More on Impossibility of Virtual Black-Box Obfuscation in Idealized Models

The celebrated work of Barak et al. (Crypto’01) ruled out the possibility of virtual blackbox (VBB) obfuscation for general circuits. The recent work of Canetti, Kalai, and Paneth (TCC’15) extended this impossibility to the random oracle model as well assuming the existence of trapdoor permutations (TDPs). On the other hand, the works of Barak et al. (Crypto’14) and Brakerski-Rothblum (TCC’14) showed that general VBB obfuscation is indeed possible in idealized graded encoding models. The recent work of Pass and Shelat (Cryptology ePrint 2015/383) complemented this result by ruling out general VBB obfuscation in idealized graded encoding models that enable evaluation of constant-degree polynomials in finite fields. In this work extend the above two impossibly results for general VBB obfuscation in idealized models. In particular we prove the following two results both assuming the existence of TDPs: • There is no general VBB obfuscation in the generic group model of Shoup (Eurocrypt’97) for any abelian group. By applying our techniques to the setting of Pass and Shelat we extend their result to any (even non-commutative) finite ring. • There is no general VBB obfuscation even in the random trapdoor permutation oracle model. Our proof extends to any number of levels of hierarchical trapdoors.