International Association
for Cryptologic Research

This paper analyzes the authenticated encryption algorithm ACORN, a candidate in the CAESAR cryptographic competition. We identify weaknesses in the state update function of ACORN which result in collisions in the internal state of ACORN. This paper shows that for

a given set of key and initialization vector values we can construct two distinct input messages which result in a collision in the ACORN internal state. Using a standard PC the collision can be found almost instantly when the secret key is known.