Some observations on zap and its applications

Abstract

In this paper we make some observations on the zaps and their applications developed by Dwork and Naor [13]. We clarify the relations among public-coin witness indistinguishability (WI), public-coin honest verifier zero-knowledge (HVZK) and public-coin special honest verifier zero-knowledge (SHVZK). Specifically, we observe that the existence of zaps under the existence of one-way permutations actually strictly separates public-coin WI and public-coin SHVZK assuming NP (unknown character) BPP. We also show that public-coin HVZK does not implies WI assuming the existence of one-way permutations. For zap-based applications, we present an improved Dwork-Naor 2-round timed deniable authentication scheme that improves the communication and computation complexity of the original protocol presented by Dwork and Naor [13]. Specifically, in the improved protocol the first message (from the verifier to the authenticator) is independent on the message to be authenticated by the authenticator. © Springer-Verlag Berlin Heidelberg 2004.