Paper 2025/772

Publicly Auditable Garbled Circuit

Abstract

Advanced cryptographic primitives such as zero-knowledge proofs (ZKPs) have seen an emerging trend in adoption by blockchain applications. However, ZKPs only allow a single prover to convince a verifier of a correct evaluation of a computation over the prover's secret input. This is insufficient if the applications require shared private state from more than one party. In this case, one can adopt secure multiparty computation (MPC), which allows a set of mutually distrustful parties to jointly compute a global function while keeping their respective inputs private. Generic MPC, including secret-sharing and garbled-circuit techniques that allow the participants, without deep expertise in cryptography, to define the computation as a circuit, is an important tool in privacy-preserving protocols. Yet, generic MPC mostly focuses on malicious security in which the computed result is verifiable only among the computing parties, but not publicly verifiable. In blockchain applications, public verifiability is important, as the consensus protocol is not just among the computing parties but also public verifiers. Several works have attempted to bridge this gap, albeit not in the blockchain realm, i.e., publicly auditable MPC. Nevertheless, they are non-constant round protocols and only for secret-sharing MPC, i.e., round complexity scales linearly with the circuit multiplicative depth, while round latency is an important cost metric for blockchains. We address this problem by providing a Publicly Auditable Garbled Circuit protocol that is maliciously secure, publicly auditable, and constant-round. Our protocol is efficient, with minimal overhead in terms of the number of rounds, communication, and public transcript size.