Cryptography with Constant Input Locality

We study the following natural question: Which cryptographic primitives (if any) can be realized by functions with constant input locality, namely functions in which every bit of the

input

influences only a constant number of bits of the output? This continues the study of cryptography in low complexity classes. It was recently shown (Applebaum et al., FOCS 2004) that, under standard cryptographic assumptions, most cryptographic primitives can be realized by functions with constant

output

locality, namely ones in which every bit of the

output

is influenced by a constant number of bits from the input.

We (almost) characterize what cryptographic tasks can be performed with constant input locality. On the negative side, we show that primitives which require some form of non-malleability (such as digital signatures, message authentication, or non-malleable encryption)

cannot

be realized with constant input locality. On the positive side, assuming the intractability of certain problems from the domain of error correcting codes (namely, hardness of decoding a random linear code or the security of the McEliece cryptosystem), we obtain new constructions of one-way functions, pseudorandom generators, commitments, and semantically-secure public-key encryption schemes whose input locality is constant. Moreover, these constructions also enjoy constant

output locality

. Therefore, they give rise to cryptographic hardware that has constant-depth, constant fan-in and constant

fan-out

. As a byproduct, we obtain a pseudorandom generator whose output and input locality are both optimal (namely, 3).