How to Go Beyond the Black-Box Simulation Barrier

The simulation paradigm is central to cryptography. A simulator is an algorithm that tries to simulate the interaction of the adversary with an honest party, without knowing the private input of this honest party. Almost all known simulators use the adversary’s algorithm as a black-box. We present the first constructions of non-black-box simulators. Using these new non-black-box techniques we obtain several results that were previously shown to be impossible to obtain using black-box simulators. Specifically, assuming the existence of collision-resistent hash functions, we construct a new zero-knowledge argument system for NP that satisfies the following properties: 1. This system has a constant number of rounds with negligible soundness error. 2. It remains zero knowledge even when composed concurrently n times, where n is the security parameter. Simultaneously obtaining Properties 1 and 2 has been proven to be impossible to achieve using black-box simulators. 3. It is an Arthur-Merlin (public coins) protocol. Simultaneously obtaining Properties 1 and 3 has also been proven to be impossible to achieve with a black-box simulator. 4. It has a simulator that runs in strict polynomial time, rather than in expected polynomial time. All previously known zero-knowledge arguments satisfying Property 1 utilized expected polynomial-time simulators. Following this work it was shown that simultaneously obtaining Properties 1 and 4 is also impossible to achieve with a black-box simulator.