Microsoft

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Security ACE Working Group CBOR Web Token CWT Proof-of-Possession Holder-of-Key This specification describes how to declare in a CBOR Web Token (CWT) that the presenter of the CWT possesses a particular proof-of-possession key and how the recipient can cryptographically confirm proof of possession of the key by the presenter. Being able to prove possession of a key is also sometimes described as the presenter being a holder-of-key. This specification is a profile of "Proof-of-Possession Key Semantics for JSON Web Tokens (JWTs)" (RFC 7800), but using CBOR and CWTs rather than JSON and JWTs.

This specification describes how a CBOR Web Token can declare that the presenter of the CWT possesses a particular proof-of-possession (PoP) key and how the recipient can cryptographically confirm proof of possession of the key by the presenter. Proof of possession of a key is also sometimes described as the presenter being a holder-of-key. This specification is a profile of "Proof-of-Possession Key Semantics for JSON Web Tokens (JWTs)" , but using CBOR and CWTs rather than JSON and JWTs .

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

This specification uses terms defined in the CBOR Web Token , , and Concise Binary Object Representation (CBOR) specifications. These terms are defined by this specification: Party that creates the CWT and binds the proof-of-possession key to it. Party that proves possession of a private key (for asymmetric key cryptography) or secret key (for symmetric key cryptography) to a recipient. Party that receives the CWT containing the proof-of-possession key information from the presenter.

By including a cnf (confirmation) claim in a CWT, the issuer of the CWT declares that the presenter possesses a particular key and that the recipient can cryptographically confirm that the presenter has possession of that key. The value of the cnf claim is a JSON object and the members of that object identify the proof-of-possession key. The presenter can be identified in one of several ways by the CWT depending upon the application requirements. If the CWT contains a sub (subject) claim , the presenter is normally the subject identified by the CWT. (In some applications, the subject identifier will be relative to the issuer identified by the iss (issuer) claim .) If the CWT contains no sub claim, the presenter is normally the issuer identified by the CWT using the iss claim. The case in which the presenter is the subject of the CWT is analogous to Security Assertion Markup Language (SAML) 2.0 SubjectConfirmation usage. At least one of the sub and iss claims MUST be present in the CWT. Some use cases may require that both be present.

The cnf claim is used in the CWT to contain members used to identify the proof-of-possession key. Other members of the cnf object may be defined 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). The set of confirmation members that a CWT must contain to be considered valid is context dependent and is outside the scope of this specification. Specific applications of CWTs will require implementations to understand and process some confirmation members in particular ways. However, in the absence of such requirements, all confirmation members that are not understood by implementations MUST be ignored. This specification establishes the IANA "CWT Confirmation Methods" registry for these members in and registers the members defined by this specification. Other specifications can register other members used for confirmation, including other members for conveying proof-of-possession keys using different key representations. The cnf claim value MUST represent only a single proof-of-possession key; thus, at most one of the COSE_Key and Encrypted_COSE_Key confirmation values defined below may be present. Note that if an application needs to represent multiple proof-of-possession keys in the same CWT, one way for it to achieve this is to use other claim names, in addition to cnf, to hold the additional proof-of-possession key information. These claims could use the same syntax and semantics as the cnf claim. Those claims would be defined by applications or other specifications and could be registered in the IANA "CBOR Web Token Claims" registry .

When the key held by the presenter is an asymmetric private key, the COSE_Key member is a COSE_Key representing the corresponding asymmetric public key. The following example demonstrates such a declaration in the CWT Claims Set of a CWT:

The COSE_Key MUST contain the required key members for a COSE_Key of that key type and MAY contain other COSE_Key members, including the kid (Key ID) member. The COSE_Key member MAY also be used for a COSE_Key representing a symmetric key, provided that the CWT is encrypted so that the key is not revealed to unintended parties. The means of encrypting a CWT is explained in . If the CWT is not encrypted, the symmetric key MUST be encrypted as described below.

When the key held by the presenter is a symmetric key, the Encrypted_COSE_Key member is an encrypted COSE_Key representing the symmetric key encrypted to a key known to the recipient using COSE_Encrypt or COSE_Encrypt0. The following example illustrates a symmetric key that could subsequently be encrypted for use in the Encrypted_COSE_Key member:

The COSE_Key representation is used as the plaintext when encrypting the key. The COSE_Key could, for instance, be encrypted using a COSE_Encrypt0 representation using the AES-CCM-16-64-128 algorithm. The following example CWT Claims Set of a CWT illustrates the use of an encrypted symmetric key as the Encrypted_COSE_Key member value:

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 issuer of a CWT declares that the presenter possesses a particular key and that the recipient can cryptographically confirm proof of possession of the key by the presenter by including a cnf claim in the CWT whose value is a JSON object with the JSON object containing a kid member identifying the key. The following example demonstrates such a declaration in the CWT Claims Set of a CWT:

The content of the kid value is application specific. For instance, some applications may choose to use a cryptographic hash of the public key value as the kid value.

