| ACE | M. Jones |
| Internet-Draft | Microsoft |
| Intended status: Standards Track | L. Seitz |
| Expires: August 25, 2019 | RISE SICS |
| G. Selander | |
| Ericsson AB | |
| S. Erdtman | |
| Spotify | |
| H. Tschofenig | |
| ARM Ltd. | |
| February 21, 2019 |
Proof-of-Possession Key Semantics for CBOR Web Tokens (CWTs)
draft-ietf-ace-cwt-proof-of-possession-06
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. Being able to prove possession of a key is also sometimes described as being the holder-of-key. This specification provides equivalent functionality to "Proof-of-Possession Key Semantics for JSON Web Tokens (JWTs)" (RFC 7800), but using CBOR and CWTs rather than JSON and JWTs.
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This specification describes how a CBOR Web Token (CWT) [RFC8392] can declare that the presenter of the CWT possesses a particular proof-of-possession (PoP) key. Proof of possession of a key is also sometimes described as being the holder-of-key. This specification provides equivalent functionality to "Proof-of-Possession Key Semantics for JSON Web Tokens (JWTs)" [RFC7800], but using CBOR [RFC7049] and CWTs [RFC8392] rather than JSON [RFC8259] and JWTs [JWT].
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 BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.
This specification uses terms defined in the CBOR Web Token (CWT) [RFC8392], CBOR Object Signing and Encryption (COSE) [RFC8152], and Concise Binary Object Representation (CBOR) [RFC7049] specifications.
These terms are defined by this specification:
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 CBOR map and the members of that map identify the proof-of-possession key.
The presenter can be identified in one of several ways by the CWT, depending upon the application requirements. For instance, some applications may use the CWT sub (subject) claim [RFC8392], to identify the presenter. Other applications may use the iss claim to identify the presenter. In some applications, the subject identifier might be relative to the issuer identified by the iss (issuer) claim [RFC8392]. The actual mechanism used is dependent upon the application. The case in which the presenter is the subject of the CWT is analogous to Security Assertion Markup Language (SAML) 2.0 [OASIS.saml-core-2.0-os] SubjectConfirmation usage.
The cnf claim in the CWT is used to carry confirmation methods. Some of them use proof-of-possession keys while others do not. This design 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).
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 Section 7.2 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. At most one of the COSE_Key and Encrypted_COSE_Key confirmation values defined in Figure 1 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 [IANA.CWT.Claims].
/--------------------+-----+-------------------------------\ | Name | Key | Value type | |--------------------+-----+-------------------------------| | COSE_Key | 1 | COSE_Key | | Encrypted_COSE_Key | 2 | COSE_Encrypt or COSE_Encrypt0 | | kid | 3 | binary string | \--------------------+-----+-------------------------------/
Figure 1: Summary of the cnf names, keys, and value types
When the key held by the presenter is an asymmetric private key, the COSE_Key member is a COSE_Key [RFC8152] representing the corresponding asymmetric public key. The following example (using CBOR diagnostic notation) demonstrates such a declaration in the CWT Claims Set of a CWT:
{
/iss/ 1 : "coaps://server.example.com",
/aud/ 3 : "coaps://client.example.org",
/exp/ 4 : 1361398824,
/cnf/ 8 :{
/COSE_Key/ 1 :{
/kty/ 1 : /EC/ 2,
/crv/ -1 : /P-256/ 1,
/x/ -2 : h'd7cc072de2205bdc1537a543d53c60a6acb62eccd890c7fa27c9
e354089bbe13',
/y/ -3 : h'f95e1d4b851a2cc80fff87d8e23f22afb725d535e515d020731e
79a3b4e47120'
}
}
}
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 [RFC8392]. If the CWT is not encrypted, the symmetric key MUST be encrypted as described in Section 3.3.
When the key held by the presenter is a symmetric key, the Encrypted_COSE_Key member is an encrypted COSE_Key [RFC8152] representing the symmetric key encrypted to a key known to the recipient using COSE_Encrypt or COSE_Encrypt0.
The following example (using CBOR diagnostic notation, with linebreaks for readability) illustrates a symmetric key that could subsequently be encrypted for use in the Encrypted_COSE_Key member:
{
/kty/ 1 : /Symmetric/ 4,
/alg/ 3 : /HMAC256/ 5,
/k/ -1 : h'6684523ab17337f173500e5728c628547cb37df
e68449c65f885d1b73b49eae1'
}
The COSE_Key representation is used as the plaintext when encrypting the key.
The following example CWT Claims Set of a CWT (using CBOR diagnostic notation, with linebreaks for readability) illustrates the use of an encrypted symmetric key as the Encrypted_COSE_Key member value:
{
/iss/ 1 : "coaps://server.example.com",
/sub/ 2 : "24400320",
/aud/ 3: "s6BhdRkqt3",
/exp/ 4 : 1311281970,
/iat/ 5 : 1311280970,
/cnf/ 8 : {
/COSE_Encrypt0/ 2 : [
/protected header/ h'A1010A' /{ \alg\ 1:10 \AES-CCM-16-64-128\}/,
/unprotected header/ { / iv / 5: h'636898994FF0EC7BFCF6D3F95B'},
/ciphertext/ h'0573318A3573EB983E55A7C2F06CADD0796C9E584F1D0E3E
A8C5B052592A8B2694BE9654F0431F38D5BBC8049FA7F13F'
]
}
}
The example above was generated with the key:
h'6162630405060708090a0b0c0d0e0f10'
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 CBOR map with the CBOR map containing a kid member identifying the key.
