Proving you're over 21 without proving who you are. Per-disclosure key derivation, sealed-identity age verification, cross-disclosure unlinkability.
Kai is 24. He holds his phone to the scanner at a bar entrance. The scanner receives one bit of truth: "over 21: verified." It does not receive his name, birthdate, address, license number, or photo.
Three weeks later, Kai enters a bar in a different city. The scanner there also sees "over 21: verified." The two venues cannot combine their records to reconstruct where Kai has been. Each disclosure is keyed independently. Cross-venue correlation is structurally impossible, not just prohibited by policy.
The venue sees what it legally needs to see in order to serve Kai. Nothing beyond that is structurally accessible.
Age gates, prescription dispensing, professional credential verification, voter enrollment, background check workflows — any context where the attribute matters and the full identity record does not.
ABT-I uses Ed25519 digital signatures for authentication of every event at every party. Per-event symmetric keys are derived via HKDF with the event identifier as salt and held in hardware-backed secure storage (Apple iOS Keychain, Android Keystore, or equivalent platform secure-storage). Envelope encryption is performed at the first-party endpoint before any ciphertext leaves the device or origin. Each tier authority’s ciphertext contains only that tier’s authored projection — information not relevant to a tier authority is authored out before encryption, not redacted after. The registry maintains a hash-chained log where each entry’s hash includes the prior entry’s hash, providing tamper-evident integrity across the chain. Forward-only tier activation: registration of a new tier authority causes inclusion of an active tier layer in subsequent envelopes only. Existing envelopes are not retroactively modified. Cryptographic boundary at the first party. Plaintext never moves. Per-tier projection authored at envelope construction. Registry-routed restoration requires structural participation by all three parties.
Cryptography researchers studying envelope encryption, tier-bounded ciphertext, deterministic key derivation, and signed receipt chains in identity attribute disclosure.
Privacy researchers studying architectural privacy enforcement, unlinkability, purpose limitation, retention through cessation, and consumer-controlled key custody.
Consumer protection advocates seeking architectural alternatives to policy-based privacy enforcement. Cryptographic structural enforcement, not vendor trust.
Policy researchers examining cryptographic enforcement of storage limitation (GDPR Article 5(1)(e)), data minimization (GDPR Article 5(1)(c)), and consumer protection requirements.
Decentralized identity researchers studying selective disclosure, verifiable credentials, attribute-based credentials, cross-relying-party unlinkability, age-verification systems, KYC-without-identification, and subject-controlled per-disclosure cryptographic projections.
