A CBOR Tag for Unprotected CWT Claims Sets

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Table of Contents

1. Introduction

A CBOR Web Token (CWT) as specified by [RFC8392] is always wrapped in a CBOR Object Signing and Encryption (COSE, [RFC8152]) envelope. COSE provides -- amongst other things -- the end-to-end data origin authentication and integrity protection employed by RFC 8392 and 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) and 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 secured 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. Example Use Cases

Use cases involving the conveyance of Claims, in particular, remote attestation procedures (RATS, see [I-D.ietf-rats-architecture]) 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 [I-D.ietf-teep-architecture]) or 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 further describe the RATS usage scenario 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 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 Remote ATtestation procedureS (RATS) Secure Channels, the following subsection 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. IANA Considerations

In the registry [IANA.cbor-tags], IANA is requested to allocate the tag in Table 1 from the FCFS space, with the present document as the specification reference.¶

5. Security Considerations

The security considerations of [RFC8949] apply. The security considerations of [RFC8392] need to be applied analogously, replacing the role of COSE with that of the Secured 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 Remote ATtestation procedureS (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.¶

Appendix A. CDDL

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

This specification proposes using the definitions in Figure 1 for the 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.¶

Claims-Set = {
 * $$Claims-Set-Claims
 * Claim-Label .feature "extended-claims-label" => any
}
Claim-Label = 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

; [RFC8693]
$$Claims-Set-Claims //= ( 9: CWT-scope ) ; scope
; TO DO: understand what this means:
; scope The scope of an access token as defined in [RFC6749].
; scope 9 byte string or text string [IESG] [RFC8693, Section 4.2]
CWT-scope = bytes / text

; [RFC-ietf-ace-oauth-authz-45, Section 5.10]
$$Claims-Set-Claims //= ( 38: CWT-ace-profile ) ; ace_profile
CWT-ace-profile = $CWT-ACE-Profiles /
  int .feature "ace_profile-extend"
; fill in from IANA registry
;   https://www.iana.org/assignments/ace/ace.xhtml#ace-profiles :
$CWT-ACE-Profiles /= 1 ; coap_dtls

$$Claims-Set-Claims //= ( 39: CWT-cnonce ) ; cnonce
CWT-cnonce = bytes

$$Claims-Set-Claims //= ( 40: CWT-exi ) ; exi
CWT-exi = uint ; in seconds (5.10.3)

;;; insert CDDL from 9052-to-be to complete these CDDL definitions.

Appendix B. Example

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:¶

 <TBD601>(
   {
     / 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'
   }
 )

Acknowledgements

Laurence Lundblade suggested some improvements to the CDDL.¶

Authors' Addresses