The following example (using CBOR diagnostic notation) demonstrates such a declaration in the CWT Claims Set of a CWT:
{
/iss/ 1 : "coaps://server.example.com",
/aud/ 3 : "coaps://client.example.org",
/exp/ 4 : 1361398824,
/cnf/ 8 : {
/kid/ 2 : h'dfd1aa976d8d4575a0fe34b96de2bfad'
}
}
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.
Note that the use of a Key ID to identify a proof-of-possesion key needs to be carefully circumscribed, as described below and in Section 6. Where the Key ID is not a cryptographic value derived from the key or where all of the parties involved are not validating the cryptographic derivation, it is possible to get into situations where the same Key ID is being used for multiple keys. The implication of this is that a recipient may have multiple keys known to it that have the same Key ID, and thus it might not know which proof-of-possession key is associated with the CWT.
In the world of constrained Internet of Things (IoT) devices, there is frequently a restriction on the size of Key IDs, either because of table constraints or a desire to keep message sizes small. These restrictions are going to protocol dependent. For example, DTLS can use a Key ID of any size. However, if the key is being used with COSE encrypted message, then the length of the key needs to be minimized and may have a limit as small as one byte.
Note that the value of a Key ID is not always the same for different parties. When sending a COSE encrypted message with a shared key, the Key ID may be different on both sides of the conversation, with the appropriate one being included in the message based on the recipient of the message.
For symmetric keys, the Key ID is normally going to be generated by the CWT issuer. This means that enforcing a rule that Key ID values only match if CWTs have the same issuer works for matching Key IDs between CWTs. In this case, the issuer can ensure that there are no collisions between currently active symmetric keys for all CWTs that it has issued. This allows for a recipient to use the pair of issuer and Key ID for matching keys.
For asymmetric keys, the Key ID value is normally going to be generated by the CWT recipient, thus the possibility of collisions is greater. For instance, recipients might start by assigning a Key ID of 0, given that Key IDs are frequently only needed to be unique and meaningful to the recipient. This problem can be addressed in a couple of different ways, depending on how the Key ID value is going to be used:
Proof of possession is often 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 [RFC8392] 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 [JWT], as it provides additional protections. 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.
CBOR Web Tokens with proof-of-possession keys are used in context of an architecture, such as the ACE OAuth Framework [I-D.ietf-ace-oauth-authz], in which protocols are used by a presenter to request these tokens and to subsequently use them with recipients. To avoid replay attacks when the proof-of-possession tokens are sent to presenters, a security protocol, which uses mechansims such as nonces or timestamps, has to be utilized. Note that a discussion of the architecture or specific protocols that CWT proof-of-possession tokens are used with is beyond the scope of this specification.
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.
As described in Section 6 (Key Identification) and Appendix D (Notes on Key Selection) of [JWS], it is important to make explicit trust decisions about the keys. Proof-of-possession signatures made with keys not meeting the application's trust criteria MUST NOT be relied upon.
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 use of CWTs with proof-of-possession keys requires additional information to be shared between the involved parties in order to ensure correct processing. The recipient needs to be able to use credentials to verify the authenticity, integrity, and potentially the confidentiality of the CWT and its content. This requires the recipient to know information about the issuer. Likewise, there needs to be agreement between the issuer and the recipient about the claims being used (which is also true of CWTs in general).
When an issuer creates a CWT containing a Key ID claim, it needs to make sure that it does not issue another CWT containing the same Key ID with a different content, or for a different subject, within the lifetime of the CWTs, unless intentionally desired. Failure to do so may allow one party to impersonate another party, with the potential to gain additional privileges. Likewise, if PoP keys are used for multiple different kinds of CWTs in an application and the PoP keys are identified by Key IDs, care must be taken to keep the keys for the different kinds of CWTs segregated so that an attacker cannot cause the wrong PoP key to be used by using a valid Key ID for the wrong kind of CWT.
The following registration procedure is used for all the registries established by this specification.
Values are registered on a Specification Required [RFC8126] 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 [IANA.CWT.Claims] established by [RFC8392].
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.
| [IANA.CWT.Claims] | IANA, "CBOR Web Token Claims" |
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997. |
| [RFC7049] | Bormann, C. and P. Hoffman, "Concise Binary Object Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049, October 2013. |
| [RFC8126] | Cotton, M., Leiba, B. and T. Narten, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 8126, DOI 10.17487/RFC8126, June 2017. |
| [RFC8152] | Schaad, J., "CBOR Object Signing and Encryption (COSE)", RFC 8152, DOI 10.17487/RFC8152, July 2017. |
| [RFC8174] | Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017. |
| [RFC8392] | Jones, M., Wahlstroem, E., Erdtman, S. and H. Tschofenig, "CBOR Web Token (CWT)", RFC 8392, DOI 10.17487/RFC8392, May 2018. |
Thanks to the following people for their reviews of the specification: Roman Danyliw, Michael Richardson, and Jim Schaad.
Ludwig Seitz and Goeran Selander worked on this document as part of the CelticPlus project CyberWI, with funding from Vinnova.
[[ to be removed by the RFC Editor before publication as an RFC ]]
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