Cryptanalysis of FlexAEAD

This paper analyzes the internal keyed permutation of FlexAEAD which is a round-1 candidate of the NIST LightWeight Cryptography Competition. In our analysis, we report an iterated truncated differential leveraging on a particular property of the AES S-box that becomes useful due to the particular nature of the diffusion layer of the round function. The differential holds with a low probability of 2^-7 for one round which allows it to penetrate the same number of rounds as claimed by the designers, but with a much lower complexity. Moreover, it can be easily extended to a key-recovery attack at a little extra cost. We further report a Super-Sbox construction in the internal permutation, which is exploited using the Yoyo game to devise a 6-round deterministic distinguisher and a 7-round key recovery attack for the 128-bit internal permutation. Similar attacks can be mounted for the 64-bit and 256-bit variants. All these attacks outperform the existing results of the designers as well as other third-party results. The iterated truncated differentials can be tweaked to mount forgery attacks similar to the ones given by Eichlseder et al Success probabilities of all the reported distinguishing attacks are shown to be high. All practical attacks have been experimentally verified. To the best of our knowledge, this work reports the first key-recovery attack on the internal keyed permutation of FlexAEAD

The Exchange Attack: How to Distinguish Six Rounds of AES with 288.22^{88.2} chosen plaintexts

In this paper we present exchange-equivalence attacks which is a new cryptanalytic attack technique suitable for SPN-like block cipher designs. Our new technique results in the first secret-key chosen plaintext distinguisher for 6-round AES. The complexity of the distinguisher is about $2^{88.2}$ in terms of data, memory and computational complexity. The distinguishing attack for AES reduced to six rounds is a straight-forward extension of an exchange attack for 5-round AES that requires $2^{30}$ in terms of chosen plaintexts and computation. This is also a new record for AES reduced to five rounds. The main result of this paper is that AES up to at least six rounds is biased when restricted to exchange-invariant sets of plaintexts

Towards Key-Dependent Integral and Impossible Differential Distinguishers on 5-Round AES

Reduced-round AES has been a popular underlying primitive to design new cryptographic schemes and thus its security including distinguishing properties deserves more attention. At Crypto\u2716, a key-dependent integral distinguisher on 5-round AES was put forward, which opened up a new direction to take more insights into the distinguishing properties of AES. After that, two key-dependent impossible differential (ID) distinguishers on 5-round AES were proposed at FSE\u2716 and CT-RSA\u2718, respectively. It is strange that the current key-dependent integral distinguisher requires significantly higher complexities than the key-dependent ID distinguishers, even though they are constructed with the same property of MixColumns ($2^{128} \gg 2^{98.2}$). Proposers of the 5-round key-dependent distinguishers claimed that the corresponding integral and ID distinguishers can only work under chosen-ciphertext and chosen-plaintext settings, respectively, which is very different from the situations of traditional key-independent distinguishers. In this paper, we first construct a novel key-dependent integral distinguisher on 5-round AES with $2^{96}$ chosen plaintexts, which is much better than the previous key-dependent integral distinguisher that requires the full codebook proposed at Crypto\u2716. Secondly, we show that both distinguishers are valid under either chosen-plaintext setting or chosen-ciphertext setting, which is different from the claims of previous cryptanalysis. However, under different settings, complexities of key-dependent integral distinguishers are very different while those of the key-dependent ID distinguishers are almost the same. We analyze the reasons for it

MixColumns Coefficient Property and Security of the AES with A Secret S-Box

The MixColumns operation is an important component providing diffusion for the AES. The branch number of it ensures that any continuous four rounds of the AES have at least 25 active S-Boxes, which makes the AES secure against the differential and linear cryptanalysis. However, the choices of the coefficients of the MixColumns matrix may undermine the AES security against some novel-type attacks. A particular property of the AES MixColumns matrix coefficient has been noticed in recent papers that \emph{each row or column of the matrix has elements that sum to zero}. Several attacks have been developed taking advantage of the coefficient property. In this paper we investigate further the influence of the specific coefficient property on the AES security. Our target, which is also one of the targets of the previous works, is a 5-round AES variant with a secret S-Box. We will show how we take advantage of the coefficient property to extract the secret key directly without any assistance of the S-Box information. Compared with the previous similar attacks, the present attacks here are the best in terms of the complexity under the chosen-plaintext scenario

