| Internet-Draft | Unprotected CWT Claims Sets | August 2023 |
| Birkholz, et al. | Expires 3 February 2024 | [Page] |
CBOR Web Token (CWT, RFC 8392) Claims Sets sometimes do not need the protection afforded by wrapping them into COSE, as is required for a true CWT. This specification defines a CBOR tag for such unprotected CWT Claims Sets (UCCS) and discusses conditions for its proper use.¶
This note is to be removed before publishing as an RFC.¶
Status information for this document may be found at https://datatracker.ietf.org/doc/draft-ietf-rats-uccs/.¶
Discussion of this document takes place on the Remote ATtestation procedureS (rats) Working Group mailing list (mailto:rats@ietf.org), which is archived at https://mailarchive.ietf.org/arch/browse/rats/. Subscribe at https://www.ietf.org/mailman/listinfo/rats/.¶
Source for this draft and an issue tracker can be found at https://github.com/ietf-rats-wg/draft-ietf-rats-uccs.¶
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A CBOR Web Token (CWT) as specified by [RFC8392] is always wrapped in a CBOR Object Signing and Encryption (COSE, [RFC9052]) envelope. COSE provides -- amongst other things -- end-to-end data origin authentication and integrity protection employed by RFC 8392 as well as optional encryption for CWTs. Under the right circumstances (Section 3), though, a signature providing proof for authenticity and integrity can be provided through the transfer protocol and thus omitted from the information in a CWT without compromising the intended goal of authenticity and integrity. In other words, if communicating parties have a pre-existing security association, they can reuse it to provide authenticity and integrity for their messages, enabling the basic principle of using resources parsimoniously. Specifically, if a mutually secured channel is established between two remote peers, and if that secure channel provides the required properties (as discussed below), it is possible to omit the protection provided by COSE, creating a use case for unprotected CWT Claims Sets. Similarly, if there is one-way authentication, the party that did not authenticate may be in a position to send authentication information through this channel that allows the already authenticated party to authenticate the other party.¶
This specification allocates a CBOR tag to mark Unprotected CWT Claims Sets (UCCS) as such and discusses conditions for its proper use in the scope of Remote Attestation Procedures (RATS [RFC9334]) in the usage scenario that is the conveyance of Evidence from an Attester to a Verifier.¶
This specification does not change [RFC8392]: A true CWT does not make use of the tag allocated here; the UCCS tag is an alternative to using COSE protection and a CWT tag. Consequently, within the well-defined scope of a secure channel, it can be acceptable and economic to use the contents of a CWT without its COSE container and tag it with a UCCS CBOR tag for further processing within that scope -- or to use the contents of a UCCS CBOR tag for building a CWT to be signed by some entity that can vouch for those contents.¶
The term Claim is used as in [RFC7519].¶
The terms Claim Key, Claim Value, and CWT Claims Set are used as in [RFC8392].¶
The terms Attester, Attesting Environment, Evidence, Relying Party and Verifier are used as in [RFC9334].¶
Unprotected CWT Claims Set(s); CBOR map(s) of Claims as defined by the CWT Claims Registry that are composed of pairs of Claim Keys and Claim Values.¶
[NIST-SP800-90Ar1] defines a Secure Channel as follows:¶
For the purposes of the present document, we focus on a protected communication channel used for conveyance that can ensure the same qualities associated for UCCS conveyance as CWT conveyance without any additional protection. Note that this means that, in specific cases, the Secure Channel as defined here does not itself provide mutual authentication. See Section 3.¶
All terms referenced or defined in this section are capitalized in the remainder of this document.¶
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.¶
Usage scenarios involving the conveyance of Claims, in particular RATS, require a standardized data definition and encoding format that can be transferred and transported using different communication channels. As these are Claims, [RFC8392] is a suitable format. However, the way these Claims are secured depends on the deployment, the security capabilities of the device, as well as their software stack. For example, a Claim may be securely stored and conveyed using a device's Trusted Execution Environment (TEE, see [RFC9397]) or a Trusted Platform Module (TPM, see [TPM2]). Especially in some resource constrained environments, the same process that provides the secure communication transport is also the delegate to compose the Claim to be conveyed. Whether it is a transfer or transport, a Secure Channel is presumed to be used for conveying such UCCS. The following sections elaborate on Secure Channel characteristics in general and further describe RATS usage scenarios and corresponding requirements for UCCS deployment.¶
