HFIPay: Privacy-Preserving, Cross-Chain Cryptocurrency Payments to Human-Friendly Identifiers
For cryptocurrency users, HFIPay provides a privacy-preserving payment method to human-friendly identifiers, addressing a practical usability bottleneck without relying on a trusted third party for fund security.
HFIPay enables sending cryptocurrency to human-friendly identifiers (e.g., email) without exposing the recipient's on-chain activity to anyone who knows the identifier. It achieves enumeration resistance and pre-claim unlinkability via off-chain relay resolution, blinded bindings, and zero-knowledge proofs, with a verified-quote deployment making the binding sender-verifiable.
Sending cryptocurrency to an email address or phone number should be as simple as a bank transfer, yet naive schemes that map identifiers directly to blockchain addresses expose the recipient's balances and transaction history to anyone who knows the identifier. HFIPay separates private routing, sender-side quote verification, and on-chain claim authorization. A relay resolves the human-friendly identifier off-chain and commits only a per-intent blinded binding rho_i plus the quoted payment tuple; the chain sees neither the identifier nor a reusable recipient tag. In a verified-quote deployment, the relay returns a sender-verifiable off-chain proof linking rho_i to an attested binding-key commitment, so the relay cannot substitute a different recipient before funding. To claim, the recipient proves in zero knowledge -- via ZK-ACE -- that the funded intent's blinded binding matches a handle derived from the same deterministic identity, authorizing release of the quoted asset and amount to a chosen destination. We formalize two privacy goals: enumeration resistance and pre-claim unlinkability, and distinguish a baseline deployment (relay trusted for binding correctness) from the verified-quote deployment (binding is sender-verifiable without a public registry). When composed with an NVM runtime, the same mechanism extends to cross-chain settlement. The result is a relay-assisted but non-custodial architecture: relays are privacy and availability dependencies, but cannot redirect funds.