Cryptography in Constant Parallel Time

Benny Applebaum, Technion

We study the parallel time-complexity of basic cryptographic primitives such as one-way functions (OWFs) and pseudorandom generators (PRGs). Specifically, we consider the possibility of computing instances of these primitives by NC0 circuits, in which each output bit depends on a constant number of input bits. Despite previous efforts in this direction, there has been no convincing theoretical evidence supporting this possibility, which was posed as an open question in several previous works.

We essentially settle this question by providing strong evidence for the possibility of cryptography in NC0. Our main result is that every "moderately easy" OWF (resp., PRG), say computable in NC1, can be compiled into a corresponding OWF (resp., low-stretch PRG) in which each output bit depends on only four input bits. The existence of OWF and PRG in NC1 is a relatively mild assumption, implied by most number-theoretic or algebraic intractability assumptions commonly used in cryptography. A similar compiler can also be obtained for other cryptographic primitives such as one-way permutations, encryption, signature, commitment, and collision-resistant hashing.

Our results make use of the machinery of randomizing polynomials, which was originally motivated by questions in the domain of information-theoretic secure multiparty computation. By extending this tool to the computational setting we obtain additional results regarding NC0 cryptography. In particular, we show that even some relatively complex cryptographic primitives, including (stateless) symmetric encryption and digital signatures, are NC0-reducible to a PRG. No parallel reductions of this type were previously known, even in NC. Our reductions make a non-black-box use of the underlying PRG.

Joint work with Yuval Ishai and Eyal Kushilevitz (FOCS 2004, CCC 2005)

Gates 4B (opposite 490) Thursday 06/16/05 1630 hrs