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**2001/063**

**Title:**

Resettably-Sound Zero-Knowledge and its Applications

**Authors:**
Boaz Barak, Oded Goldreich, Shafi Goldwasser, Yehuda Lindell

**Abstract:**

Resettably-sound proofs and arguments remain sound even when the

prover can reset the verifier, and so force it to use the same

random coins in repeated executions of the protocol. We show that

resettably-sound zero-knowledge {\em arguments} for NP exist

if collision-resistant hash functions exist. In contrast,

resettably-sound zero-knowledge {\em proofs} are possible only

for languages in P/poly.

We present two applications of resettably-sound zero-knowledge

arguments. First, we construct resettable zero-knowledge arguments

of knowledge for NP, using a natural relaxation of the

definition of arguments (and proofs) of knowledge. We note that,

under the standard definition of proofs of knowledge, it is

impossible to obtain resettable zero-knowledge arguments of

knowledge for languages outside BPP. Second, we construct a

constant-round resettable zero-knowledge argument for NP in the

public-key model, under the assumption that collision-resistant

hash functions exist. This improves upon the sub-exponential

hardness assumption required by previous constructions.

We emphasize that our results use non-black-box zero-knowledge

simulations. Indeed, we show that some of the results are {\em

impossible} to achieve using black-box simulations. In

particular, only languages in BPP have resettably-sound

arguments that are zero-knowledge with respect to black-box

simulation.

**ePrint:**
https://eprint.iacr.org/2001/063

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