Security considerations

• Replay. Bind the verifier nonce into the holder proof; enforce single-submission and short session TTLs. x402 uses a per-payment nonce to prevent double-spend.

• Custody. No user-held keys; one HSM-held holder key; issuer keys held only by issuers. Password-gated signing avoids long-lived tokens.

• Data minimisation. Selective disclosure + range proofs mean the verifier learns only that constraints are satisfied, not the underlying values.

• On-chain trust anchor. Verification keys and revocation status resolve on-chain, not from the untrusted agent path.

• Auditability. Every payment is a settled Zetrix transaction (txHash); every verification produces a signed, HMAC-authenticated result.

• Fail-closed. Missing/invalid signatures, expired sessions, or revoked credentials must deny access.