How do we do it?

• users generate a ZKP to verify the signature on their biometric passport.
• country Signing Certificate Authorities (CSCAs), as listed in the ICAO registry, are published in Merkle trees on-chain.
• anyone can submit a ZKP attesting that they own a valid passport signed by an official certificate.
• computation is saved by storing all Document Signing Certificates (intermediary certificates) in a Merkle tree.
• your deterministic nullifier is hidden via a Semaphore-style design that adds entropy using a secret known only to you.
• prove your identity by generating a proof of inclusion in an identity tree.
• disclose only what you want from your passport information (nationality, range check over the age, etc.)
• use sparse Merkle trees enable proofs of non-inclusion: demonstrating you’re not on the OFAC list without revealing your name, date of birth, or nationality

What we learned along the way
- countries use different signature algorithm/hash function.
- supporting emerging markets requires handling low-end devices (< 2 GB RAM) and unreliable internet connections.
- groth16 proving keys become very large for signature algorithms based on big elliptic curves (>5gb).
- current proving libraries are often single-threaded; the main performance bottleneck is witness calculation.