Impossible-Differential and Boomerang Cryptanalysis of Round-Reduced Kiasu-BC

Kiasu-BC is a tweakable block cipher proposed by Jean et al. at ASIACRYPT 2014 alongside their TWEAKEY framework. The cipher is almost identical to the AES-128 except for the tweak, which renders it an attractive primitive for various modes of operation and applications requiring tweakable block ciphers. Therefore, studying how the additional tweak input affects security compared to that of the AES is highly valuable to gain trust in future instantiations. This work proposes impossible-differential and boomerang attacks on eight rounds of Kiasu-BC in the single-key model, using the core idea that the tweak input allows to construct local collisions. While our results do not threat the security of the full-round version, they help concretize the security of Kiasu-BC in the single-key model

Cryptanalysis of Tweakable Block Ciphers and Forkciphers

INTERNSHIP REPORTInternational audienc

Cryptanalysis of Forkciphers

International audienceThe forkcipher framework was designed in 2018 by Andreeva et al. for authenticated encryption of short messages. Two dedicated ciphers were proposed in this framework: ForkAES based on the AES (and its tweakable variant Kiasu-BC), and ForkSkinny based on Skinny. The main motivation is that the forked ciphers should keep the same security as the underlying ciphers, but offer better performances thanks to the larger output. Recent cryptanalysis results at ACNS '19 have shown that ForkAES actually offers a reduced security margin compared to the AES with an 8-round attack, and this was taken into account in the design of ForkSkinny. In this paper, we present new cryptanalysis results on forkciphers. First we improve the previous attack on ForkAES in order to attack the full 10 rounds. This is the first attack challenging the security of full ForkAES. Then we present the first analysis of ForkSkinny, showing that the best attacks on Skinny can be extended to one round for most ForkSkinny variants, and up to three rounds for ForkSkinny-128-256. This allows to evaluate the security degradation between ForkSkinny and the underlying block cipher. Our analysis shows that all components of a forkcipher must be carefully designed: the attack against ForkAES uses the weak diffusion of the middle rounds in reconstruction queries (going from one ciphertext to the other), but the attack against ForkSkinny uses a weakness of the tweakey schedule in encryption queries (when one branch of the tweakey schedule is skipped)

Zero-Correlation Attacks on Tweakable Block Ciphers with Linear Tweakey Expansion

The design and analysis of dedicated tweakable block ciphers is a quite recent and very active research field that provides an ongoing stream of new insights. For instance, results of Kranz, Leander, and Wiemer from FSE 2017 show that the addition of a tweak using a linear tweak schedule does not introduce new linear characteristics. In this paper, we consider – to the best of our knowledge – for the first time the effect of the tweak on zero-correlation linear cryptanalysis for ciphers that have a linear tweak schedule. It turns out that the tweak can often be used to get zero-correlation linear hulls covering more rounds compared to just searching zero-correlation linear hulls on the data-path of a cipher. Moreover, this also implies the existence of integral distinguishers on the same number of rounds. We have applied our technique on round reduced versions of Qarma, Mantis, and Skinny. As a result, we can present – to the best of our knowledge – the best attack (with respect to number of rounds) on a round-reduced variant of Qarma