SD-JWT VC

An SD-JWT VC is the credential format OpenCredentials issues for selective disclosure: a compact, issuer-signed JWT carrying digests of claims, followed by ~-separated disclosures that reveal individual claims, optionally ending in a holder key-binding JWT. The holder chooses which disclosures to attach when presenting, so they can prove exactly one fact — "my email domain is tinycloud.xyz" — without exposing the address or any other claim. This is the wire format the policy engine verifies when it gates a delegation on credential evidence.

Role

Selective disclosure is what makes credentials usable as authorization evidence without leaking data. A capability is minimal by construction; a credential should be too. SD-JWT lets OpenCredentials issue a rich credential once (every claim digested into the signed JWT) while the holder presents only the slice a Policy demands. It is the privacy hinge of the credentials→policy-engine pipeline, sitting in Layer 2.

Mechanics

The format follows IETF SD-JWT (sha-256 as SD_ALG):

• Structure — <issuer-JWT>~<disclosure_1>~…~<disclosure_n>~[<key-binding-JWT>]. The issuer JWT is a normal 3-segment compact JWT signed by the witness (did:web:issuer.tinycloud.xyz, EdDSA / Ed25519). Each disclosure is base64url of a JSON array [salt, claimName, value]; the signed payload holds only the digests of these, so a verifier confirms a disclosed claim by hashing the disclosure and matching the digest.
• Parsing (js/opencredentials-client/src/sd_jwt.ts, parse_sd_jwt) — splits on ~; the first segment is the JWT, a trailing 3-segment value is treated as the key-binding JWT, and the middle segments parse as disclosures ([salt, claimName, value]).
• Presentation (present_sd_jwt) — re-emits jwt ~ selected-disclosures [~ key-binding-jwt], defaulting to all disclosures but accepting a chosen subset. This is how a holder drops every disclosure except the email-domain one.
• Issue/verify (Rust) — rust/opencredentials_sd_jwt/src/lib.rs provides the issuer/holder/verifier side (Ed25519Signer/Ed25519Verifier over JWKs); the policy engine's verifier reuses this machinery via the opencredentials-verify crate.

Shape

SD-JWT  = <compact-JWT> "~" *( <disclosure> "~" ) [ <key-binding-JWT> ]
disclosure = base64url( JSON [ salt, claimName, value ] )
// policy-engine presentation wrapper:
PresentedEvidence.presentation = { "sdJwt": "<SD-JWT string>" }

The TinyCloud credential type wired into authorization is opencredentials.email/v1 (the email VCT), which supports a full-email disclosure and a separate email-domain disclosure — and the policy engine specifically requires the domain disclosure, rejecting a presentation that reveals the full email but not the domain.

Relationships

Issued by the witness service as part of OpenCredentials; the W3C-VC profile the policy engine expects (w3c.vc/credential/v1); its disclosures are the holder-side privacy control feeding policy evidence; signed by a did:web issuer and optionally key-bound to a holder DID; an L2 format.

Example

An issued opencredentials.email/v1 SD-JWT for sam@tinycloud.xyz contains digested claims including email and email_domain. To satisfy a Policy requiring emailDomains: ["tinycloud.xyz"], the holder calls present_sd_jwt selecting only the email_domain disclosure and wraps it as { sdJwt: "…" }. The verifier confirms the witness signature, hashes the one disclosure against the signed digest, sees domain tinycloud.xyz, and grants — the full address sam@ was never transmitted.

Status & drift

in-progress. The SD-JWT parse/present client (sd_jwt.ts) and Rust issue/verify (opencredentials_sd_jwt) are implemented, and the email-domain selective disclosure is the path verified end-to-end by the policy engine. Key-binding JWTs are parsed/preserved by the client but are not the gate the email-domain policy flow relies on. Other VCTs beyond opencredentials.email/v1 are design-intent. See contradictions.

Sources

• OpenCredentials: js/opencredentials-client/src/sd_jwt.ts (parse_sd_jwt/present_sd_jwt, ~-format, disclosure shape, key-binding detection), rust/opencredentials_sd_jwt/src/lib.rs (SD_ALG = sha-256, Ed25519Signer/Ed25519Verifier)