We consider the problem of constructing a general protocol for secure two-party computation in a way that preserves security under concurrent composition. In our treatment, we focus on the case where an a-priori bound on the number of concurrent sessions is specified before the protocol is constructed (a.k.a. bounded concurrency). We make no set-up assumptions.

Lindell (STOC 2003) has shown that any protocol for bounded-concurrent secure two-party computation, whose security is established via black-box simulation, must have round complexity that is strictly larger than the bound on the number of concurrent sessions. In this paper, we construct a (non black-box) protocol for realizing bounded-concurrent secure two-party computation in a constant number of rounds. The only previously known protocol for realizing the above task required more rounds than the pre-specified bound on the number of sessions (despite usage of non black-box simulation techniques).

Our constructions rely on the existence of enhanced trap-door permutations, as well as on the existence of hash functions that are collision-resistant against subexponential sized circuits.