Paper 2025/2194

Turning Simulation into Construction: New Uses of NIZK Simulators

Abstract

Simulation is a fundamental technique in cryptography, central to security proofs, probably most well-known is its use for showing zero-knowledge. In this paper, we consider simulations no longer being merely a proof technique, but to constructively use simulators of non-interactive zero-knowledge proofs (NIZKs) as an integral component of cryptographic designs. We have occasionally seen such a use, e.g., in designated verifier signatures (DVS), deniable encryption, or the recent concept of anamorphic encryption. However, it was never explicity explored as a stand-alone concept, which is the goal of this paper. By embedding simulated proofs directly into concrete systems, we unlock cryptographic functionalities that were previously out of reach under standard assumptions or required prohibitively complex techniques. In other words, by incorporating simulated proofs into the ``real'' world, rather than the simulated one, we achieve conceptually more elegant primitives. As a primer, we construct a secure signature scheme whose security hinges on a simulated proof of a false statement, i.e., the ludicrous statement $1 = 2$. To illustrate the broader potential of this approach, we present new and simple constructions of chameleon hash functions with strong privacy guarantees (e.g., full indistinguishability), that do not require a trusted setup. Additionally, we present a very simple DVS with tight security proofs and a strengthened notion of non-transferability. Based on the zero-knowledge guarantees of the underlying NIZKs, the resulting constructions achieve privacy even if the adversary is allowed to choose the random coins to set up the cryptographic material. To model this, we introduce the notion of trapdoor-detectable zero-knowledge, which may be of independent interest.