Paper 2026/406 Original: eprint.iacr.org/2026/406 ↗

Putting the OPTI in Round Optimal IA-MPC in the Plain Model

Abstract

We advance the study of best-possible security guarantees of two-round secure multiparty computation in the plain model. While Goel et al. (TCC '21) showed the infeasibility of Identifiable Abort in two rounds in the plain model with a standard honest majority (i.e. $t < n/2$), the effect of relaxing the setting to an honest supermajority has not been established. In this work, we close this gap. When $t < n/3$, we show that two rounds are sufficient to achieve identifiable abort. Notably, ours is the first two-round general-purpose MPC construction in the plain model tolerating more than one corruption that achieves a guarantee stronger than unanimous abort. We obtain our construction via a new compiler that lifts any protocol with unanimous abort to one that achieves identifiable abort. A core building block for our compiler is Oblivious Public Transfer with Intermediaries (OPTI), a new primitive that we introduce which may be of independent interest.