Decentralized Trust Graph (DTG) Credentials

NOTE: This is an early implementation of the DTG Core Credentials specification (v1.0, Working Draft 02), which supersedes the earlier v0.3 proposal draft.

See the First Person Project Whitepaper for more information.

This library supports both W3C VC 1.1 and 2.0 specifications.

See CHANGELOG.md for release history.

Examples

cargo run --example sign_and_verify   # create, sign, verify one credential
cargo run --example data_room         # a whole data room, end to end

data_room runs the room story in one process with real DIDs, real signed credentials, real AEAD and real chain verification: a room issues its owner a VAC, invites a member by VIC, completes the VMC pair on their acknowledgement, seals a record, watches that member equip an agent with strictly less authority than they hold themselves, then appoints a service to act in that member's name by VDC — the same member, two credentials, and a verifier that can always tell which it was shown — rotates the epoch on removal, and finally prints exactly what the host can see, which is ciphertext, an epoch number, and nothing else.

The credential types each have their own tests; tests/authority_chain.rs and tests/delegation_chain.rs are mostly attacks, since what makes a VAC or a VDC safe is a verifier refusing a chain that widens.

Credential Type Hierarchy

All credentials inherit from the abstract DTGCredential.

VerifiableCredential
└── DTGCredential
    ├── MembershipCredential (VMC)
    ├── RelationshipCredential (VRC)
    ├── DelegationCredential (VDC)
    ├── InvitationCredential (VIC)
    ├── PersonaCredential (VPC)
    ├── EndorsementCredential (VEC)
    ├── WitnessCredential (VWC)
    └── AuthorityCredential (VAC)

Two of those confer rather than assert, and a verifier has to be able to tell which it was shown:

	Question it answers	The act is attributed to
VAC (authority)	may this party do this thing, as itself?	the party itself
VDC (delegation)	may this party act in another's name?	the entity it stands in for

Neither implies the other, and a VDC never supplies authority the delegator did not itself hold. See Authority and Delegation.

NOTE: The relationship card (R-Card) is not a DTGCredential subtype. It was reclassified as a verifiable data structure (VDS) in Working Draft 01, to be defined by the planned DTG Verifiable Data Structures specification. The RCard type, CredentialSubjectRCard and new_rcard() are deprecated in this library and will be removed in a future release.

Trust Task Context

Credentials issued inside a multi-step trust task exchange may carry a taskContext property holding the threadId of that exchange. It is REQUIRED on a WitnessCredential — deserializing a VWC without one fails with DTGCredentialError::MissingTaskContext — and OPTIONAL on every other type.

A credential without a taskContext must be interpretable standing alone. A credential with one must not be read as proof that the trust task completed unless the matching outcome evidence is also present and verified.

let vwc = DTGCredential::new_vwc(
  issuer, subject, valid_from, valid_until,
  "thread-abc-123".to_string(), // taskContext
  digest, witness_context,
);

assert_eq!(vwc.task_context(), Some("thread-abc-123"));

Digests

A credential can be referenced by another through a digestMultibase of it: a member-issued VMC digests the membership grant it acknowledges, a VWC digests the edge credential it attests, an attenuated VAC digests the VAC it narrows, and a VDC digests the delegation it derives from or the grant it accepts. All five use the same computation.

// A credential you received: digest the JSON as it arrived.
let digest = dtg_credentials::digest_multibase_json(&grant_json)?;

// A credential this library just built: `digest_multibase()` is equivalent.
let digest = grant.digest_multibase()?;

That is the SHA-256 of the credential canonicalized with JCS (RFC 8785) and excluding its top-level proof, wrapped in a sha2-256 multihash and encoded base58btc with a multibase z prefix — the encoding VC Data Integrity §2.6 defines for digestMultibase. Leaving proof out binds the digest to what the credential says rather than to one signature over it, so a reference survives its referent being re-signed, and the digest can be computed before signing.

Important

Digest what you received, not what you parsed, wherever you still hold the bytes. DTGCommon now models credentialStatus and preserves unmodelled top-level members through a round trip, so for most credentials the two agree — but a timestamp is normalized on the way out, and 2026-01-06T10:00:00.000+00:00 hashes differently from the 2026-01-06T10:00:00Z this library re-emits. digest_multibase() is safe for a credential built in-process; anything that arrived from elsewhere goes through digest_multibase_json().

verify_digest() checks that a credential's digest matches the one it names:

if vwc.verify_digest(&vrc)? {
  println!("this VWC attests that VRC");
}

It compares decoded bytes, not strings — the specification requires it, because one digest has more than one spelling. digests_match() and decode_digest_multibase() are exposed for callers doing the comparison themselves.

For a membership pair, prefer acknowledges() — it checks the digest and that the two halves are of the right types and name the same parties in mirrored roles. See Membership edges. accepts() is its counterpart for a delegation edge.

