| Internet-Draft | Unprotected CWT Claims Sets | November 2024 |
| Birkholz, et al. | Expires 7 May 2025 | [Page] |
This document defines the Unprotected CWT Claims Set (UCCS), a data format for representing a CBOR Web Token (CWT) Claims Set without protecting it by a signature, message authentication code (MAC), or encryption. UCCS enables the use of CWT claims in environments where protection is provided by other means, such as secure communication channels or trusted execution environments. This specification defines a CBOR tag for UCCS and describes the UCCS format, its encoding, and processing considerations, and discusses security implications of using unprotected claims sets.¶
(This editors' note will be removed by the RFC editor:)
The present revision (–12) contains remaining document changes
based on feedback from the IESG evaluation and has been submitted
as input to IETF 121.¶
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.¶
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.¶
Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.¶
Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."¶
This Internet-Draft will expire on 7 May 2025.¶
Copyright (c) 2024 IETF Trust and the persons identified as the document authors. All rights reserved.¶
This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License.¶
A CBOR Web Token (CWT) as specified by [RFC8392] is always wrapped in a CBOR Object Signing and Encryption (COSE, [STD96]) envelope. COSE provides -- among 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 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]: An actual RFC 8392 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 as CWT without having the COSE protection available: mutual authentication, integrity protection, confidentiality. (Replay protection can be added by including a nonce claim such as Nonce (claim 10 [IANA.cwt]).) Examples include conveyance via PCIe (Peripheral Component Interconnect Express) IDE (Integrity and Data Encryption) or a TLS tunnel.¶
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 [BCP14] (RFC2119) (RFC8174) when, and only when, they appear in all capitals, as shown here.¶
Section 2 briefly discusses use cases for UCCS. Section 3 addresses general characteristics of secure channels, followed by a specific discussion of using them in the context of RATS Conceptual Message Conveyance in Section 4, and finally some more forward-looking considerations for using UCCS in other RATS contexts in Section 5. Conventional sections (IANA Considerations, Security Considerations, Normative References, and Informative References) follow. The normative Appendix A provides a formal definition of the structure of UCCS as no formal definition of CWT Claims Sets was provided in [RFC8392]. This employs the Concise Data Definition Language (CDDL) [RFC8610], using its ability to also describe the structurally similar Unprotected JWT Claims Sets [RFC7519] (UJCS) in the same definition. Appendix B provides an (informative) example for CBOR-Tagged UCCS. The normative Appendix C provides CDDL rules that add UCCS-format tokens to Entity Attestation Tokens (EATs, see [I-D.ietf-rats-eat]) using its predefined extension points.¶
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, 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, 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 [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 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 a detailed usage scenario for UCCS in the context of RATS in conjunction with its attendant security requirements. The use of UCCS tag CPA601 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 on the authenticity and integrity properties of the Secure Channel used for conveying the UCCS to it.¶
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 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 UCCS emerge from the Secure Channel and into the receiver, 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 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 Entity Attestation Tokens (EATs, see [I-D.ietf-rats-eat]) 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.)¶
This section discusses two additional usage scenarios for UCCS in the context of RATS.¶
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 local inter-process communication, 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, when a privacy preserving Secure Channel is employed for conveying a conceptual message, the receiver cannot correlate the message with the senders of other received UCCS messages beyond the information the Secure Channel authentication provides.¶
An 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 RFC 8949 [STD94], 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 |
|---|---|---|
