Zero-knowledge proofs for finite field arithmetic, or: Can zero-knowledge be for free?

Abstract

We present a general method for constructing commitment schemes based on existence of q-one way group homomorphisms, in which elements in a finite prime field GF(q) can be committed to. A receiver of commitments can non-interactively check whether committed values satisfy linear equations. Multiplicative relations can be verified interactively with exponentially small error, while communicating only a constant number of commitments. Particular assumptions sufficient for our commitment schemes include: the RSA assumption, hardness of discrete log in a prime order group, and polynomial security of Diffie-Hellman encryption. Based on these commitments, we give efficient zero-knowledge proofs and arguments for arithmetic circuits over finite prime fields, namely given such a circuit, show in zero-knowledge that inputs can be selected leading to a given output. For a field GF(q), where q is an m-bit prime, a circuit of size O(m), and error probability 2 −m, our protocols require communication of O(m 2) bits. We then look at the Boolean Circuit Satisfiability problem and give non-interactive zero-knowledge proofs and arguments with preprocessing. In the proof stage, the prover can prove any circuit of size n he wants by sending only one message of size O(n) bits. As a final application, we show that Shamirs (Shens) interactive proof system for the (IP-complete) QBF problem can be transformed to a zero-knowledge proof system with the same asymptotic communication complexity and number of rounds.