Faster and Smoother - VSH Revisited

We reconsider the provably collision resistant Very Smooth Hash and propose a small change in the design aiming to improve both performance and security. While the original proofs of security based on hardness of factoring or discrete logarithms are preserved, we can base the security on the k-sum problem studied by Wagner and more recently by Minder Sz. Sinclair. The new approach allows to output shorter digests and brings the speed of Fast VSH closer to the range of "classical" hash functions. The modified VSH is likely to remain secure even if factoring and discrete logarithms are easy, while this would have a devastating effect on the original versions. This observation leads us to propose a variant that operates modulo a power of two to increase the speed even more. A function that offers an equivalent of 128-bit collision resistance runs at 68.5 MB/s on a 2.4 CHz Intel Core 2 CPU, more than a third of the speed of SH A-256.