Comment by nine_k

Indeed. But a federated database is fine, too; this is how Visa and MasterCard work.

Imagine having a bunch of ID cards in you wallet, like you already have (driver's license, library card, office access card, store loyalty card) that all have interoperable smartcard interface, and a QR code of their built-in public key.

They would be much like contactless bank cards you also keep in your wallet.

Banks and phone network operators are uniquely positioned to sell a validation service for such cards, being highly connected and already having data about their existing customers, which would be an easy initial audience pool.