Square Attack on 7-Round Kiasu-BC

Kiasu-BC is a tweakable block cipher presented within the TWEAKEY framework at AsiaCrypt 2014. Kiasu-BC is almost identical to AES-128, the only difference to AES-128 is the tweak addition, where the 64-bit tweak is xored to the first two rows of every round-key. The security analysis of the designers focuses primarily on related-key related-tweak differential characteristics and meet-in-the-middle attacks. For other attacks, they conclude that the security level of Kiasu-BC is similar to AES-128. In this work, we provide the first third-party analysis of Kiasu-BC. We show that we can mount Square attacks on up to 7-round Kiasu-BC with a complexity of about 2 encryptions, which improves upon the best published 7-round attacks for AES-128. Furthermore, we show that such attacks are applicable to the round-reduced ΘCB3-like mode of the CAESAR candidate Kiasu6=. To be specific, we show a key-recovery attack on 7-round Kiasu 6= with a complexity of about 2 encryptions.