Key-Recovery Attacks Against the MAC Algorithm Chaskey

Chaskey is a Message Authentication Code (MAC) for 32bit microcontrollers proposed by Mouha et. al at SAC 2014. Its underlying blockcipher uses an Even-Mansour construction with a permutation based on the ARX methodology. In this paper, we present key-recovery attacks against Chaskey in the single and multi-user setting. These attacks are based on recent work by Fouque, Joux and Mavromati presented at Asiacrypt 2014 on Even-Mansour based constructions. We first show a simple attack on the classical single-user setting which confirms the security properties of Chaskey. Then, we describe an attack in the multi-user setting and we recover all keys of 2 users by doing 2 queries per user. Finally, we show a variant of this attack where we are able to recover keys of two users in a smaller group of 2 users.