Status of This Memo

This is an Internet Standards Track document.¶

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Table of Contents

1.Introduction

A CBOR Web Token (CWT) as specified by[RFC8392] is always wrapped in aCBOR Object Signing and Encryption (COSE) envelope[STD96].Among other things, COSE provides end-to-end data originauthentication and integrity protection employed by[RFC8392] as well asoptional encryption for CWTs.Under the right circumstances (Section 3), a signature providing proof for authenticity and integrity can beprovided through the transfer protocol and thus omitted from theinformation in a CWT without compromising the intended goal of authenticityand integrity.In other words, if communicating parties have a preexisting securityassociation, they can reuse it to provide authenticity and integrityfor their messages, enabling the basic principle of using resourcesparsimoniously.Specifically, if a mutually secured channel is established between tworemote peers, and if that secure channel provides the requiredproperties (as discussed below), it is possible to omit the protectionprovided by COSE, creating a use case for unprotected CWT Claims Sets.Similarly, if there is one-way authentication, the party that did notauthenticate may be in a position to send authentication information throughthis channel that allows the already authenticated party to authenticate theother party; this effectively turns the channel into a mutuallysecured 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 ofRemote Attestation Procedures (RATS[RFC9334]) for theconveyance of RATS Conceptual Messages.¶

This specification does not change[RFC8392]: A CWT as defined by[RFC8392] does not make use ofthe tag allocated here; the UCCS tag is an alternative to using COSEprotection and a CWT tag.Consequently, within the well-defined scope of a secure channel, itcan be acceptable and economic to use the contents of a CWT withoutits COSE container and tag it with a UCCS CBOR tag for furtherprocessing within that scope -- or to use the contents of a UCCS CBORtag for building a CWT to be signed by some entity that can vouch forthose 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 thatcan be transferredand transported using different communication channels. As these areClaims, the Claims Sets defined in[RFC8392] area suitable format. However, the way these Claims are secured depends on the deployment, the securitycapabilities of the device, as well as their software stack. For example, a Claim may be securelystored and conveyed using a device's Trusted Execution Environment (TEE)[RFC9397] ora Trusted Platform Module (TPM)[TPM2].Especially in some resource-constrained environments,the same process that provides the secure communicationtransport is also the delegate to compose the Claim to be conveyed. Whether it is a transferor transport, a Secure Channel is presumed to be used for conveying such UCCS. The following sectionselaborate on Secure Channel characteristics in general and further describe RATS usage scenarios andcorresponding requirements for UCCS deployment.¶

3.Characteristics of a Secure Channel

A Secure Channel for the conveyance of UCCS needs to provide the securityproperties 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". Section3.2 of RFC 8725[BCP225] states:¶

[...] if a JWT is cryptographicallyprotected end-to-end by a transport layer, such as TLS usingcryptographically current algorithms, there may be no need to apply anotherlayer of cryptographic protections to the JWT. In such cases, the use ofthe "none" algorithm can be perfectly acceptable.¶

The security considerations discussed, e.g., in Sections2.1,3.1, and3.2 of RFC 8725[BCP225] apply in an analogous way to the use of UCCS aselaborated on in this document.In particular, the need to "Use Appropriate Algorithms" (Section3.2 of RFC 8725[BCP225]) includes choosing appropriate cryptographicalgorithms for setting up and protecting the Secure Channel.For instance, their cryptographic strength should be at least asstrong as any cryptographic keys the Secure Channel will be used forto protect in transport.Table 5 inSection 7.2 provides references to some more securityconsiderations for specific cryptography choices that are discussed inthe COSE initial algorithms specification[RFC9053].¶

Secure Channels are often set up in a handshake protocol that mutuallyderives a session key, where the handshake protocol establishes the (identity and thus) authenticity of one or both ends of the communication.The session key canthen be used to provide confidentiality and integrity of the transfer ofinformation inside the Secure Channel.(Where the handshake did not provide a mutually secure channel,further authentication information can be conveyed by the party notyet authenticated, leading to a mutually secured channel.)A well-known example of such aSecure Channel setup protocol is the TLS[RFC8446] handshake; theTLS record protocol can then be used for secure conveyance.¶

