Zkboo: Faster Zero-Knowledge For Boolean Circuits

In this paper we describe ZKBoo(1), a proposal for practically efficient zero-knowledge arguments especially tailored for Boolean circuits and report on a proof-of-concept implementation. As an highlight, we can generate (resp. verify) a non-interactive proof for the SHA-1 circuit in approximately 13ms (resp. 5ms), with a proof size of 444KB.Our techniques are based on the "MPC-in-the-head" approach to zero-knowledge of Ishai et al. (IKOS), which has been successfully used to achieve significant asymptotic improvements. Our contributions include:A thorough analysis of the different variants of IKOS, which highlights their pros and cons for practically relevant soundness parameters;A generalization and simplification of their approach, which leads to faster Sigma-protocols (that can be made non-interactive using the Fiat-Shamir heuristic) for statements of the form "I know x such that y = phi (x)" (where phi is a circuit and y a public value);A case study, where we provide explicit protocols, implementations and benchmarking of zero-knowledge protocols for the SHA-1 and SHA-256 circuits.