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

A Concise Binary Object Representation (CBOR) Tag for Unprotected CBOR Web Token Claims Sets (UCCS) Fraunhofer SIT

Rheinstrasse 75 Darmstadt 64295 Germany henk.birkholz@ietf.contact

Qualcomm Technologies Inc.

279 Farnborough Road Farnborough GU14 7LS United Kingdom jodonogh@qti.qualcomm.com

Cisco Systems

3550 Cisco Way San Jose CA 95134 United States of America ncamwing@cisco.com

Universität Bremen TZI

Postfach 330440 Bremen D-28359 Germany +49-421-218-63921 cabo@tzi.org

SEC rats Secure Channel RATS Conceptual Message Conveyance Privacy Confidentiality CDDL EAT UJCS CWT 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 its processing considerations. It also discusses security implications of using unprotected claims sets.

Introduction A CBOR Web Token (CWT) as specified by is always wrapped in a CBOR Object Signing and Encryption (COSE) envelope . Among other things, COSE provides end-to-end data origin authentication and integrity protection employed by as well as optional encryption for CWTs. Under the right circumstances (), 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 ) for the conveyance of RATS Conceptual Messages. This specification does not change : A CWT as defined by 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.

Terminology The term Claim is used as in . The terms Claim Key, Claim Value, and CWT Claims Set are used as in . The terms Attester, Attesting Environment, Evidence, Relying Party and Verifier are used as in .

UCCS:: 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.
Secure Channel:: defines a Secure Channel as follows: "A path for transferring data between two entities or components that ensures confidentiality, integrity and replay protection, as well as mutual authentication between the entities or components. The secure channel may be provided using approved cryptographic, physical or procedural methods, or a combination thereof." For the purposes of the present document, we focus on a protected communication channel used for conveyance that can ensure the same qualities as a CWT without having COSE protection available, which includes mutual authentication, integrity protection, and confidentiality. (Replay protection can be added by including a nonce claim such as Nonce (claim 10 ).) 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 BCP 14 when, and only when, they appear in all capitals, as shown here.

Structure of This Document briefly discusses use cases for UCCS. addresses general characteristics of secure channels, followed by a specific discussion of using them in the context of RATS Conceptual Message Conveyance in , and more forward-looking considerations for using UCCS in other RATS contexts are discussed in . This is followed by the , , , and . The normative provides a formal definition of the structure of UCCS, as no formal definition of CWT Claims Sets was provided in . This employs the Concise Data Definition Language (CDDL) , using its ability to also describe in the same definition the structurally similar use of JWT Claims Sets , without any protective wrapper (such as JWS) applied, as Unprotected JWT Claims Sets (UJCS). provides an (informative) example for CBOR-Tagged UCCS. The normative provides CDDL rules that add UCCS-format tokens to Entity Attestation Tokens (EATs) using its predefined extension points.

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 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) or a Trusted Platform Module (TPM) . 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.

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 using the algorithm "none". Section of 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 , , and of RFC 8725 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 of RFC 8725 ) 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. in provides references to some more security considerations for specific cryptography choices that are discussed in the COSE initial algorithms specification . 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 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.

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 , 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 or a TPM . 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 ) nested in other EATs (Section Nested Tokens of ), 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.)

Considerations for Using UCCS in Other RATS Contexts This section discusses two additional usage scenarios for UCCS in the context of RATS.

Delegated Attestation 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", ). 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 ().

Privacy Preservation A Secure Channel that 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 to convey 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 () 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).

IANA Considerations

CBOR Tag Registration In the "CBOR Tags" registry as defined in Section of RFC 8949 , IANA has allocated the tag in from the Specification Required space (1+2 size), with the present document as the specification reference. Values for Tags

Tag	Data Item	Semantics
601	map (Claims-Set as per of [RFC9781])	Unprotected CWT Claims Set [RFC9781]

Media-Type application/uccs+cbor Registration IANA has added the following to the "Media Types" registry . Media Type Registration

Name	Template	Reference
uccs+cbor	application/uccs+cbor	of RFC 9781

Type name:

application

Subtype name:

uccs+cbor

Required parameters:

N/A

Optional parameters:

N/A

Encoding considerations:

binary (CBOR data item)

Security considerations:

of RFC 9781

Interoperability considerations:

none

Published specification:

RFC 9781

Applications that use this media type:

Applications that transfer Unprotected CWT Claims Set(s) (UCCS) over Secure Channels

Fragment identifier considerations:

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".)

