Paper 2024/614

Non-interactive Blind Signatures: Post-quantum and Stronger Security

Abstract

Blind signatures enable a receiver to obtain signatures on messages of its choice without revealing any message to the signer. Round-optimal blind signatures are designed as a two-round interactive protocol between a signer and receiver. Incidentally, the choice of message is not important in many applications, and is routinely set as a random (unstructured) message by a receiver. With the goal of designing more efficient blind signatures for such applications, Hanzlik (Eurocrypt '23) introduced a new variant called non-interactive blind signatures (NIBS). These allow a signer to asynchronously generate partial signatures for any recipient such that only the intended recipient can extract a blinded signature for a random message. This bypasses the two-round barrier for traditional blind signatures, yet enables many known applications. Hanzlik provided new practical designs for NIBS from bilinear pairings. In this work, we propose new enhanced security properties for NIBS as well as provide multiple constructions with varying levels of security and concrete efficiency. We propose a new generic paradigm for NIBS from circuit-private leveled homomorphic encryption achieving optimal-sized signatures (i.e., same as any non-blind signature) at the cost of large public keys. We also investigate concretely efficient NIBS with post-quantum security, satisfying weaker level of privacy as proposed by Hanzlik.