Proof-Of-Possession Semantics for JSON Web Tokens (JWTs)
draft-ietf-oauth-proof-of-possession-02

OAuth Working Group                                             M. Jones
Internet-Draft                                                 Microsoft
Intended status: Standards Track                              J. Bradley
Expires: September 10, 2015                                Ping Identity
                                                           H. Tschofenig
                                                             ARM Limited
                                                           March 9, 2015

        Proof-Of-Possession Semantics for JSON Web Tokens (JWTs)
                draft-ietf-oauth-proof-of-possession-02

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
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   This Internet-Draft will expire on September 10, 2015.

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
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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.  Proof-Of-Possession Representation . . . . . . . . . . . . . .  4
     3.1.  Proof-of-Possession of an Asymmetric Key . . . . . . . . .  5
     3.2.  Proof-of-Possession of a Symmetric Key . . . . . . . . . .  5
     3.3.  Proof-of-Possession Using a Key ID . . . . . . . . . . . .  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  . . . . . . . . . . . . . . . . . .  9
     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 . . . . . . . . . . . . . . . . . . . . . . . . 12

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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 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 entire JWT is then
   integrity protected by the issuer (the authorization server).  The
   JWT is then sent to the presenter.  Since the presenter is unable to
   obtain the encrypted symmetric key from the JWT itself, 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 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 OAuth 2.0
   authorization server (the 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.  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.  The asymmetric key mechanism described
   above is conceptually similar to a certificate.

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   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)
   [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 different
      from the issuer.

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

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 presenter 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) or "kid" (key ID)
   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

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   "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 presenter 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:

     {
      "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.  Proof-of-Possession of a Symmetric Key

   When the key held by the presenter 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:

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     {
      "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",
      "cty": "jwk+json"
     }

   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,
      "cnf":{
        "jwk":
          "eyJhbGciOiJSU0ExXzUiLCJlbmMiOiJBMTI4Q0JDLUhTMjU2IiwiY3R5Ijoi
           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.  Proof-of-Possession Using a Key ID

   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 presenter of a JWT declares that it possesses a

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   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",
      "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" 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 defines a registry for these members in
   Section 5.2 and registers the "jwk" and "kid" members within the
   registry.

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.

   Note that another means of proving possession of the key when it is a

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   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.

   Values are registered with a Specification Required [RFC5226] after a

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   three-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: oauth-pop-reg-review@ietf.org. ]]

   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@ietf.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
   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
   Expert(s).

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"

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   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., "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
      description").

   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 3.1 of [[ this document ]]

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   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)",
              draft-ietf-jose-json-web-encryption (work in progress),
              January 2015.

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

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

   [I-D.ietf-oauth-pop-architecture]
              Hunt, P., ietf@justin.richer.org, i., 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,
              "Assertions and Protocol for the OASIS Security Assertion
              Markup Language (SAML) V2.0", OASIS Standard saml-core-
              2.0-os, March 2005.

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Appendix A.  Acknowledgements

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

Appendix B.  Document History

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

   -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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