Note

digest() and digest_json() are deprecated. They emit the Working Draft 01 sha256:<lowercase hex> form, which Working Draft 02 replaced. They are kept so a caller migrating can recompute an old digest to compare against one they stored; new code uses digest_multibase() / digest_multibase_json().

On the wire, digestMultibase is what this library emits, and the old property name digest is still accepted when parsing. A credential carrying an old value parses and then fails to compare, with InvalidDigest rather than a silent mismatch.

Membership edges

Membership is a pair of VMCs, not a single directed credential:

	issuer	credentialSubject.id	digestMultibase
Community-issued (the grant)	community	member	MUST be absent
Member-issued (the acknowledgement)	member	community	MUST be present

The member-issued half is the member's consent artifact. A community can always issue a credential naming somebody as a member; what it cannot do is produce the acknowledgement, because that needs the member's signature. So an unconsented membership claim is unprovable — a community that cannot show the acknowledgement is visibly asserting a membership nobody agreed to.

// Community side: grant membership.
let grant = DTGCredential::new_vmc(
  community_did, member_did, valid_from, valid_until, personhood,
).with_id(format!("urn:uuid:{}", Uuid::new_v4()));
grant.sign(&community_key, None).await?;

// Member side: acknowledge it. `grant_json` is the JSON the community sent —
// the wire form, not a parse of it. The parties are read off the grant, so the
// two halves cannot disagree about who they are between.
let mut ack = DTGCredential::new_member_vmc(&grant_json, Utc::now(), None)?
  .with_id(format!("urn:uuid:{}", Uuid::new_v4()));
ack.sign(&member_key, None).await?;

// Either side: is this edge complete?
assert!(ack.acknowledges(&grant)?);

acknowledges() checks the binding — types, mirrored parties, and the digest. It deliberately does not check either credential's proof or validity window: proof verification needs a resolver this crate does not hold, and whether a window is current is a question about an instant the caller chooses. An edge is complete when both halves are valid and bound; this covers the binding.

Because the digest covers the grant's claims, a re-issued grant carries a different digest and the earlier acknowledgement no longer matches it. Renewal therefore forces re-acknowledgement rather than letting a stale consent carry over to a membership the member never agreed to.

Authority (VAC)

A VAC states what a party may do within a scope some node governs. Its holder can narrow it without involving the governing party — which is what lets a member equip an agent with four hours of read-only access instead of lending it their own standing authority.

// The governing party grants Bob read+write+curate for a month.
let root = DTGCredential::new_vac(
  room_did, bob_did, room_did.clone(),
  vec!["read".into(), "write".into(), "curate".into()],
  now, now + Duration::days(30),   // validUntil is REQUIRED on a VAC
)?;

// Bob equips his agent with strictly less, bound to that agent.
let agent = root.attenuate(
  agent_did, vec!["read".into()], now, now + Duration::hours(4), Some(agent_did),
)?;

attenuate() refuses anything that would widen, but the verifier's check is the authoritative one — nothing stops another implementation building the JSON by hand. authority::verify_chain is where the security of this credential lives:

let permitted = verify_chain(
  &[agent, root],   // leaf first; the holder presents every link
  room_did, room_did, "read", agent_did, Utc::now(),
)?;

Anyone can mint a well-formed VAC naming any scope and any actions, and it will verify perfectly as a credential. What makes it worthless is that its chain does not reach the party governing the scope. A verifier that checks only the credential it was handed has verified nothing.

parent is a digest, not an identifier. So there is nothing a verifier could be induced to fetch, verification never depends on network availability, and a link binds to the exact claims its issuer narrowed from — re-issuing a parent with different claims orphans its children, while re-proofing it leaves them alone. For a VAC that arrived from a counterparty, use attenuate_from_json() and give it the bytes you received.

Note

Three upstream changes to the VAC are not implemented yet: revocation via credentialStatus, cascading to everything attenuated below (PR #39); a maxAttenuation ceiling (PR #40); and a key-control demonstration at invocation, which removes audience as redundant (PR #41). audience is kept until that lands rather than being removed twice. A verified chain is not by itself evidence that the party presenting it is the leaf's subject.

Delegation (VDC)

A VDC establishes that one party may act in another's name. It is not authority, and the distinction decides which credential to reach for: ask whose name the act is performed in. The actor's own — that is a VAC. Another entity's — that is a VDC.

Like membership, a delegation is a pair:

	issuer	credentialSubject.id	carries
Grant	delegator	delegate	scope, optionally maxDepth
Acceptance	delegate	delegator	accepts only

// Alice appoints her agent, permitting one further hop.
let grant = DTGCredential::new_vdc(
  alice_did, agent_did, now, now + Duration::days(90),
  vec!["schedule:read".into(), "schedule:propose".into()],
  Some(1),                          // maxDepth; None or 0 prohibits re-delegation
)?;

// The agent accepts. `grant_json` is the wire form, not a parse of it.
let acceptance = DTGCredential::new_delegate_vdc(&grant_json, now, valid_until)?;
assert!(acceptance.accepts(&grant)?);

The acceptance is required. A grant alone establishes what the delegator appointed, not what the delegate agreed to — and a delegator cannot produce the countersignature. It is also why a party holding only the delegate's key cannot manufacture new appointments.

