International Association
for Cryptologic Research

<p>At CHES 2017, Banik et al. proposed a lightweight block cipher GIFT consisting of two versions GIFT-64 and GIFT-128. Recently, there are lots of authenticated encryption schemes that adopt GIFT-128 as their underlying primitive, such as GIFT-COFB and HyENA. To promote a comprehensive perception of the soundness of the designs, we evaluate their security against differential-linear cryptanalysis.</p><p>For this, automatic tools have been developed to search differential-linear approximation for the ciphers based on S-boxes. With the assistance of the automatic tools, we find 13-round differential-linear approximations for GIFT-COFB and HyENA. Based on the distinguishers, 18-round key-recovery attacks are given for the message processing phase and initialization phase of both ciphers. Moreover, the resistance of GIFT-64/128 against differential-linear cryptanalysis is also evaluated. The 12-round and 17-round differential-linear approximations are found for GIFT-64 and GIFT-128 respectively, which lead to 18-round and 19-round key-recovery attacks respectively. Here, we stress that our attacks do not threaten the security of these ciphers. </p>

The SHA-3 standard consists of four cryptographic hash functions, called SHA3-224, SHA3-256, SHA3-384 and SHA3-512, and two extendable-output functions (XOFs), called SHAKE128 and SHAKE256. In this paper, we study the collision resistance of the SHA-3 instances. By analyzing the nonlinear layer, we introduce the concept of maximum difference density subspace, and develop a new target internal difference algorithm by probabilistic linearization. We also exploit new strategies for optimizing the internal differential characteristic. Further more, we figure out the expected size of collision subsets in internal differentials, by analyzing the collision probability of the digests rather than the intermediate states input to the last nonlinear layer. These techniques enhance the analysis of internal differentials, leading to the best collision attacks on four round-reduced variants of the SHA-3 instances. In particular, the number of attacked rounds is extended to 5 from 4 for SHA3-384, and to 6 from 5 for SHAKE256.

The KECCAK hash function was selected by NIST as the winner of the SHA-3 competition in 2012 and became the SHA-3 hash standard of NIST in 2015. On account of SHA-3’s importance in theory and applications, the analysis of its security has attracted increasing attention. In the SHA-3 family, SHA3-512 shows the strongest resistance against collision attacks: the theoretical attacks of SHA3-512 only extend to four rounds by solving polynomial systems with 64 times faster than the birthday attack. Yet for the SHA-3 instance SHAKE256, there are no results on collision attacks that we are aware of in the literatures. In this paper, we study the collision attacks against round-reduced SHA-3. Inspired by the work of Dinur, Dunkelman and Shamir in 2013, we propose a variant of birthday attack and improve the internal differential cryptanalysis by abstracting new concepts such as differential transition conditions and difference conditions table. With the help of these techniques, we develop new collision attacks on round-reduced SHA-3 using conditional internal differentials. More exactly, the initial messages constrained by linear conditions pass through the first two rounds of internal differential, and their corresponding inputs entering the last two rounds are divided into different subsets for collision search according to the values of linear conditions. Together with an improved target internal difference algorithm (TIDA), collision attacks on up to 5 rounds of all the six SHA-3 functions are obtained. In particular, collision attacks on 4-round SHA3-512 and 5-round SHAKE256 are achieved with complexity of $2^{237}$ and $2^{185}$ respectively. As far as we know, this is the best collision attack on reduced SHA3-512, and it is the first collision attack on reduced SHAKE256.

The stream cipher ChaCha is one of the most widely used ciphers in the real world, such as in TLS, SSH and so on. In this paper, we study the security of ChaCha via differential cryptanalysis based on probabilistic neutrality bits (PNBs). We introduce the \textit{syncopation} technique for the PNB-based approximation in the backward direction, which significantly amplifies its correlation by utilizing the property of ARX structure. In virtue of this technique, we present a new and efficient method for finding a good set of PNBs. A refined framework of key-recovery attack is then formalized for round-reduced ChaCha. The new techniques allow us to break 7.5 rounds of ChaCha without the last XOR and rotation, as well as to bring faster attacks on 6 rounds and 7 rounds of ChaCha.

