**A Black-Box Approach to Post-Quantum Zero-Knowledge in Constant Rounds**

*Nai-Hui Chia and Kai-Min Chung and Takashi Yamakawa*

**Abstract: **In a recent seminal work, Bitansky and Shmueli (STOC '20) gave the first construction of a constant round zero-knowledge argument for NP secure against quantum attacks.
However, their construction has several drawbacks compared to the classical counterparts.
Specifically, their construction only achieves computational soundness, requires strong assumptions of quantum hardness of learning with errors (QLWE assumption) and the existence of quantum fully homomorphic encryption (QFHE), and relies on non-black-box simulation.

In this paper, we resolve these issues at the cost of weakening the notion of zero-knowledge to what is called $\epsilon$-zero-knowledge. Concretely, we construct the following protocols:

- We construct a constant round interactive proof for NP that satisfies statistical soundness and black-box $\epsilon$-zero-knowledge against quantum attacks assuming the existence of collapsing hash functions, which is a quantum counterpart of collision-resistant hash functions. Interestingly, this construction is just an adapted version of the classical protocol by Goldreich and Kahan (JoC '96) though the proof of $\epsilon$-zero-knowledge property against quantum adversaries requires novel ideas.

- We construct a constant round interactive argument for NP that satisfies computational soundness and black-box $\epsilon$-zero-knowledge against quantum attacks only assuming the existence of post-quantum one-way functions.

At the heart of our results is a new quantum rewinding technique that enables a simulator to extract a committed message of a malicious verifier while simulating verifier's internal state in an appropriate sense.

**Category / Keywords: **foundations / quantum cryptography, zero knowledge

**Date: **received 4 Nov 2020, last revised 6 Nov 2020

**Contact author: **takashi yamakawa obf at gmail com

**Available format(s): **PDF | BibTeX Citation

**Version: **20201110:123348 (All versions of this report)

**Short URL: **ia.cr/2020/1384

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