Research on State Collisions of Authenticated Cipher ACORN

This paper analyzes the authentication security of lightweight authenticated cipher ACORN, a second-round candidate of the CAESAR competition. We first put forward the necessary and sufficient conditions for state collisions of ACORN. Then we point out the errors of the designer's analysis and utilize a reverse derivation method to find proper input differences to the registers which can ensure that the state differences get eliminated in certain rounds. Last but not least, we redefine the differential properties of the overall feedback function to compute the probability of state collisions more precisely and improve the security margin of ACORN given by the designer.