In CRYPTO 2019, Gohr shows that well-trained neural networks can perform cryptanalytic distinguishing tasks superior to traditional differential distinguishers. Moreover, applying an unorthodox key guessing strategy, an 11-round key-recovery attack on a modern block cipher Speck32/64 improves upon the published state-of-the-art result. This calls into the next questions. To what extent is the advantage of machine learning (ML) over traditional methods, and whether the advantage generally exists in the cryptanalysis of modern ciphers? To answer the first question, we devised ML-based key-recovery attacks on more extended round-reduced Speck32/64. We achieved an improved 12-round and the first practical 13-round attacks. The essential for the new results is enhancing a classical component in the ML-based attacks, that is, the neutral bits. To answer the second question, we produced various neural distinguishers on round-reduced Simon32/64 and provided comparisons with their pure differential-based counterparts.

The differential-linear cryptanalysis is an important cryptanalytic tool in cryptography, and has been extensively researched since its discovery by Langford and Hellman in 1994. There are nevertheless very few methods to study the middle part where the differential and linear trail connect, besides the Differential-Linear Connectivity Table (Bar-On et al., EUROCRYPT 2019) and the experimental approach. In this paper, we study differential-linear cryptanalysis from an algebraic perspective. We first introduce a technique called Differential Algebraic Transitional Form (DATF) for differential-linear cryptanalysis, then develop a new theory of estimation of the differential-linear bias and techniques for key recovery in differential-linear cryptanalysis. The techniques are applied to the CAESAR finalist ASCON, the AES finalist SERPENT, and the eSTREAM finalist Grain v1. The bias of the differential-linear approximation is estimated for ASCON and SERPENT. The theoretical estimates of the bias are more accurate than that obtained by the DLCT, and the techniques can be applied with more rounds. Our general techniques can also be used to estimate the bias of Grain v1 in differential cryptanalysis, and have a markedly better performance than the Differential Engine tool tailor-made for the cipher. The improved key recovery attacks on round-reduced variants of these ciphers are then proposed. To the best of our knowledge, they are thus far the best known cryptanalysis of SERPENT, as well as the best differential-linear cryptanalysis of ASCON and the best initialization analysis of Grain v1. The results have been fully verified by experiments. Notably, security analysis of SERPENT is one of the most important applications of differential-linear cryptanalysis in the last two decades. The results in this paper update the differential-linear cryptanalysis of SERPENT-128 and SERPENT-256 with one more round after the work of Biham, Dunkelman and Keller in 2003.

The Keccak hash function is the winner of the SHA-3 competition (2008–2012) and became the SHA-3 standard of NIST in 2015. In this paper, we focus on practical collision attacks against round-reduced SHA-3 and some Keccak variants. Following the framework developed by Dinur et al. at FSE 2012 where 4-round collisions were found by combining 3-round differential trails and 1-round connectors, we extend the connectors to up to three rounds and hence achieve collision attacks for up to 6 rounds. The extension is possible thanks to the large degree of freedom of the wide internal state. By linearizing S-boxes of the first round, the problem of finding solutions of 2-round connectors is converted to that of solving a system of linear equations. When linearization is applied to the first two rounds, 3-round connectors become possible. However, due to the quick reduction in the degree of freedom caused by linearization, the connector succeeds only when the 3-round differential trails satisfy some additional conditions. We develop dedicated strategies for searching differential trails and find that such special differential trails indeed exist. To summarize, we obtain the first real collisions on six instances, including three round-reduced instances of SHA-3 , namely 5-round SHAKE128 , SHA3 -224 and SHA3 -256, and three instances of Keccak contest, namely Keccak [1440, 160, 5, 160], Keccak [640, 160, 5, 160] and Keccak [1440, 160, 6, 160], improving the number of practically attacked rounds by two. It is remarked that the work here is still far from threatening the security of the full 24-round SHA-3 family.