Privacy-Preserving Post-quantum Credentials for Digital Payments

Digital payments and decentralized systems enable new financial products and services for users. A core challenge stems from the need to protect users from fraud and abuse while retaining privacy in individual transactions. Proposed herein is a pseudonymous credential scheme for use in payment systems. The scheme is privacy-preserving, efficient for practical applications, and hardened against quantum-compute attacks. A constant-round, interactive, zero-knowledge proof of knowledge (ZK-POK), relying on a one-way function and an asymmetric encryption primitive, both of which need to support at most one homomorphic addition, is presented. The scheme is instantiated with SWIFFT as a post-quantum one-way function and Ring Learning With Errors (RLWE) as a post-quantum asymmetric encryption primitive, with the protocol deriving its quantum-hardness from the properties of the underlying primitives. Performance of the ZK-POK instantiated with the chosen primitive was evaluated to reveal a memory footprint of 85 kB to achieve 200 bits of security. Comparison reveals that our scheme is more efficient than equivalent, state-of-the-art post-quantum schemes. A practical, interactive, credential mechanism was constructed from the proposed building blocks, in which users are issued pseudonymous credentials against their personally identifiable information (PII) that can be used to register with financial service providers without revealing personal information. The protocol is shown to be secure and free of information leakage, preserving the user's privacy regardless of the number of registrations.