On the Salsa20 Core Function

In this paper, we point out some weaknesses in the Salsa20 core function that could be exploited to obtain up to 231 collisions for its full (20 rounds) version. We first find an invariant for its main building block, the quarterround function, that is then extended to the rowround and columnround functions. This allows us to find an input subset of size 232 for which the Salsa20 core behaves exactly as the transformation f(x) = 2x. An attacker can take advantage of this for constructing 231 collisions for any number of rounds. We finally show another weakness in the form of a differential characteristic with probability one that proves that the Salsa20 core does not have 2nd preimage resistance.

• Type: Conference
• Year: 2008
• Where: FSE
• Authors: Julio César Hernández Castro, Juan M. Estévez-Tapiador, Jean-Jacques Quisquater
• Keywords: Collisions, Core Function, Cryptology, FSE 2008, Main Building Block