International Association
for Cryptologic Research

In this paper, we study multi-party non-interactive key ex-change (NIKE) in the fine-grained setting. More precisely, we propose three multi-party NIKE schemes in three computation models, namely, the bounded parallel-time, bounded time, and bounded storage models. Their security is based on a very mild assumption (e.g. NC1 \subsetneq \oplus L/poly) or even without any complexity assumption. This improves the recent work of Afshar, Couteau, Mahmoody, and Sadeghi (EUROCRYPT 2023) that requires idealized assumptions, such as random oracles or generic groups. Additionally, we show that all our constructions satisfy a natural desirable property that we refer to as extendability, and we give generic transformations from extendable multi-party NIKE to multi-party identity-based NIKEs in the fine-grained settings.

In this work, we propose a simple framework of constructing efficient non-interactive zero-knowledge proof (NIZK) systems for all NP. Compared to the state-of-the-art construction by Groth, Ostrovsky, and Sahai (J. ACM, 2012), our resulting NIZK system reduces the proof size and proving and verification cost without any trade-off, i.e., neither increasing computation cost, CRS size nor resorting to stronger assumptions. Furthermore, we extend our framework to construct a batch argument (BARG) system for all NP. Our construction remarkably improves the efficiency of BARG by Waters and Wu (Crypto 2022) without any tradeoff.