On Strong Simulation and Composable Point Obfuscation

The Virtual Black Box (VBB) property for program obfuscators provides a strong guarantee: anything computable by an efficient adversary, given the obfuscated program, can also be computed by an efficient simulator, with only oracle access to the program. However, we know how to achieve this notion only for very restricted classes of programs. This work studies a simple relaxation of VBB: allow the simulator unbounded computation time, while still allowing only polynomially many queries to the oracle. We demonstrate the viability of this relaxed notion, which we call Virtual Grey Box (VGB), in the context of composable obfuscators for point programs: it is known that, with respect to VBB, if such obfuscators exist, then there exist multi-bit point obfuscators (also known as “digital lockers”) and subsequently also very strong variants of encryption that are resilient to various attacks, such as key leakage and key-dependent-messages. However, no composable VBB-obfuscators for point programs have been shown. We show composable VGB -obfuscators for point programs under a strong variant of the Decision Diffie–Hellman assumption. We show that VGB (instead of VBB) obfuscation still suffices for the above applications, as well as for new applications. This includes extensions to the public key setting and to encryption schemes with resistance to certain related key attacks (RKA).