A New Approach to Round-Optimal Secure Multiparty Computation.

We present a new approach towards constructing roundoptimal secure multiparty computation (MPC) protocols against malicious adversaries without trusted setup assumptions. Our approach builds on ideas previously developed in the context of covert multiparty computation [Chandran et al., FOCS' 07] even though we do not seek covert security. Using our new approach, we obtain the following results: - A five round MPC protocol based on the Decisional Diffie-Hellman (DDH) assumption. - A four round MPC protocol based on one-way permutations and sub-exponentially secure DDH. This result is optimal in the number of rounds. Previously, no four-round MPC protocol for general functions was known and five-round protocols were only known based on indistinguishability obfuscation (and some additional assumptions) [Garg et al., EUROCRYPT'16].