Preventing Relay Attacks in Offline Digital Currency and CBDC Payments via Fail-Closed, Execution-Time Cryptographic Authorization

Abstract - Fail-Closed Cryptographic Authorization for Secure Offline Payments Offline payment systems — including contactless cards, mobile wallets, and proposed offline Central Bank Digital Currency (CBDC) modes — typically infer authorization from physical proximity, signal timing, device presence, or post-transaction fraud analytics. These approaches provide only probabilistic security. They remain structurally vulnerable to relay and tunneling attacks, depend on behavioral monitoring to detect abuse, and often rely on persistent identifiers or profiling to manage fraud risk. This invention introduces a fail-closed, execution-time cryptographic enforcement architecture for offline value transfer. Instead of assuming that proximity implies authorization, payment execution is permitted only if a binary authorization predicate evaluates to valid immediately before irreversible value transfer. The system is applicable to secure elements, Trusted Execution Environments (TEEs), payment-capable consumer devices, terminals and validators, and offline CBDC implementations that require strong fraud resistance, privacy preservation, and deterministic authorization without continuous connectivity. Offline security does not require surveillance.It requires execution-time cryptographic control.