Improved Key Recovery Attacks on Reduced-Round AES with Practical Data an d Memory Complexities

Determining the security of AES is a central problem in cryptanalysis, but progress in this area had been slow and only a handful of cryptanalytic techniques led to significant advancements. At Eurocrypt 2017 Grassi et al. presented a novel type of distinguisher for AES-like structures, but so far all the published attacks which were based on this distinguisher were inferior to previously known attacks in their complexity. In this paper we combine the technique of Grassi et al. with several other techniques in a novel way to obtain the best known key recovery attack on 5-round AES in the single-key model, reducing its overall complexity from about $2^{32}$ to less than $2^{22}$. Extending our techniques to 7-round AES, we obtain the best known attacks on AES-192 which use practical amounts of data and memory, breaking the record for such attacks which was obtained in 2000 by the classical Square attack

The Retracing Boomerang Attack

Boomerang attacks are extensions of differential attacks, that make it possible to combine two unrelated differential properties of the first and second part of a cryptosystem with probabilities $p$ and $q$ into a new differential-like property of the whole cryptosystem with probability $p^2q^2$ (since each one of the properties has to be satisfied twice). In this paper we describe a new version of boomerang attacks which uses the counterintuitive idea of throwing out most of the data (including potentially good cases) in order to force equalities between certain values on the ciphertext side. This creates a correlation between the four probabilistic events, which increases the probability of the combined property to $p^2q$ and increases the signal to noise ratio of the resultant distinguisher. We call this variant a retracing boomerang attack since we make sure that the boomerang we throw follows the same path on its forward and backward directions. To demonstrate the power of the new technique, we apply it to the case of 5-round AES. This version of AES was repeatedly attacked by a large variety of techniques, but for twenty years its complexity had remained stuck at $2^{32}$. At Crypto\u2718 it was finally reduced to $2^{24}$ (for full key recovery), and with our new technique we can further reduce the complexity of full key recovery to the surprisingly low value of $2^{16.5}$ (i.e., only 90,000 encryption/decryption operations are required for a full key recovery on half the rounds of AES). In addition to improving previous attacks, our new technique unveils a hidden relationship between boomerang attacks and two other cryptanalytic techniques, the yoyo game and the recently introduced mixture differentials

Cryptanalysis of ForkAES

Forkciphers are a new kind of primitive proposed recently by Andreeva et al. for efficient encryption and authentication of small messages. They fork the middle state of a cipher and encrypt it twice under two smaller independent permutations. Thus, forkciphers produce two output blocks in one primitive call. Andreeva et al. proposed ForkAES, a tweakable AES-based forkcipher that splits the state after five out of ten rounds. While their authenticated encrypted schemes were accompanied by proofs, the security discussion for ForkAES was not provided, and founded on existing results on the AES and KIASU-BC. Forkciphers provide a unique interface called reconstruction queries that use one ciphertext block as input and compute the respective other ciphertext block. Thus, they deserve a careful security analysis. This work fosters the understanding of the security of ForkAES with three contributions: (1) We observe that security in reconstruction queries differs strongly from the existing results on the AES. This allows to attack nine out of ten rounds with differential, impossible-differential and yoyo attacks. (2) We observe that some forkcipher modes may lack the interface of reconstruction queries, so that attackers must use encryption queries. We show that nine rounds can still be attacked with rectangle and impossible-differential attacks. (3) We present forgery attacks on the AE modes proposed by Andreeva et al. with nine-round ForkAES