On the Concrete Complexity of Zero-Knowledge Proofs

The fact that there are zero-knowledge proofs for all languages in NP has, potentially, enormous implications to cryptography. For cryptographers, the issue is no longer "which languages in NP have zero-knowledge proofs" but rather "which languages in NP have practical zero-knowledge proofs". Thus, the concrete complexity of zero-knowledge proofs for different languages must be established. In this paper, we study the concrete complexity of the known general methods for constructing zero-knowledge proofs. We establish that circuit-based methods have the potential of producing proofs which can be used in practice. Then we introduce several techniques which greatly reduce the concrete complexity of circuit-based proofs. In order to show that our protocols yield proofs of knowledge, we show how to extend the Feige-Fiat-Shamir definition for proofs of knowledge to the model of Brassard-Chaum-Cr'epeau. Finally, we present techniques for improving the efficiency of protocols which involve ar..