Strict Polynomial-time in Simulation and Extraction

Boaz Barak and Yehuda Lindell


The notion of efficient computation is usually identified in cryptography and complexity with probabilistic polynomial time. However, until recently, in order to obtain constant-round zero-knowledge proofs and proofs of knowledge (for NP), one had to allow simulators and knowledge-extractors to run in time which is only polynomial on the average (i.e., expected polynomial time). Whether or not allowing expected polynomial-time is necessary for obtaining constant-round zero-knowledge proofs and proofs of knowledge, has been posed as an important open question. This question is interesting not only for its theoretical ramifications, but also because expected polynomial time simulation is not closed under composition. Therefore, in some cases security is not maintained when a protocol that utilizes expected polynomial time simulation (or extraction) is used as a part of a larger protocol. A partial answer to the question of the necessity (or non-necessity) of expected polynomial-time was provided recently by Barak, who gave the first constant-round zero-knowledge argument with a strict (in contrast to expected) polynomial-time simulator. His was also the first protocol that is not black-box zero-knowledge. That is, the simulator in his protocol utilizes the description of the code of the verifier in an essential way.

In this paper, we completely resolve the question of expected polynomial-time in zero-knowledge arguments and arguments of knowledge. First, we show that there exist constant-round zero-knowledge arguments of knowledge with strict polynomial-time extractors. As in the simulator of Barak's zero-knowledge protocol, the extractor for our proof of knowledge is not black-box and uses the code of the prover in an essential way. On the negative side, we show that non-black-box techniques are essential to both strict polynomial-time simulation and extraction. That is, we show that no constant-round zero-knowledge argument (or proof) can have a strict polynomial-time black-box simulator. Similarly, we show that no constant-round zero-knowledge argument (or proof) of knowledge can have a strict polynomial-time black-box knowledge extractor. Thus, for constant-round black-box zero-knowledge arguments (resp., arguments of knowledge), it is imperative that the simulator (resp., extractor) be allowed to run in expected polynomial-time.

Postscript, gzipped Postscript.

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