . . . . . . . . . . . . . . . .   9 . 10
   Appendix A.  Open Issues . . . . . . . . . . . . . . . . . . . . . 10
   Appendix B.  Acknowledgements  . . . . . . . . . . . . . . . . . . 10
   Appendix C.  Document History  . . . . . . . . . . . . . . . . . . 10
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11

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.

   [[ Editorial Note: This paragraph needs to be updated to provide more
   context and possibly also to describe the use of asymmetric keys
   instead.  It's not clear that the symmetric case is as useful or
   valuable, and it is certainly more complicated.]]  Envision the
   following use case: An OAuth 2.0 authorization server 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 authorization server and the recipient.  The JWT is then
   sent to the presenter.  Since the presenter is unable to obtain the
   encrypted symmetric key, the authorization server 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 utilize
   the JWT to at 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 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.

1.1.  Notational Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   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]

   These terms are defined by this specification:

      Party that possesses the key identified by the JWT.

3.  Proof-Of-Possession Representation

   The presenter of a JWT declares that it possesses a particular key
   and that the recipient can cryptographically confirm proof-of-
   possession of the key by the issuer by including a "cnf"
   (confirmation) claim in the JWT whose value is a JSON object, with
   the JSON object containing a "jwk" (JSON Web Key) member identifying
   the key.

   The presenter can be identified in one of two ways by the JWT,
   depending upon the application requirements.  If the JWT contains a
   "sub" (subject) claim, the presenter is 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 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, and in some
   use cases, both MUST be present.

3.1.  Proof-of-Possession of an Asymmetric Key

   When the key held by the issuer is an asymmetric private key, the
   value of 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:


   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)

3.2.  Proof-of-Possession of a Symmetric Key

   When the key held by the issuer is a symmetric key, the value of the
   "jwk" member is an encrypted JSON Web Key (JWK) [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 6 of the JSON Web Key [JWK] specification.

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


   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:


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

      "iss": "https://server.example.com",
      "sub": "24400320",
      "aud": "s6BhdRkqt3",
      "nonce": "n-0S6_WzA2Mj",
      "exp": 1311281970,
      "iat": 1311280970,
           andrK2pzb24ifQ. ... (remainder of JWE omitted for brevity)"


   Note that the case in which the "jwk" claim contains an unencoded JWK
   value and the case in which it contains an encrypted JWK value can be
   distinguished by the type of the member value.  In the first case,
   the value is a JSON object containing the JWK and in the second case,
   the value is a string containing the JWE JSON Serialization of the
   encrypted JWK representation.

3.3.  Confirmation

   The "cnf" (confirmation) claim is used in the JWT to contain the
   "jwk" 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 defines a registry for these members in
   Section 5.2 and registers the "jwk" member within the registry.

3.4.  Specifics Intentionally Not Specified

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

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

   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.hunt-oauth-pop-architecture]. [I-D.ietf-oauth-pop-architecture].

5.  IANA Considerations

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

   Values are registered with a Specification Required [RFC5226] after a
   two-week review period on the [TBD]@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 Expert(s)
   may approve registration once they are satisfied that such a
   specification will be published.

   Registration requests must be sent to the [TBD]@ietf.org mailing list
   for review and comment, with an appropriate subject (e.g., "Request
   for access token type: example"). [[ Note to the RFC Editor: The name
   of the mailing list should be determined in consultation with the
   IESG and IANA.  Suggested name: jwt-reg-review. ]]
   Within the review period, the Designated Expert(s) 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@iesg.org mailing list) for resolution.

   Criteria that should be applied by the Designated Expert(s) 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 Expert(s)
   and should direct all requests for registration to the review mailing

   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

5.1.  JSON Web Token Claims Registration

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

5.1.1.  Registry Contents

   o  Claim Name: "cnf"
   o  Claim Description: Confirmation
   o  Change Controller: IESG
   o  Specification Document(s): Section 3.3 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., "example").  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 Expert(s) state that
      there is a compelling reason to allow an exception in this
      particular case.

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

   Change Controller:
      For Standards Track RFCs, state "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(s) that specify the parameter,
      preferably including URI(s) that can be used to retrieve copies of
      the document(s).  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 or Encrypted JSON
      Web Key
   o  Change Controller: IESG
   o  Specification Document(s): Section 3 of [[ this document ]]

6.  References

6.1.  Normative References

   [JWE]      Jones, M. and J. Hildebrand, "JSON Web Encryption (JWE)",
              draft-ietf-jose-json-web-encryption (work in progress),
              July 2014.
              January 2015.

   [JWK]      Jones, M., "JSON Web Key (JWK)", draft-ietf-jose-json-web-
              draft-ietf-jose-json-web-key (work in progress), July 2014.
              January 2015.

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

   [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


              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-02 draft-ietf-oauth-pop-architecture-00 (work
              in progress), June July 2014.

              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.

Appendix A.  Open Issues

   In some conversations, we have said that it is the issuer of the JWT
   that possesses the key, and in some conversations, we have said that
   it is the presenter of the JWT that possesses the key.  Which
   description should we use?

Appendix B.  Acknowledgements

   The authors wish to thank James Manger for his review of the

Appendix C.  Document History

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


   o  Used the same section structuring conventions as the JWT
   o  Reverted some changes introduced in -01 without adequate prior
   o  Applied some editorial corrections.

   o  Updated affiliation.
   o  Various editorial changes.
   o  Updates to the security considerations section based on review
      feedback by James Manager.
   o  Included the kid element in the examples (as requested by James
   o  Expanded the introduction section.
   o  Moved the terminology/RFC2119 boilerplate text from the
      introduction to a separate terminology section. references.


   o  Wrote  Created the first draft. initial working group draft from

Authors' Addresses

   Michael B. Jones

   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

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