(2026-02) Putting the OPTI in Round Optimal IA-MPC in the Plain Model
2026-02-27
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. ), the effect of relaxing the setting to that of Selective Identifiable Abort, or an honest supermajority (i.e. ), has not been established.
In this work, we close this gap. In the honest supermajority case, we show a positive result: two rounds are sufficient to achieve identifiable abort, and thus also selective 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.
When a third or more of the parties might be corrupt, we show a negative result: three rounds are necessary to achieve Selective Identifiable Abort with straight-line simulation.