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.

All of the security considerations that are discussed in also apply here. In addition, proof of possession introduces its own unique security issues. Possessing a key is only valuable if it is kept secret. Appropriate means must be used to ensure that unintended parties do not learn private key or symmetric key values. Applications utilizing proof of possession should also utilize audience restriction, as described in Section 4.1.3 of , as it provides different protections. Proof of possession can be used by recipients to reject messages from unauthorized senders. Audience restriction can be used by recipients to reject messages intended for different recipients. A recipient might not understand the cnf claim. Applications that require the proof-of-possession keys communicated with it to be understood and processed must ensure that the parts of this specification that they use are implemented. Proof of possession via encrypted symmetric secrets is subject to replay attacks. This attack can, for example, be avoided when a signed nonce or challenge is used since the recipient can use a distinct nonce or challenge for each interaction. Replay can also be avoided if a sub-key is derived from a shared secret that is specific to the instance of the PoP demonstration. As is the case with other information included in a CWT, 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 CWT learns about the entity that created the CWT since this will be important for any policy decisions. Integrity protection prevents an adversary from changing any elements conveyed within the CWT payload. Special care has to be applied when carrying symmetric keys inside the CWT since those not only require integrity protection but also confidentiality protection.

A proof-of-possession key can be used as a correlation handle if the same key is used with multiple parties. Thus, for privacy reasons, it is recommended that different proof-of-possession keys be used when interacting with different parties.

The following registration procedure is used for all the registries established by this specification. Values are registered on a Specification Required basis after a three-week review period on the cwt-reg-review@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 Experts may approve registration once they are satisfied that such a specification will be published. [[ Note to the RFC Editor: The name of the mailing list should be determined in consultation with the IESG and IANA. Suggested name: cwt-reg-review@ietf.org. ]] Registration requests sent to the mailing list for review should use an appropriate subject (e.g., "Request to Register CWT Confirmation Method: example"). 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 Experts include 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 evaluating the security properties of the item being registered and whether the registration makes sense. 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 Experts.

This specification registers the cnf claim in the IANA "CBOR Web Token Claims" registry established by .

Claim Name: cnf Claim Description: Confirmation JWT Claim Name: cnf CBOR Key Value: TBD (maybe 8) CBOR Major Type: 5 Change Controller: IESG Specification Document(s): of [[ this document ]]

This specification establishes the IANA "CWT Confirmation Methods" registry for CWT cnf member values. The registry records the confirmation method member and a reference to the specification that defines it.

The human-readable name requested (e.g., "kid"). Brief description of the confirmation method (e.g., "Key Identifier"). Claim Name of the equivalent JWT confirmation method value, as registered in . CWT claims should normally have a corresponding JWT claim. If a corresponding JWT claim would not make sense, the Designated Experts can choose to accept registrations for which the JWT Claim Name is listed as "N/A". Key value for the confirmation method. The key value MUST be an integer in the range of 1 to 65536. CBOR major type and, when applicable, minor type for the claim. For Standards Track RFCs, list the "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 or documents that specify the parameter, preferably including URIs that can be used to retrieve copies of the documents. An indication of the relevant sections may also be included but is not required.

Confirmation Method Name: COSE_Key Confirmation Method Description: COSE_Key Representing Public Key JWT Confirmation Method Name: jwk CBOR Key Value: 1 CBOR Major Type: 5 Change Controller: IESG Specification Document(s): of [[ this document ]] Confirmation Method Name: Encrypted_COSE_Key Confirmation Method Description: Encrypted COSE_Key JWT Confirmation Method Name: jwe CBOR Key Value: 2 CBOR Major Type: 4 (with an optional 6 tag prefix) Change Controller: IESG Specification Document(s): of [[ this document ]] Confirmation Method Name: kid Confirmation Method Description: Key Identifier JWT Confirmation Method Name: kid CBOR Key Value: 3 CBOR Major Type: 2 Change Controller: IESG Specification Document(s): of [[ this document ]]

Microsoft

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

erik@wahlstromstekniska.se

Spotify AB

erdtman@spotify.com

ARM Ltd.

Hannes.Tschofenig@arm.com

IANA Microsoft

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

Ping Identity

ve7jtb@ve7jtb.com

Nomura Research Institute

n-sakimura@nri.co.jp

Internet2

cantor.2@osu.edu

Nokia

John.Kemp@nokia.com

RSA Security

rphilpott@rsasecurity.com

Sun Microsystems

eve.maler@sun.com

IANA

TBD

Convert the examples from JSON/JWT to CBOR/CWT.

[[ to be removed by the RFC Editor before publication as an RFC ]] -00 Created the initial draft from RFC 7800.