### Always Have a Backup Plan: Fully Secure Synchronous MPC with Asynchronous Fallback

Erica Blum, Chen-Da Liu-Zhang, and Julian Loss

##### Abstract

Protocols for secure Multi-Party Computation (MPC) can be classified according to the underlying communication model. Two prominent communication models considered in the literature are the synchronous and asynchronous models, which considerably differ in terms of the achievable security guarantees. Synchronous MPC protocols can achieve the optimal corruption threshold $n/2$ and allow every party to give input, but become completely insecure when synchrony assumptions are violated. On the other hand, asynchronous MPC protocols remain secure under arbitrary network conditions, but can tolerate only $n/3$ corruptions and parties with slow connections unavoidably cannot give input. A natural question is whether there exists a protocol for MPC that can tolerate up to $t_s < n/2$ corruptions under a synchronous network and $t_a < n/3$ corruptions even when the network is asynchronous. We answer this question by showing tight feasibility and impossibility results. More specifically, we show that such a protocol exists if and only if $t_a + 2t_s < n$ and the number of inputs taken into account under an asynchronous network is at most $n-t_s$.

Available format(s)
Category
Cryptographic protocols
Publication info
Keywords
multi-party computationsynchronousasynchronous
Contact author(s)
lichen @ inf ethz ch
History
Short URL
https://ia.cr/2020/740

CC BY

BibTeX

@misc{cryptoeprint:2020/740,
author = {Erica Blum and Chen-Da Liu-Zhang and Julian Loss},
title = {Always Have a Backup Plan: Fully Secure Synchronous MPC with Asynchronous Fallback},
howpublished = {Cryptology ePrint Archive, Paper 2020/740},
year = {2020},
note = {\url{https://eprint.iacr.org/2020/740}},
url = {https://eprint.iacr.org/2020/740}
}

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