Proof-of-personhood

The third family of Sybil-resistance: a mechanism that tries to establish that an account is backed by one distinct living human — the one-to-many uniqueness question ("one human, one enrollment"), which is harder than the one-to-one authentication a passkey performs. The term was introduced by Bryan Ford and colleagues at a EuroS&P workshop in 2017. Approaches include biometrics (World ID's iris scan), social vouching (BrightID), and aggregation (Human Passport). Crucially, proof-of-personhood is not Sybil-immune: as the DeSoc paper concedes, an attacker can recruit or rent real humans, each of whom passes the test once but on the attacker's behalf. And in the agentic era it is increasingly the wrong question — see Accountability.