A Secure Channel for the conveyance of UCCS needs to provide the security properties that would otherwise be provided by COSE for a CWT. In this regard, UCCS is similar in security considerations to JWTs [RFC8725] using the algorithm "none". RFC 8725 states:¶
[...] if a JWT is cryptographically protected end-to-end by a transport layer, such as TLS using cryptographically current algorithms, there may be no need to apply another layer of cryptographic protections to the JWT. In such cases, the use of the "none" algorithm can be perfectly acceptable.¶
The security considerations discussed, e.g., in Sections 2.1, 3.1, and 3.2 of [RFC8725] apply in an analogous way to the use of UCCS as elaborated on in this document.¶
Secure Channels are often set up in a handshake protocol that mutually derives a session key, where the handshake protocol establishes the (identity and thus) authenticity of one or both ends of the communication. The session key can then be used to provide confidentiality and integrity of the transfer of information inside the Secure Channel. A well-known example of a such a Secure Channel setup protocol is the TLS [RFC8446] handshake; the TLS record protocol can then be used for secure conveyance.¶
As UCCS were initially created for use in RATS Secure Channels, the following section provides a discussion of their use in these channels. Where other environments are intended to be used to convey UCCS, similar considerations need to be documented before UCCS can be used.¶
This section describes three detailed usage scenarios for UCCS in the context of RATS.¶
For the purposes of this section, the Verifier is the receiver of the UCCS and the Attester is the provider of the UCCS.¶
Secure Channels can be transient in nature. For the purposes of this specification, the mechanisms used to establish a Secure Channel are out of scope.¶
As a minimum requirement in the scope of RATS Claims, the Verifier MUST authenticate the Attester as part of the establishment of the Secure Channel. Furthermore, the channel MUST provide integrity of the communication from the Attester to the Verifier. If confidentiality is also required, the receiving side MUST be authenticated as well; this can be achieved if the Verifier and the Attester mutually authenticate when establishing the Secure Channel.¶
The extent to which a Secure Channel can provide assurances that UCCS originate from a trustworthy Attesting Environment depends on the characteristics of both the cryptographic mechanisms used to establish the channel and the characteristics of the Attesting Environment itself.¶
A Secure Channel established or maintained using weak cryptography may not provide the assurance required by a Relying Party of the authenticity and integrity of the UCCS.¶
Ultimately, it is up to the Verifier's policy to determine whether to accept a UCCS from the Attester and to the type of Secure Channel it must negotiate. While the security considerations of the cryptographic algorithms used are similar to COSE, the considerations of the Secure Channel should also adhere to the policy configured at each of the Attester and the Verifier. However, the policy controls and definitions are out of scope for this document.¶
Where the security assurance required of an Attesting Environment by a Relying Party requires it, the Attesting Environment SHOULD be implemented using techniques designed to provide enhanced protection from an attacker wishing to tamper with or forge UCCS. A possible approach might be to implement the Attesting Environment in a hardened environment such as a TEE [RFC9397] or a TPM [TPM2].¶
When UCCS emerge from the Secure Channel and into the Verifier, the security properties of the secure channel no longer protect the UCCS, which now are subject to the same security properties as any other unprotected data in the Verifier environment. If the Verifier subsequently forwards UCCS, they are treated as though they originated within the Verifier.¶
As with EATs nested in other EATs (Section 4.2.18.3 (Nested Tokens) of [I-D.ietf-rats-eat]), the Secure Channel does not endorse fully formed CWTs transferred through it. Effectively, the COSE envelope of a CWT (or a nested EAT) shields the CWT Claims Set from the endorsement of the secure channel. (Note that EAT might add a nested UCCS Claim, and this statement does not apply to UCCS nested into UCCS, only to fully formed CWTs.)¶
Another usage scenario is that of a sub-Attester that has no signing keys (for example, to keep the implementation complexity to a minimum) and has a Secure Channel, such as a local IPC, to interact with a lead Attester (see Composite Device, Section 3.3 of [RFC9334]). The sub-Attester produces a UCCS with the required CWT Claims Set and sends the UCCS through the Secure Channel to the lead Attester. The lead Attester then computes a cryptographic hash of the UCCS and protects that hash using its signing key for Evidence, for example, using a Detached-Submodule-Digest or Detached EAT Bundle (Section 5 of [I-D.ietf-rats-eat]).¶
