Multi-property Preserving Combiners for Hash Functions

A robust combiner for hash functions takes two candidate implementations and constructs a hash function which is secure as long as at least one of the candidates is secure. So far, hash function combiners only aim at preserving a single property such as collision-resistance or pseudorandomness. However, when hash functions are used in protocols like TLS they are often required to provide several properties simultaneously. We therefore put forward the notion of multi-property preserving combiners, clarify some aspects on different definitions for such combiners, and propose a construction that provably preserves collision resistance, pseudorandomness, “random-oracle-ness”, target collision resistance and message authentication according to our strongest notion.