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Zaps and their applications

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2 Author(s)
Dwork, C. ; Compaq Syst. Res. Centre, Palo Alto, CA, USA ; Naor, M.

A zap is a two-round, witness-indistinguishable protocol in which the first round, consisting of a message from the verifier to the prover, can be fixed “once-and-for-all” and applied to any instance, and where the verifier does not use any private coins. We present a zap for every language in NP, based on the existence of non-interactive zero-knowledge proofs in the shared random string model. The zap is in the standard model, and hence requires no common guaranteed random string. We introduce and construct verifiable pseudo-random bit generators (VPRGs), and give a complete existential characterization of both noninteractive zero-knowledge proofs and zaps in terms of approximate VPRGs. We present several applications for zaps; In the timing model of C. Dwork et al. (1998) and using moderately hard functions, we obtain 3-round concurrent zero knowledge and 2-round concurrent deniable authentication (the latter protocol also operates in the resettable model of R. Canetti et al. (2000)). In the standard model we obtain 2-round oblivious transfer using public keys (3-round otherwise). We note that any zap yields resettable 2-round witness-indistinguishability and obtain a 3-round timing-based resettable zero-knowledge argument system for any language in NP

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Foundations of Computer Science, 2000. Proceedings. 41st Annual Symposium on

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