Cache attack on recursive structure of MISTY1

Abstract

Side-channel attacks exploit information from physical implementations of cryptographic systems. Cache attacks have improved at recovering information by combining observations of the victim's cache access and knowledge of the cipher's structure. Cache attacks have been implemented for most Feistel- and SPN-structured block cipher algorithms, but the security of algorithms for special structures has seen little attention. We perform a Flush+Reload attack on MISTY1, a class of block cipher with a recursive structure. The FL function is performed before the plaintext input S- box and after the ciphertext output S-box, making it difficult to attack the first and last rounds. However, the key scheduling part of MISTY1 leaks many bits of the key, which, together with the leakage of partial bits of the round key during encryption, is sufficient to recover it. We design an algorithm that can recover the MISTY1 128- bit key after observing one encryption and then use the leakage during encryption to reduce its complexity. We experiment on 32- and 64-byte cache line environments. An adversary needs to observe as little as 5 encryptions to recover the 128-bit key in 0.035 s in the first case and 10 encryptions to recover the key in 2.1 h in the second case.