Additional information:

Deprecated alias names for this type:: N/A
Magic number(s):: N/A
File extension(s):: .uccs
Macintosh file type code(s):: N/A

Person and email address to contact for further information:

RATS WG mailing list (rats@ietf.org)

Intended usage:

COMMON

Restrictions on usage:

none

Author/Change controller:

IETF

Media-Type application/ujcs+json Registration IANA has added the following to the "Media Types" registry . JSON Media Type Registration

Name	Template	Reference
ujcs+json	application/ujcs+json	of RFC 9781

Type name:

application

Subtype name:

ujcs+json

Required parameters:

N/A

Optional parameters:

N/A

Encoding considerations:

binary (UTF-8)

Security considerations:

of RFC 9781

Interoperability considerations:

none

Published specification:

RFC 9781

Applications that use this media type:

Applications that transfer Unprotected JWT Claims Set(s) (UJCS) over Secure Channels

Fragment identifier considerations:

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".)

Additional information:

Deprecated alias names for this type:: N/A
Magic number(s):: N/A
File extension(s):: .ujcs
Macintosh file type code(s):: N/A

Person and email address to contact for further information:

RATS WG mailing list (rats@ietf.org)

Intended usage:

COMMON

Restrictions on usage:

none

Author/Change controller:

IETF

Content-Format Registration IANA has registered the following in the "CoAP Content-Formats" registry within the "Constrained RESTful Environments (CoRE) Parameters" registry group . Content-Format Registration

Content Type	Content Coding	ID	Reference
application/uccs+cbor	-	601	of RFC 9781

Security Considerations The security considerations of apply. The security considerations of 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". 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.

General Considerations Implementations of Secure Channels are often separate from the application logic that has security requirements on them. Similar security considerations to those described in 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.

Algorithm-Specific Security Considerations provides references to some security considerations of specific cryptography choices that are discussed in . Algorithm-Specific Security Considerations

Algorithm	Reference
AES-CBC-MAC
AES-GCM
AES-CCM
ChaCha20/Poly1305

References Normative References Concise Binary Object Representation (CBOR) Tags IANA CBOR Web Token (CWT) Claims IANA Informative References Media Types IANA Constrained RESTful Environments (CoRE) Parameters IANA Trusted Platform Module 2.0 Library Trusted Computing Group Version 184 Recommendation for Random Number Generation Using Deterministic Random Bit Generators

CDDL The Concise Data Definition Language (CDDL), as defined in and , provides an easy and unambiguous way to express structures for protocol messages and data formats that use CBOR or JSON. does not define CDDL for CWT Claims Sets. The CDDL model in shows how to use CDDL for defining the CWT Claims Set defined in . These CDDL rules have been built such that they also can describe Claims sets by disabling feature "cbor" and enabling feature "json".

CDDL definition for Claims-Set any } Claim-Label = CBOR-ONLY / 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 ) $$Claims-Set-Claims //= ( cti-claim-label => bytes ) iss-claim-label = JC<"iss", 1> sub-claim-label = JC<"sub", 2> aud-claim-label = JC<"aud", 3> exp-claim-label = JC<"exp", 4> nbf-claim-label = JC<"nbf", 5> iat-claim-label = JC<"iat", 6> cti-claim-label = CBOR-ONLY<7> ; jti in JWT: different name and text JSON-ONLY = J .feature "json" CBOR-ONLY = C .feature "cbor" JC = JSON-ONLY / CBOR-ONLY ]]> Specifications that define additional Claims should also supply additions to the $$Claims-Set-Claims socket, e.g.: 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 can be used for a UJCS:

Example This appendix is informative. The example CWT Claims Set from can be turned into a UCCS by enclosing it with a tag number 601:

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

Acknowledgements suggested some improvements to the CDDL. provided a very useful review.