Second Preimages for Iterated Hash Functions and Their Implications on MACs

In this article, we focus on second preimages for iterated hash functions. More precisely, we introduce the notion of a b-block bypass which is closely related to the notion of second preimage but specifies additional properties. We will then give two examples of iterated hash functions to which this notion applies: a double-block length hash function and a single-block length hash function. Furthermore, we look at NMAC and HMAC and show the implications of a b-block bypass regarding forgery attacks. As a result it turns out that the impact of second preimages for NMAC and HMAC heavily depends on how the second preimages are constructed.