Garbled RAM Revisited, Part II

In EUROCRYPT 2013, Lu and Ostrovsky proposed the notion of Garbled RAM (GRAM) programs. These GRAM programs are analogous to the classic result of Yao’s garbled circuits: a large encrypted memory can first be provided to evaluator, and then a program can separately be garbled and sent to an evaluator to securely execute while learning nothing but the output of the program and its running time. The key feature of GRAM is that it harnesses the natural complexity-theoretic power that Random Access Memory (RAM) programs have over circuits, such as in binary search or randomized algorithms, where program can be executed in a garbled way without “unrolling” it into a circuit. The candidate Lu-Ostrovsky GRAM scheme proposed in that paper is based on the existence of one-way functions, but due to an implicit reliance on a complex “circular” use of Yao garbled circuits, the scheme requires an additional circularity assumptions and may not be secure otherwise. In this paper, we show how to construct efficient GRAM without circularity and based solely on the existence of any one-way function. The novel approach that allows us to break the circularity is a modification of the Goldreich-Goldwasser-Micali (PRF) construction. More specifically, we modify the PRF to allow PRF-keys to be “adaptively revoked” during run-time at the additive cost of roughly log n per revocation. Then, to improve the overhead of this scheme, we apply a delicate recursion technique that bootstraps mini-GRAM schemes into larger, more powerful ones while still avoiding circularity in the hybrid arguments. This results in secure GRAM with overhead of poly(κ)(min(t, n )) for any constant > 0 where n is the size of memory and t is the running time. In a companion work (Part I), Gentry, Halevi, Raykova, and Wichs show an alternative approach using identity-based encryption to solve the circularity problem. Their scheme achieves overhead of poly(κ)polylog(n) assuming the existence of IBE.