Impossibility of VBB obfuscation with ideal constant-degree graded encodings

Abstract

A celebrated result by Barak et al. (Crypto’01) shows the impossibility of general-purpose virtual black-box (VBB) obfuscation in the plain model. A recent work by Canetti, Kalai, and Paneth (TCC’15) extends this impossibility result to the random oracle model (assuming trapdoor permutations). In contrast, Brakerski-Rothblum (TCC’14) and Barak et al. (Euro- Crypt’14) show that in idealized graded encoding models, general-purpose VBB obfuscation indeed is possible; these constructions require graded encoding schemes that enable evaluating high-degree (polynomial in the size of the circuit to be obfuscated) polynomials on encodings. We show a complementary impossibility of general-purpose VBB obfuscation in idealized graded encoding models that enable only evaluation of constant-degree polynomials (assuming trapdoor permutations).