A Secure Channel which preserves the privacy of the Attester may provide security properties equivalent to COSE, but only inside the life-span of the session established. In general, a Verifier cannot correlate UCCS received in different sessions from the same Attesting Environment based on the cryptographic mechanisms used when a privacy preserving Secure Channel is employed.¶
In the case of Remote Attestation, the Attester must consider whether any UCCS it returns over a privacy preserving Secure Channel compromises the privacy in unacceptable ways. As an example, the use of the EAT UEID Claim Section 4.2.1 of [I-D.ietf-rats-eat] in UCCS over a privacy preserving secure channel allows a verifier to correlate UCCS from a single Attesting Environment across many Secure Channel sessions. This may be acceptable in some use-cases (e.g., if the Attesting Environment is a physical sensor in a factory) and unacceptable in others (e.g., if the Attesting Environment is a user device belonging to a child).¶
In the CBOR Tags registry [IANA.cbor-tags] as defined in Section 9.2 of [RFC8949], IANA is requested to allocate the tag in Table 1 from the Specification Required space (1+2 size), with the present document as the specification reference.¶
| Tag | Data Item | Semantics |
|---|---|---|
| TBD601 | map (Claims-Set as per Appendix A of [RFCthis]) | Unprotected CWT Claims Set [RFCthis] |
The security considerations of [RFC8949] apply. The security considerations of [RFC8392] need to be applied analogously, replacing the function of COSE with that of the Secure Channel.¶
Section 3 discusses security considerations for Secure Channels, in which UCCS might be used. This document provides the CBOR tag definition for UCCS and a discussion on security consideration for the use of UCCS in RATS. Uses of UCCS outside the scope of RATS are not covered by this document. The UCCS specification -- and the use of the UCCS CBOR tag, correspondingly -- is not intended for use in a scope where a scope-specific security consideration discussion has not been conducted, vetted and approved for that use.¶
Implementations of Secure Channels are often separate from the application logic that has security requirements on them. Similar security considerations to those described in [RFC9052] for obtaining the required levels of assurance include:¶
The Verifier needs to ensure that the management of key material used to establish or protect the Secure Channel is acceptable. This may include factors such as:¶
Section 3.2.1 of [RFC9053] contains a detailed explanation of these considerations.¶
Section 4.1.1 of [RFC9053] contains a detailed explanation of these considerations.¶
Section 4.2.1 of [RFC9053] contains a detailed explanation of these considerations.¶
Section 4.3.1 of [RFC9053] contains a detailed explanation of these considerations.¶
[RFC8392] does not define CDDL for CWT Claims Sets.¶
This specification proposes using the definitions in Figure 1 for the CWT Claims Set defined in [RFC8392]. Note that these definitions have been built such that they also can describe [RFC7519] Claims sets by disabling feature "cbor" and enabling feature "json", but this flexibility is not the subject of the present specification.¶
UCCS = #6.601(Claims-Set)
Claims-Set = {
* $$Claims-Set-Claims
* Claim-Label .feature "extended-claims-label" => any
}
Claim-Label = CBOR-ONLY<int> / text
string-or-uri = text
$$Claims-Set-Claims //= ( iss-claim-label => string-or-uri )
$$Claims-Set-Claims //= ( sub-claim-label => string-or-uri )
$$Claims-Set-Claims //= ( aud-claim-label => string-or-uri )
$$Claims-Set-Claims //= ( exp-claim-label => ~time )
$$Claims-Set-Claims //= ( nbf-claim-label => ~time )
$$Claims-Set-Claims //= ( iat-claim-label => ~time )
$$Claims-Set-Claims //= ( cti-claim-label => bytes )
iss-claim-label = JC<"iss", 1>
sub-claim-label = JC<"sub", 2>
aud-claim-label = JC<"aud", 3>
exp-claim-label = JC<"exp", 4>
nbf-claim-label = JC<"nbf", 5>
iat-claim-label = JC<"iat", 6>
cti-claim-label = CBOR-ONLY<7> ; jti in JWT: different name and text
JSON-ONLY<J> = J .feature "json"
CBOR-ONLY<C> = C .feature "cbor"
JC<J,C> = JSON-ONLY<J> / CBOR-ONLY<C>
Specifications that define additional Claims should also supply additions to the $$Claims-Set-Claims socket, e.g.:¶
; [RFC8747]
$$Claims-Set-Claims //= ( 8: CWT-cnf ) ; cnf
CWT-cnf = {
(1: CWT-COSE-Key) //
(2: CWT-Encrypted_COSE_Key) //
(3: CWT-kid)
}
CWT-COSE-Key = COSE_Key
CWT-Encrypted_COSE_Key = COSE_Encrypt / COSE_Encrypt0
CWT-kid = bytes
;;; insert CDDL from RFC9052 to complete these CDDL definitions.
;# include RFC9052
¶
The example CWT Claims Set from Appendix A.1 of [RFC8392] can be turned into an UCCS by enclosing it with a tag number TBD601:¶
601(
{
/ iss / 1: "coap://as.example.com",
/ sub / 2: "erikw",
/ aud / 3: "coap://light.example.com",
/ exp / 4: 1444064944,
/ nbf / 5: 1443944944,
/ iat / 6: 1443944944,
/ cti / 7: h'0b71'
}
)
¶
Laurence Lundblade suggested some improvements to the CDDL. Carl Wallace provided a very useful review.¶