Microsoft

mbj@microsoft.com http://self-issued.info/

Ping Identity

ve7jtb@ve7jtb.com http://www.thread-safe.com/

ARM Limited

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

Security OAuth Working Group RFC Request for Comments I-D Internet-Draft JSON Web Token JWT Proof of Possession Holder of Key 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.

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. Envision the following use case. An OAuth 2.0 authorization server generates a JWT and places a 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 element). When the presenter needs to utilize the JWT to a 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 indeed interacts 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 a cryptographically protected messages exchanged with the presenter.

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 . Unless otherwise noted, all the protocol parameter names and values are case sensitive. This specification uses terms defined in the JSON Web Token (JWT) , JSON Web Key (JWK) , and JSON Web Encryption (JWE) specifications.

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 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. In cases where the presenter shall be anonymous only the iss (issuer) claim may be present identifying the party that issued the JWT.

When the key held by the issuer is an asymmetric key pair, the value of the jwk member is a JSON Web Key (JWK) representing the public key. The example shown in demonstrates such a declaration in the JWT Claims Set of a JWT:

The JWK MUST contain the required elements of a JWK (as needed for the particular type) and MAY contain other JWK elements. It is highly recommended to include the kid (key ID) element in the JWK as well since it allows the presenter and the recipient to later use this key id to refer to the exchanged 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) encrypted with a key known to the recipient. The rules for encrypting a JWK are found in Section 6 of the JSON Web Key specification. Note that the JWE compact serialization is used. The example shown in illustrates a symmetric key that is subsequently encrypted for use in the jwk member:

The UTF-8 encoding of the JWK shown in would be used as the JWE Plaintext. An example JWE header is shown in . It provides the necessary information for encrypting the JWK.

The example in illustrates a JWT with the encrypted symmetric key as the jwk claim value:

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.

The cnf (confirmation) claim is used in the JWT to contain the jwk element 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 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 and registers the jwk member within the registry.

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. For a protocol that applies the mechanisms described in this document to the OAuth 2.0 context please take a look at .

All of the normal security issues, especially in relationship to comparing URIs and dealing with unrecognized values, that are discussed in JWT also apply here. 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 do not only require integrity protection but also confidentiality protection. 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 symmetric key or the private key (in case of an asymmetric crypto-system). 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 .

The following registration procedure is used for all the registries established by this specification. Values are registered with a Specification Required 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 judgement of the other Expert(s).

This specification registers the cnf claim in the IANA JSON Web Token Claims registry defined in .

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

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.

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. Brief description of the confirmation method (e.g., "Example description"). 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. 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.

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

We would like to thanks James Manger for his review feedback.

[[ to be removed by the RFC Editor before publication as an RFC ]] -01 Updated affiliation. Various editorial changes. Updates to the security consideration section based on review feedback by James Manager. Included the kid element in the examples (as requested by James Manger). Expanded the introduction section. Moved the terminology/RFC2119 boilerplate text from the introduction to a separate terminology section. -00 Wrote the first draft.