How stateful replay ledgers make x402 payment gates secure against reuse attacks

Developer paygate402 is a Go middleware library built around the x402 HTTP payment protocol, which requires a server to respond with a 402 status and payment terms before a client resends the request with an X-PAYMENT header. The library deliberately delegates signature verification and fund settlement to an external facilitator component, keeping the middleware itself focused only on scheme and network matching. The critical security insight behind the design is that an X-PAYMENT header is a self-contained bearer token whose signature remains valid on any subsequent replay, meaning any proxy, log, or retry loop that captures it could reuse it indefinitely. The only defense against replay attacks is a server-side ledger that records every payment already settled, making this the sole stateful component in an otherwise stateless protocol flow. To protect the payment offer itself, the library uses a signed, nonce-bearing quote with a fixed TTL, whose signature covers fields in a canonical length-prefixed format to prevent boundary-manipulation forgery.