Paper 2013/745

Asynchronous MPC with a Strict Honest Majority Using Non-equivocation

Michael Backes, Fabian Bendun, Ashish Choudhury, and Aniket Kate

Abstract

Multiparty computation (MPC) among n parties can tolerate up to t<n/2 active corruptions in a synchronous communication setting; however, in an asynchronous communication setting, the resiliency bound decreases to only t<n/3 active corruptions. We improve the resiliency bound for asynchronous MPC (AMPC) to match synchronous MPC using non-equivocation. Non-equivocation is a message authentication mechanism to restrict a corrupted sender from making conflicting statements to different (honest) parties. It can be implemented using an increment-only counter and a digital signature oracle, realizable with trusted hardware modules readily available in commodity computers and smartphone devices. A non-equivocation mechanism can also be transferable and allow a receiver to verifiably transfer the authenticated statement to other parties. In this work, using transferable non-equivocation, we present an AMPC protocol tolerating t<n/2 faults. From a practical point of view, our AMPC protocol requires fewer setup assumptions than the previous AMPC protocol with t<n/2 by Beerliovä-Trub\'ıniovä, Hirt and Nielsen [PODC 2010]: unlike their AMPC protocol, it does not require any synchronous broadcast round at the beginning of the protocol and avoids the threshold homomorphic encryption setup assumption. Moreover, our AMPC protocol is also efficient and provides a gain of \Theta(n) in the communication complexity per multiplication gate, over the AMPC protocol of Beerliovä-Trub\'ıniovä et al. In the process, using non-equivocation, we also define the first asynchronous verifiable secret sharing (AVSS) scheme with t<n/2, which is of independent interest to threshold cryptography.

Note: Non-equivocation definition and protocols pseudocodes are refined.