As UCCS were initially created for use in RATS Secure Channels, the followingsection provides a discussion oftheir use in these channels. Where other environments are intended to beused to convey UCCS, similar considerations need to be documented beforeUCCS can be used.¶

4.UCCS in RATS Conceptual Message Conveyance

This section describes a detailed usage scenario for UCCS in thecontext of RATS in conjunction with its attendant securityrequirements.The use of UCCS tag 601 outside of the RATS contextMUST 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 thisspecification, the mechanisms used to establish a Secure Channel are out ofscope.¶

In the scope of RATS Claims, the receiverMUSTauthenticate the sender as part of the establishment of the Secure Channel.Furthermore, the channelMUST provide integrity of the communication between thecommunicating RATS roles.For data confidentiality[RFC4949], the receiving sideMUST beauthenticated as well. This is achieved if the sender and receivermutually authenticate when establishing the Secure Channel.The quality of the receiver's authentication and authorization willinfluence whether the sender can disclose the UCCS.¶

The extent to which a Secure Channel can provide assurances that UCCSoriginate from a trustworthy Attesting Environment depends on thecharacteristics of both the cryptographic mechanisms used to establish thechannel and the characteristics of the Attesting Environment itself.The assurance provided to a Relying Party depends, among others, on the authenticityand integrity properties of the Secure Channel used for conveyingthe UCCS to the Relying Party.¶

Ultimately, it is up to the receiver's policy to determine whether to accepta 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 similarto COSE, the considerations of the Secure Channel should also adhere to the policyconfigured at each of end of the Secure Channel. However, the policy controlsand definitions are out of scope for this document.¶

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

When a UCCS emerges from the Secure Channel and into the receiver, the securityproperties of the secure channel no longer protect the UCCS, which is now subject to the same security propertiesas 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 thesame way it governs UCCS.As with EATs (see[RFC9711]) nested in other EATs (Section4.2.18.3 (Nested Tokens) of[RFC9711]), the SecureChannel does not endorse fully formed CWTs transferred through it.Effectively, the COSE envelope of a CWT (or a nested EAT) shields theCWT 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 tofully formed CWTs.)¶

5.Considerations for Using UCCS in Other RATS Contexts

This section discusses two additional usage scenarios for UCCS in thecontext 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; inparticular, "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 whichUCCS might be used.This document provides the CBOR tag definition for UCCS and a discussionon security consideration for the use of UCCS in RATS. Uses of UCCS outside the scope ofRATS are not covered by this document. The UCCS specification -- and theuse of the UCCS CBOR tag, correspondingly -- is not intended for use in ascope where a scope-specific security consideration discussion has notbeen 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 theprotocol identified by ALPN)MUST specify the roles of the endpointsin a fashion that the security properties of conveying UCCS via aSecure 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 expressstructures for protocol messages and data formats that use CBOR orJSON.¶

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

The CDDL model inFigure 1 shows how to use CDDLfor defining the CWT Claims Set defined in[RFC8392].These CDDL ruleshave been built such that they also can describe[RFC7519] Claims sets bydisabling feature "cbor" and enabling feature "json".¶

UCCS-Untagged = Claims-SetUCCS-Tagged = #6.601(UCCS-Untagged)Claims-Set = { * $$Claims-Set-Claims * Claim-Label .feature "extended-claims-label" => any}Claim-Label = CBOR-ONLY<int> / textstring-or-uri = text$$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 textJSON-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 supplyadditions to the $$Claims-Set-Claims socket, e.g.:¶

; [RFC8747]$$Claims-Set-Claims //= ( 8: CWT-cnf ) ; cnfCWT-cnf = {  (1: CWT-COSE-Key) //  (2: CWT-Encrypted_COSE_Key) //  (3: CWT-kid)}CWT-COSE-Key = COSE_KeyCWT-Encrypted_COSE_Key = COSE_Encrypt / COSE_Encrypt0CWT-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 JWTClaims Set", or a "UJCS".The CDDL definition ofClaims-Set inFigure 1 can be used for a UJCS:¶

UJCS = Claims-Set

Appendix B.Example

This appendix is informative.¶

The example CWT Claims Set fromAppendix A.1 of [RFC8392] can be turned intoa 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 Section4.2.18 (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