From the Impossibility of Obfuscation to a New Non-Black-Box Simulation Technique

The introduction of a non-black-box simulation technique by Barak (FOCS 2001) has been a major landmark in cryptography, breaking the previous barriers of black-box impossibility. Barak's techniques were subsequently extended and have given rise to various powerful applications. We present the first non-black-box simulation technique that does not rely on Barak's technique (or on non-standard assumptions). Our technique is based on essentially different tools: {\em it does not invoke universal arguments}, nor does it rely on collision-resistant hashing. Instead, the main ingredient we use is the impossibility of general program obfuscation (Barak et al., CRYPTO 2001). Using our technique, we construct a new resettably-sound zero-knowledge (\rsZK) protocol. \rsZK protocols remain sound even against cheating provers that can repeatedly reset the verifier to its initial state and random tape. Indeed, for such protocols black-box simulation is impossible. Our $\rsZK$ protocol is the first to be based solely on semi-honest oblivious transfer and does not rely on collision-resistant hashing, in addition, our protocol does not use $\PCP$ machinery. In the converse direction, we show a generic transformation from any $\rsZK$ protocol to a family of functions that cannot be obfuscated.