Cryptology ePrint Archive: Report 2018/895

Weak Zero-Knowledge Beyond the Black-Box Barrier

Nir Bitansky and Dakshita Khurana and Omer Paneth

Abstract: The round complexity of zero-knowledge protocols is a long-standing open question, yet to be settled under standard assumptions. So far, the question has appeared equally challenging for relaxations such as weak zero-knowledge and witness hiding. Protocols satisfying these relaxed notions under standard assumptions have at least four messages, just like full-fledged zero knowledge. The difficulty in improving round complexity stems from a fundamental barrier: none of these notions can be achieved in three messages via reductions (or simulators) that treat the verifier as a black box.

We introduce a new non-black-box technique and use it to obtain the first protocols that cross this barrier under standard assumptions. Our main results are:

\begin​{itemize} \item Weak zero-knowledge for $NP $in two messages, assuming quasipolynomially-secure fully-homomorphic encryption and other standard primitives (known from quasipolynomial hardness of Learning with Errors), as well as subexponentially-secure one-way functions.

\item Weak zero-knowledge for $NP$ in three messages under standard polynomial assumptions (following for example from fully-homomorphic encryption and factoring).


We also give, under polynomial assumptions, a two-message witness-hiding protocol for any language $L \in NP$ that has a witness encryption scheme. This protocol is also publicly verifiable.

Our technique is based on a new {\em homomorphic trapdoor paradigm}, which can be seen as a non-black-box analog of the classic Feige-Lapidot-Shamir trapdoor paradigm.

Category / Keywords: cryptographic protocols / zero knowledge, non-black-box techniques, fully-homomorphic encryption

Original Publication (with minor differences): STOC 2019

Date: received 23 Sep 2018, last revised 31 Jul 2019

Contact author: nbitansky at gmail com

Available format(s): PDF | BibTeX Citation

Version: 20190731:084849 (All versions of this report)

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