Asynchronous Secure Multiparty Computation in Constant Time

In the setting of secure multiparty computation, a set of mutually distrusting parties wish to securely compute a joint function. It is well known that if the communication model is asynchronous, meaning that messages can be arbitrarily delayed by an unbounded (yet finite) amount of time, secure computation is feasible if and only if at least twothirds of the parties are honest, as was shown by Ben-Or, Canetti, and Goldreich [STOC'93] and by Ben-Or, Kelmer, and Rabin [PODC'94]. The running-time of all currently known protocols depends on the function to evaluate. In this work we present the first asynchronous MPC protocol that runs in constant time. Our starting point is the asynchronous MPC protocol of Hirt, Nielsen, and Przydatek [Eurocrypt'05, ICALP'08]. We integrate threshold fully homomorphic encryption in order to reduce the interactions between the parties, thus completely removing the need for the expensive king-slaves approach taken by Hirt et al.. Initially, assuming an honest majority, we construct a constant-time protocol in the asynchronous Byzantine agreement (ABA) hybrid model. Using a concurrent ABA protocol that runs in constant expected time, we obtain a constant expected time asynchronous MPC protocol, secure facing static malicious adversaries, assuming t < n/3.