Selective Disclosure CWTs (SD-CWT)

Notational Conventions The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in .

Terminology The following terminology is used throughout this document:

signature: The digital signature output.
Claim Name: The human-readable name used to identify a claim.
Claim Key: The CBOR map key used to identify a claim.
Claim Value: The CBOR map value representing the value of the claim.
CWT Claims Set: The CBOR map that contains the claims conveyed by the CWT.

Selective disclosure of claims within a CWT

Overview CBOR claims are cpommonly signed using COSE Sign1 where only one signature is placed on a particular message. There are many cases where a signer may wish to ensure that the authenticity of a message has not been compromised via a digital signature, but where they only wish to reveal some values of the original signed message. This document outlines a precise method for formulating and transmitting these messages, encompassing elements such as claims, claim keys, and necessary data structures. This kind of signed information exchange proves particularly beneficial in scenarios where a CBOR Web Token (CWT) might transit via an intermediary before undergoing third-party verification. Frequently, such a third party might necessitate, or be permitted to access only a subset of the information encapsulated in the CWT. This proposed model facilitates selective data disclosure, while still preserving the ability to validate the original signature. This representation relies on claims registered in the IANA CBOR Web Token (CWT) Claims Registry whenever possible.

Flow Diagram Figure 1: SD-CWT Issuance and Presentation Flow +------------+ | | | Issuer | | | +------------+ | Issues SD-CWT | v +------------+ | | | Holder | | | +------------+ | Presents SD-CWT | v +-------------+ | |+ | Verifiers ||+ | ||| +-------------+|| +-------------+| +-------------+

Creating an SD-CWT An SD-CWT is a CWT of the hash digests of the claim values with unique random salts and other metadata. It MUST be digitally signed using the issuer's private key. SD-CWT-CLAIMS = (METADATA, CWT-CLAIMS) SD-CWT = SD-CWT-CLAIMS | SIG(SD-CWT-CLAIMS, ISSUER-PRIV-KEY) CWT-CLAIMS is a simple object with claim names mapped to hash digests over the claim values with unique random salts: CWT-CLAIMS = ( CLAIM-NAME: HASH(SALT | CLAIM-VALUE) )* In a case where an SD-CWT is sent with all information disclose, the SD-CWT is sent together with the mapping of the plain-text claim values, the salt values, and potentially some other information. In this case, the the payload contains the CWT-CLAIMS, and the field "disclosures" contains the mapping, the salt values, and other metadata. In a case where an SD-CWT is sent with only some information discosed, only the desired claims, mappings, and salts are added to the disclosure. The CDDL fragment that represents the above text for COSE_Sign1 follows. SD-CWT = [ Headers, payload : bstr / nil, signature : bstr, disclosures: bstr / nil ]

Verifying an SD-CWT TBD - Describe verifiacation process

Holder Binding and other common scenarios

Holder Binding TBD - Discuss optioinality, mechanism, and value

Counter Signatures TBD- Discuss use with countersignatures in the unprotected header

Data Structures TBD - Describe common data structures in CDDL

Examples TBD - Provide examples

Security Considerations All security considerations from COSE and CWT SHOULD be followed. To maintain the integrity of the issued claims, the Selective Disclosure-CBOR Web Token (SD-CWT) MUST be signed by the issuer. Absence of this signature leaves the SD-CWT vulnerable to attackers, who can alter or append claims (for instance, modifying the "email" attribute to hijack the victim's account or inserting a fabricated academic qualification). The verifier is required to verify the signature on the SD-CWT to guarantee its authenticity and that no tampering has occurred post issuance. If the signature on the SD-CWT fails the verification process, the SD-CWT MUST be unequivocally rejected.

IANA Considerations

Media Type Registration This section will register the "application/sd-cwt" media type in the "Media Types" registry [IANA.MediaTypes] in the manner described in RFC 6838 , which can be used to indicate that the content is a CWT.

Type name: application
Subtype name: sd-cwt
Required parameters: n/a
Optional parameters: n/a
Encoding considerations: binary
Security considerations: See the Security Considerations section of
Interoperability considerations: n/a
Published specification: This Specification
Applications that use this media type: mesur.io, transmute
Fragment identifier considerations: n/a
Additional information: Magic number(s): n/a File extension(s): n/a Macintosh file type code(s): n/a
Person & email address to contact for further information: Michael Prorock, mprorock@mesur.io
Intended usage: COMMON
Restrictions on usage: none
Author: Michael Prorock, mprorock@mesur.io
Change controller: IESG
Provisional registration? Yes

Acknowledgements The authors would like to thank those that have worked on similar items for providing selective disclosure mechanisms in JSON, especially: Tobias Looker, Kristina Yasuda, Daniel Fett, Oliver Terbu, and Michael Jones.