A Failure-Friendly Design Principle for Hash Functions

This paper reconsiders the established Merkle-Damg̊ard design principle for iterated hash functions. The internal state size w of an iterated n-bit hash function is treated as a security parameter of its own right. In a formal model, we show that increasing w quantifiably improves security against certain attacks, even if the compression function fails to be collision resistant. We propose the wide-pipe hash, internally using a w-bit compression function, and the double-pipe hash, with w = 2n and an n-bit compression function used twice in parallel.