RFC 9781: A Concise Binary Object Representation (CBOR) Tag for Unprotected CBOR Web Token Claims Sets (UCCS)

Status of This Memo

This is an Internet Standards Track document.¶

This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841.¶

Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at https://www.rfc-editor.org/info/rfc9781.¶

Copyright Notice

Copyright (c) 2025 IETF Trust and the persons identified as the document authors. All rights reserved.¶

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1. Introduction

A CBOR Web Token (CWT) as specified by [RFC8392] is always wrapped in a CBOR Object Signing and Encryption (COSE) envelope [STD96]. Among other things, COSE provides end-to-end data origin authentication and integrity protection employed by [RFC8392] as well as optional encryption for CWTs. Under the right circumstances (Section 3), 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 preexisting 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 effectively turns the channel into a mutually secured channel.¶

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]) for the conveyance of RATS Conceptual Messages.¶

This specification does not change [RFC8392]: A CWT as defined by [RFC8392] 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.¶

2. Deployment and Usage of UCCS

Usage scenarios involving the conveyance of Claims (RATS, in particular) require a standardized data definition and encoding format that can be transferred and transported using different communication channels. As these are Claims, the Claims Sets defined in [RFC8392] are 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) [RFC9397] or a Trusted Platform Module (TPM) [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.¶

3. Characteristics of a Secure Channel

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 are similar in security considerations to JWTs [BCP225] using the algorithm "none". Section 3.2 of RFC 8725 [BCP225] 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 RFC 8725 [BCP225] apply in an analogous way to the use of UCCS as elaborated on in this document. In particular, the need to "Use Appropriate Algorithms" (Section 3.2 of RFC 8725 [BCP225]) includes choosing appropriate cryptographic algorithms for setting up and protecting the Secure Channel. For instance, their cryptographic strength should be at least as strong as any cryptographic keys the Secure Channel will be used for to protect in transport. Table 5 in Section 7.2 provides references to some more security considerations for specific cryptography choices that are discussed in the COSE initial algorithms specification [RFC9053].¶

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. (Where the handshake did not provide a mutually secure channel, further authentication information can be conveyed by the party not yet authenticated, leading to a mutually secured channel.) A well-known example of 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.¶

4. UCCS in RATS Conceptual Message Conveyance

This section describes a detailed usage scenario for UCCS in the context of RATS in conjunction with its attendant security requirements. The use of UCCS tag 601 outside of the RATS context MUST come with additional instruction leaflets and security considerations.¶

For the purposes of this section, any RATS role can be the sender or the receiver 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.¶

In the scope of RATS Claims, the receiver MUST authenticate the sender as part of the establishment of the Secure Channel. Furthermore, the channel MUST provide integrity of the communication between the communicating RATS roles. For data confidentiality [RFC4949], the receiving side MUST be authenticated as well. This is achieved if the sender and receiver mutually authenticate when establishing the Secure Channel. The quality of the receiver's authentication and authorization will influence whether the sender can disclose the UCCS.¶

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. The assurance provided to a Relying Party depends, among others, on the authenticity and integrity properties of the Secure Channel used for conveying the UCCS to the Relying Party.¶

Ultimately, it is up to the receiver's policy to determine whether to accept a UCCS from the sender and to determine 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 end of the Secure Channel. However, the policy controls and definitions are out of scope for this document.¶

Where an Attesting Environment serves as an endpoint of a Secure Channel used to convey a UCCS, the security assurance required of that Attesting Environment by a Relying Party generally calls for the Attesting Environment to be implemented using techniques designed to provide enhanced protection from an attacker wishing to tamper with or forge a UCCS originating from that Attesting Environment. A possible approach might be to implement the Attesting Environment in a hardened environment, such as a TEE [RFC9397] or a TPM [TPM2].¶

When a UCCS emerges from the Secure Channel and into the receiver, the security properties of the secure channel no longer protect the UCCS, which is now subject to the same security properties as any other unprotected data in the Verifier environment. If the receiver subsequently forwards UCCS, they are treated as though they originated within the receiver.¶

The Secure Channel context does not govern fully formed CWTs in the same way it governs UCCS. As with EATs (see [RFC9711]) nested in other EATs (Section 4.2.18.3 (Nested Tokens) of [RFC9711]), 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 a nested UCCS Claim might be added to EAT, and this statement does not apply to UCCS nested into UCCS; it only applies to fully formed CWTs.)¶

5. Considerations for Using UCCS in Other RATS Contexts

This section discusses two additional usage scenarios for UCCS in the context of RATS.¶

6. IANA Considerations

7. Security Considerations

The security considerations of [STD94] apply. The security considerations of [RFC8392] need to be applied analogously, replacing the function of COSE with that of the Secure Channel; in particular, "it is not only important to protect the CWT in transit but also to ensure that the recipient can authenticate the party that assembled the claims and created the CWT".¶

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. In order to be able to use the UCCS CBOR tag in another such scope, the secure channel and/or the application protocol (e.g., TLS and the protocol identified by ALPN) MUST specify the roles of the endpoints in a fashion that the security properties of conveying UCCS via a Secure Channel between the roles are well-defined.¶

8. References

Appendix A. CDDL

The Concise Data Definition Language (CDDL), as defined in [RFC8610] and [RFC9165], provides an easy and unambiguous way to express structures for protocol messages and data formats that use CBOR or JSON.¶

[RFC8392] does not define CDDL for CWT Claims Sets.¶

The CDDL model in Figure 1 shows how to use CDDL for defining the CWT Claims Set defined in [RFC8392]. These CDDL rules have been built such that they also can describe [RFC7519] Claims sets by disabling feature "cbor" and enabling feature "json".¶

UCCS-Untagged = Claims-Set
UCCS-Tagged = #6.601(UCCS-Untagged)

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 the required CDDL from RFC 9052 to complete these
;;; definitions.  This can be done manually or automated by a
;;; tool that implements an import directive such as:
;# import rfc9052

The above definitions, concepts, and security considerations also define a JSON-encoded Claims-Set as encapsulated in a JWT. Such an unsigned Claims-Set can be referred to as a "Unprotected JWT Claims Set", or a "UJCS". The CDDL definition of Claims-Set in Figure 1 can be used for a UJCS:¶

UJCS = Claims-Set

Appendix B. Example

This appendix is informative.¶

The example CWT Claims Set from Appendix A.1 of [RFC8392] can be turned into a UCCS by enclosing it with a tag number 601:¶

 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'
   }
 )

Appendix C. EAT

The following CDDL adds UCCS-format and UJCS-format tokens to EAT using its predefined extension points (see Section 4.2.18 (submods) of [RFC9711]).¶

$EAT-CBOR-Tagged-Token /= UCCS-Tagged
$EAT-CBOR-Untagged-Token /= UCCS-Untagged

$JSON-Selector /= [type: "UJCS", nested-token: UJCS]

Acknowledgements

Laurence Lundblade suggested some improvements to the CDDL. Carl Wallace provided a very useful review.¶

Authors' Addresses