Selective Disclosure CWTs (SD-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

Creating an SD-CWT An SD-CWT is a CWT containing the hash digests of the claim values combined with unique random salts and additional metadata for disclosed values. The SD-CWT MUST be digitally signed using the issuer's private key. CWT-CLAIMS is an object where claim names are mapped to hash digests of the claim values combined with unique random salts: In a case where an SD-CWT is sent with all information disclosed, 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 "disclosures" field in the unprotected header contains the mapping, the salt values, and any additional metadata that might be present in the unprotected header. Disclosure in the unprotected header is important so that the content type of the payload may be set appropriately, and is distinct from any disclosed information. In a case where an SD-CWT is sent with only selected information disclosed, only the disclosed claims, mappings, and salts are added to the disclosure. Disclosures are structured as a "claim-pair" with a 32 bit salt, and the byte string of the disclosed value. uint .size 4, ; 32-bit salt 2 => bstr ; disclosed value } ]]> The CDDL fragment that represents the above text for COSE_Sign1 is provided below: The issuer SHOULD take appropriate percautions to verify that the salts are unique random values to prevent vulnerability to rainbow table attacks against the hashes.

Verifying an SD-CWT To verify an SD-CWT, the recipient extracts the protected CWT claims from the payload. These CWT claims contain hash digests of the original claim values combined with unique random salts. The recipient MUST validate that the protected header values such as issuer, audience, and expiration match the expected values for this SD-CWT per the guidelines set forward in . If any items do not match the expected or allowed values per the SD-CWT MUST be rejected. The payload and other protected claims MUST then be validated according to the section "Validating a CWT" in . If the CWT is not a COSE_Sign or COSE_Sign1 the CWT MUST be rejected. If any validations according instructions for validating a COSE_Sign/COSE_Sign1 object fail, the CWT MUST be rejected. The recipient that checks for any disclosures in the unprotected header. If they are present, the the claim values and salts MUST be extracted from the unprotected header. For each disclosed claim, the hash digest MUST be recomputed from the value and salt in the unprotected header. If the hash digest does not equal the corresponding digest in the payload the SD-CWT MUST be rejected. By performing these steps, the recipient can cryptographically verify the integrity of the protected claims and verify they have not been tampered with or substituted after issuance by the trusted issuer. The disclosures provide the plaintext claim values for utilization by the recipient.

Holder Binding and other common scenarios

Holder Binding Holder binding links an SD-CWT to its intended recipient. It prevents misuse if the token is intercepted or stolen. Binding mechanisms include cryptographic key confirmation, biometric data inclusion, or embedding holder-specific claims. The description of exact mechanisms for holder binding are outside the scope of this document at the present time. While optional, holder binding enhances SD-CWT security. It enables accountability by auditing actions to specific entities. For sensitive data, it augments trust by proving the claimant owns the token. Issuers should assess their use case when considering holder binding. Where accountability and non-repudiation are critical, binding provides assurance the token reaches the intended holder. With thoughtful implementation, binding can customize SD-CWT without compromising user privacy. Proportional use of holder binding balances security, privacy and flexibility for SD-CWT applications.

Counter Signatures Counter signatures allow an SD-CWT to be endorsed by additional entities beyond the original issuer. The counter signature is applied in the unprotected header, attesting to the validity of the primary SD-CWT signature over the protected claims. Counter signatures provide multiple benefits for SD-CWT:

Verification by multiple entities, preventing repudiation by any one party.
Added integrity protection in case the original signing key is compromised.
Allows separate signers for protected claims versus unprotected disclosures.

To utilize a counter signature, the primary SD-CWT is constructed and signed as normal. Then an additional signer computes the counter signature over the entire SD-CWT, including the original signature. This counter signature is placed in the unprotected header when transmitting the SD-CWT. Recipients validate the counter signature after verifying the primary signature, ensuring endorsements by all involved entities. Care should be taken to ensure robust trust in both signature authorities when relying on counter signatures. The CDDL fragment that represents an SD-CWT with an abbreviated counter signature is below:

Data Structures TBD - Describe common data structures in CDDL

Examples TBD - Provide examples