Attacks on Hash Functions Based on Generalized Feistel: Application to Reduced-Round
In this paper we study the strength of two hash functions which are based on Generalized Feistels. We describe a new kind
of attack based on a cancellation property in the round function. This new technique allows to efficiently use the degrees
of freedom available to attack a hash function. Using the cancellation property, we can avoid the non-linear parts of the
round function, at the expense of some freedom degrees.
Our attacks are mostly independent of the round function in use, and can be applied to similar hash functions which share
the same structure but have different round functions. We start with a 22-round generic attack on the structure of Lesamnta, and adapt it to the actual round function to attack 24-round Lesamnta (the full function has 32 rounds). We follow with an attack on 9-round SHAvite-3
512 which also works for the tweaked version of SHAvite-3