Proof-Of-Possession Semantics for JSON Web Tokens (JWTs)
draft-jones-oauth-proof-of-possession-00

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

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Table of Contents

1.  Introduction
    1.1.  Notational Conventions
    1.2.  Terminology
2.  Proof-Of-Possession Representation
    2.1.  Proof-of-Possession of a Private Key
    2.2.  Proof-of-Possession of a Symmetric Key
    2.3.  Confirmation
    2.4.  Specifics Intentionally Not Specified
3.  Security Considerations
4.  IANA Considerations
    4.1.  JSON Web Token Claims Registration
        4.1.1.  Registry Contents
    4.2.  JWT Confirmation Methods Registry
        4.2.1.  Registration Template
        4.2.2.  Initial Registry Contents
5.  References
    5.1.  Normative References
    5.2.  Informative References
Appendix A.  Open Issues
Appendix B.  Document History
§  Authors' Addresses

1.  Introduction

This specification defines how to express a declaration in a JSON Web Token (JWT) [JWT] (Jones, M., Bradley, J., and N. Sakimura, “JSON Web Token (JWT),” March 2014.) 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.

1.1.  Notational Conventions

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).

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

1.2.  Terminology

This specification uses terms defined in the JSON Web Token (JWT) [JWT] (Jones, M., Bradley, J., and N. Sakimura, “JSON Web Token (JWT),” March 2014.), JSON Web Key (JWK) [JWK] (Jones, M., “JSON Web Key (JWK),” March 2014.), and JSON Web Encryption (JWE) [JWE] (Jones, M. and J. Hildebrand, “JSON Web Encryption (JWE),” March 2014.) specifications.

These terms are defined for use by this specification:

Presenter

Party that possesses the key identified by the JWT.

2.  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] (Cantor, S., Kemp, J., Philpott, R., and E. Maler, “Assertions and Protocol for the OASIS Security Assertion Markup Language (SAML) V2.0,” March 2005.) 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.

2.1.  Proof-of-Possession of a Private Key

When the key held by the issuer is a private key, the value of the jwk member is a JSON Web Key (JWK) [JWK] (Jones, M., “JSON Web Key (JWK),” March 2014.) representing the corresponding public key. The following example demonstrates such a declaration in the JWT Claims Set of a JWT:

  {
   "iss":"xas.xboxlive.com",
   "aud":"http://auth.xboxlive.com",
   "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 elements for a JWK of that key type and MAY contain other JWK elements, including the kid (key ID) element.

2.2.  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] (Jones, M., “JSON Web Key (JWK),” March 2014.) encrypted 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] (Jones, M., “JSON Web Key (JWK),” March 2014.) specification.

The following is an example symmetric key that could be encrypted for use in the jwk member:

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

The UTF-8 [RFC3629] (Yergeau, F., “UTF-8, a transformation format of ISO 10646,” November 2003.) encoding of this JWK would be used as the JWE Plaintext.

The following is an example JWE Header 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.

2.3.  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] (Cantor, S., Kemp, J., Philpott, R., and E. Maler, “Assertions and Protocol for the OASIS Security Assertion Markup Language (SAML) V2.0,” March 2005.) 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 elements in Section 4.2 (JWT Confirmation Methods Registry) and registers the jwk member within the registry.

2.4.  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 "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.

3.  Security Considerations

All of the normal security issues, especially in relationship to comparing URIs and dealing with unrecognized values, that are discussed in JWT (Jones, M., Bradley, J., and N. Sakimura, “JSON Web Token (JWT),” March 2014.) [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 secret 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.

4.  IANA Considerations

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

Values are registered with a Specification Required [RFC5226] (Narten, T. and H. Alvestrand, “Guidelines for Writing an IANA Considerations Section in RFCs,” May 2008.) 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 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).

4.1.  JSON Web Token Claims Registration

This specification registers the cnf claim in the IANA JSON Web Token Claims registry defined in [JWT] (Jones, M., Bradley, J., and N. Sakimura, “JSON Web Token (JWT),” March 2014.).

4.1.1.  Registry Contents

• Claim Name: cnf
• Claim Description: Confirmation
• Change Controller: IESG
• Specification Document(s): Section 2.3 (Confirmation) of this document

4.2.  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.

4.2.1.  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.

4.2.2.  Initial Registry Contents

• Confirmation Method Value: jwk
• Confirmation Method Description: JSON Web Key or Encrypted JSON Web Key
• Change Controller: IESG
• Specification Document(s): Section 2 (Proof-Of-Possession Representation) of [[ this document ]]

5.  References

5.1. Normative References

5.2. Informative References

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.  Document History

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

-00

• Wrote the first draft.

Authors' Addresses