Personhood Credential (PHC)

A Personhood Credential is a Membership Credential (VMC) issued by a VTC whose governance enforces two guarantees:

1. Real human personhood — the holder is a genuine person
2. Exactly one membership per person — no one can hold multiple memberships

That’s it. A PHC is structurally identical to any other VMC. There are no additional schema fields. What makes a VMC a PHC is the governance and trust registry of the issuing community, not the credential itself.

The Key Design Choice

The DTG specification (v0.3) is emphatic about this: PHC status is “determined by governance and trust registries, not by credential structure.” Issuers may optionally add "PersonhoodCredential" to the W3C type array as a non-authoritative hint, but this is not what makes it a PHC. The trust registry is the authoritative source.

This means the same credential format works across communities with very different personhood verification standards. One community might require in-person verification; another might accept video calls; a third might use web-of-trust thresholds. The credential structure is the same — the governance differs.

In the dtg-credentials Rust library, the optional type hint looks like:

{
  "type": ["VerifiableCredential", "DTGCredential", "MembershipCredential", "PersonhoodCredential"]
}

Why It Exists

Without personhood enforcement, the trust graph is vulnerable to two related threats:

• Sybil attacks — one person creating many fake identities to inflate their apparent trustworthiness
• AI agent impersonation — an AI system participating as if it were a real person, accumulating credentials and relationships that carry a false implication of human judgment behind them

PHCs are the defense at the community level: the community’s governance ensures each membership belongs to a verified, unique human.

How Personhood Is Verified

The exact mechanism is a policy question for each community. Options include:

• In-person verification at events
• Video call verification
• Existing identity document verification via an Identity Verification Provider (IDVP)
• Web of trust thresholds (e.g., N existing members vouch for you)

Lending Personhood to Relationship Proofs

A PHC’s value extends beyond the membership edge itself: it can be carried forward into ZKP proofs of VRCs. When two members of a PHC-issuing community have a VRC between them, the holder can construct a community-anchored ZKP showing:

1. Possession of the VRC
2. Possession of a VMC from the community
3. That the VRC counterparty also holds a VMC from the same community

Because that community’s VMCs are PHCs, the resulting proof carries personhood assurance for both parties without revealing their underlying DIDs. This is one proof construction available to relationships inside a shared PHC-issuing community — not a universal requirement for VRCs, which can also exist directly between individuals outside any community context.

See also: membership-credential, trust-registries, decentralized-trust-graph, dtg-credentials-overview