International Association for Cryptologic Research

International Association
for Cryptologic Research


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

Erica Blum , University of Maryland
Chen-Da Liu-Zhang , ETH Zurich
Julian Loss , University of Maryland
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Presentation: Slides
Conference: CRYPTO 2020
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 $n-t_s$.
Video from CRYPTO 2020
  title={Always Have a Backup Plan: Fully Secure Synchronous MPC with Asynchronous Fallback},
  author={Erica Blum and Chen-Da Liu-Zhang and Julian Loss},