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

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

Copyright Notice

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Table of Contents

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

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.

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 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:

  {
   "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 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 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 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

• Claim Name: cnf
• Claim Description: Confirmation
• Change Controller: IESG
• 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

• Confirmation Method Value: jwk
• Confirmation Method Description: JSON Web Key or Encrypted JSON Web Key
• Change Controller: IESG
• Specification Document(s): Section 3.1 of [[ this document ]]

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

6. References

6.1. Normative References

6.2. Informative References

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

• Defined the terms Issuer, Presenter, and Recipient and updated their usage within the document.
• Added a description of a use case using an asymmetric proof-of-possession key to the introduction.
• Added the kid (key ID) confirmation method.
• These changes address the open issues identified in the previous draft.

-01

• Updated references.

-00

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

Authors' Addresses