| CPA601 | map (Claims-Set as per Appendix A of [RFCthis]) | Unprotected CWT Claims Set [RFCthis] |
RFC-Editor: This document uses the CPA (code point allocation) convention described in [I-D.bormann-cbor-draft-numbers]. For each usage of the term "CPA", please remove the prefix "CPA" from the indicated value and replace the residue with the value assigned by IANA; perform an analogous substitution for all other occurrences of the prefix "CPA" in the document. Finally, please remove this note.¶
IANA is requested to add the following Media-Type to the "Media Types" registry [IANA.media-types].¶
| Name | Template | Reference |
|---|---|---|
| uccs+cbor | application/uccs+cbor | Section 6.2 of RFCthis |
application¶
uccs+cbor¶
n/a¶
n/a¶
binary (CBOR data item)¶
none¶
RFCthis¶
Applications that transfer Unprotected CWT Claims Set(s) (UCCS) over Secure Channels¶
The syntax and semantics of fragment identifiers is as specified for "application/cbor". (At publication of this document, there is no fragment identification syntax defined for "application/cbor".)¶
RATS WG mailing list (rats@ietf.org)¶
COMMON¶
none¶
IETF¶
IANA is requested to add the following Media-Type to the "Media Types" registry [IANA.media-types].¶
| Name | Template | Reference |
|---|---|---|
| ujcs+json | application/ujcs+json | Section 6.3 of RFCthis |
application¶
ujcs+json¶
n/a¶
n/a¶
binary (UTF-8)¶
none¶
RFCthis¶
Applications that transfer Unprotected JWT Claims Set(s) (UJCS) over Secure Channels¶
The syntax and semantics of fragment identifiers is as specified for "application/json". (At publication of this document, there is no fragment identification syntax defined for "application/json".)¶
RATS WG mailing list (rats@ietf.org)¶
COMMON¶
none¶
IETF¶
IANA is requested to register a Content-Format number in the "CoAP Content-Formats" subregistry, within the "Constrained RESTful Environments (CoRE) Parameters" registry [IANA.core-parameters], as follows:¶
| Content Type | Content Coding | ID | Reference |
|---|---|---|---|
| application/uccs+cbor | - | TBD601 | Section 6.4 of RFCthis |
RFC editor: please replace TBD601 by the number actually assigned by IANA (601 is suggested).¶
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.¶
Implementations of Secure Channels are often separate from the application logic that has security requirements on them. Similar security considerations to those described in [STD96] for obtaining the required levels of assurance include:¶
Implementations need to provide sufficient protection for private or secret key material used to establish or protect the Secure Channel.¶
Using a key for more than one algorithm can leak information about the key and is not recommended.¶
An algorithm used to establish or protect the Secure Channel may have limits on the number of times that a key can be used without leaking information about the key.¶
Evidence in a UCCS conveyed in a Secure Channel generally cannot be used to support trust in the credentials that were used to establish that secure channel, as this would create a circular dependency.¶
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:¶
Ensuring that any permissions associated with key ownership are respected in the establishment of the Secure Channel.¶
Using cryptographic algorithms appropriately.¶
Using key material in accordance with any usage restrictions such as freshness or algorithm restrictions.¶
Ensuring that appropriate protections are in place to address potential traffic analysis attacks.¶
Table 5 provides references to some security considerations of specific cryptography choices that are discussed in [RFC9053].¶
| Algorithm | Reference |
|---|---|
| AES-CBC-MAC | Section 3.2.1 of [RFC9053] |
| AES-GCM | Section 4.1.1 of [RFC9053] |
| AES-CCM | Section 4.2.1 of [RFC9053] |
| ChaCha20/Poly1305 | Section 4.3.1 of [RFC9053] |
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.¶
RFC-Editor: This document uses the CPA (code point allocation) convention described in [I-D.bormann-cbor-draft-numbers]. Please replace the number 601 in the code blocks below by the value that has been assigned for CPA601 and remove this note.¶
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", a "UJCS".
The CDDL definition of Claims-Set in Figure 1 can be used for a "UJCS":¶
UJCS = Claims-Set¶
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 CPA601:¶
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'
}
)
¶
The following CDDL adds UCCS-format and UJCS-format tokens to EAT using its predefined extension points (see Section 4.2.18 (submods) of [I-D.ietf-rats-eat]).¶
$EAT-CBOR-Tagged-Token /= UCCS-Tagged $EAT-CBOR-Untagged-Token /= UCCS-Untagged $JSON-Selector /= [type: "UJCS", nested-token: UJCS]¶
Laurence Lundblade suggested some improvements to the CDDL. Carl Wallace provided a very useful review.¶