International Association
for Cryptologic Research

The state-test technique, originally introduced in the context of impossible-differential cryptanalysis and recently used as an improvement for truncated-differential Meet-in-the-Middle attacks, has proven to be useful for reducing the complexity of attacks. In essence, the idea is to guess parts of the state instead of the key during the key-guessing stage of an attack, ultimately reducing the number of guesses needed. We generalize the idea of the state-test technique, allowing it to be applied not only to impossible-differential and (truncated-)differential Meet-in-the-Middle, but also to differential and differential-linear cryptanalysis, proposing also a new performant technique exploiting the state-test technique and probabilistic key-recovery. Additionally, we provide insights on the interaction between cipher and difference needed for the state-test technique to be applicable, finding it to be a promising option for many ciphers. To illustrate our findings, we provide 3 new applications of the state-test technique: we show how it can be used to improve the best known attack on the block cipher Pride, how it can be used to improve a step in the best known attack on Serpent, and use it to present the first known attacks on 24, 25 and 26 rounds of CRAFT (out of 32), improving by up to three rounds over the previous best ones.

In this paper, we extend the applicability of differential meet-in-the-middle attacks, proposed at Crypto 2023, to truncated differentials, and in addition, we introduce three new ideas to improve this type of attack: we show how to add longer structures than the original paper, we show how to improve the key recovery steps by introducing some probability in them, and we combine this type of attacks with the state-test technique, that was introduced in the context of impossible differential attacks. Furthermore, we have developed a MILP-based tool to automate the search for a truncated differential-MITM attack with optimized overall complexity, incorporating some of the proposed improvements. Thanks to this, we can build the best known attacks on the cipher CRAFT, reaching 23 rounds against 21 previously; we provide a new attack on 23 round SKINNY-64-192, and we improve the best attacks on SKINNY-128-384.