Re-delegation is opt-in, the opposite default from a VAC's attenuation. A delegate speaks in the principal's name, so the principal keeps the register of who may do that; a delegate needing a further delegate ordinarily asks for a fresh root delegation rather than minting one.

let sub = grant.redelegate(subagent_did, vec!["schedule:read".into()], now, until)?;

let appointed = delegation::verify_chain(&[sub, grant], alice_did, "schedule:read", Utc::now())?;
assert_eq!(appointed.principal, alice_did);   // the acts are attributed to Alice

A VDC moves the permission question; it does not answer it

verify_chain tells you the chain appoints this delegate to act in the principal's name for this act. That is one of two checks. The other — may the principal do this thing? — is yours to make, against whatever the act requires of them: membership, a governance framework, an IDVC, a VAC. This crate does not answer it, and a VDC never influences its outcome.

The reach of a delegated act is the intersection of what the principal may do and what the chain appoints for. Two consequences worth stating: nothing the delegator holds is copied to the delegate, and withdrawing the delegator's own permission stops every delegate at once, without revoking a single VDC.

Not implemented here: revocation (credentialStatus is modelled but not resolved), and invocation binding — a VDC is not a bearer token, and nothing in this crate establishes that the party presenting a chain controls the leaf's subject identifier.

End to End Example

An end-to-end example of creating, signing and verifying a DTG Credential exists in examples

cargo run --example sign_and_verify

Creating credentials

Each credential type has it's own new_*() function to create a new credential of that type.

Example:

let vpc = DTGCredential::new_vpc(issuer, subject, valid_from, valid_to);

The created DTGCredential can be serialized to JSON using serde_json allowing it to be passed into various signing libraries

Credential identifiers

A credential may carry its own top-level id — the OPTIONAL identifier of the W3C VC Data Model, distinct from credentialSubject.id, which names the subject. When present it MUST be a single URL; urn:uuid:<uuid> is the usual choice for a credential with no dereferenceable home.

The new_*() constructors leave it unset. Chain with_id() to add one:

let vmc = DTGCredential::new_vmc(issuer, subject, valid_from, valid_to, false)
  .with_id(format!("urn:uuid:{}", Uuid::new_v4()));

assert_eq!(vmc.id(), Some(...));

Issue with an id unless you know no counterparty needs one. It is the handle a holder or verifier stores the credential under, so it is what makes re-delivery of the same credential idempotent, and re-issuance of a different one recognisable as a renewal rather than a duplicate. A verifier that keys credentials by id has no way to accept one that has none.

Important

Set the id before signing. A Data Integrity proof covers the credential minus its proof, so the identifier is part of what is signed. Splicing one into the JSON after sign() produces a document whose proof no longer verifies.

Signing credentials

By default the affinidi-signing feature is enabled which allows you to sign a credential

let mut vpc = DTGCredential::new_vpc(issuer, subject, valid_from, valid_to);

vpc.sign(&signing_key).await?;

Verifying credentials

There are two ways to validate a credential:

Method 1: If you have the public key bytes that correspond to the signing key, then you can directly verify the credential:

let signing_key = Secret::generate_ed25519(None, None);
let mut vpc = DTGCredential::new_vpc(issuer, subject, valid_from, valid_to);

vpc.sign(&signing_key).await?;

vpc.verify(&signing_key.get_public_bytes())?;

Method 2: If you do not have the public key material, you are likely going to need to resolve the DID VerificationMethod and derive the public key bytes used when creating the credential.

let mut credential = serde_json::from_str(<raw_credential_string>);

// Get the proof
let proof = if let Some(proof) = &credential.credential().proof {
  proof.clone()
} else {
    bail!("credential is not signed!");
};

// Strip proof from the credential
let unsigned = DTGCommon {
  proof: None,
  ..credential.credential().clone()
};

tdk.verify_data(&unsigned, None, &proof).await?;

Common functions

You can deal with the raw credential as required.

let vrc = DTGCredential::new_vrc(issuer, subject, valid_from, valid_to);

let credential = vrc.credential();

You can determine the credential type easily using:

let vmc = DTGCredential::new_vmc(issuer, subject, valid_from, valid_to);

if let DTGCredentialType::VMC = vmc.type_() {
  // Good
}

Has this Credential been signed?

let vmc = DTGCredential::new_vmc(issuer, subject, valid_from, valid_to);

if vmc.signed() {
  println!("Credential has been signed");
} else {
  println!("Credential has not been signed");
}