Paper 2023/1739

Broadcast-Optimal Four-Round MPC in the Plain Model

Abstract

Motivated by the fact that broadcast is an expensive, but useful, resource for the realization of multi-party computation protocols (MPC), Cohen, Garay, and Zikas (Eurocrypt 2020), and subsequently Damgård, Magri, Ravi, Siniscalchi and Yakoubov (Crypto 2021), and, Damgård, Ravi, Siniscalchi and Yakoubov (Eurocrypt 2023), focused on 𝘴𝘰-𝘤𝘢𝘭𝘭𝘦𝘥 𝘣𝘳𝘰𝘢𝘥𝘤𝘢𝘴𝘵 𝘰𝘱𝘵𝘪𝘮𝘢𝘭 𝘔𝘗𝘊. In particular, the authors focus on two-round MPC protocols (in the CRS model), and give tight characterizations of which security guarantees are achievable if broadcast is available in the first round, the second round, both rounds, or not at all. This work considers the natural question of characterizing broadcast optimal MPC in the plain model where no set-up is assumed. We focus on four-round protocols, since four is known to be the minimal number of rounds required to securely realize any functionality with black-box simulation. We give a complete characterization of which security guarantees, (namely selective abort, selective identifiable abort, unanimous abort and identifiable abort) are feasible or not, depending on the exact selection of rounds in which broadcast is available.