White-Box and Asymmetrically Hard Crypto Design

In this talk we surveyed some our recent works related to the area of white-box cryptogaphy. Specifically the resource hardness framework from Asiacrypt'2017 and its relation to the incompressibility and weak-WBC

Mini-ciphers: a reliable testbed for cryptanalysis?

This paper reports on higher-order square analysis of the AES cipher. We present experimental results of attack simulations on mini-AES versions with word sizes of 3, 4, 5, 6 and 7 bits and describe the propagation of higher-order Lambda-sets inside some of these distinguishers. A possible explanation of the length of the square distinguishers uses the concept of higher-order derivatives of discrete mappings

Decomposition attack on SASASASAS

We demonstrate the first attacks on the SPN ciphers with 6, 7, 8, and 9 secret layers. In particular, we show a decomposition attack on the SASASASAS scheme when the S-box size M and the block length N satisfy the condition M^2 < N (for example, 8-bit S-box and 128-bit block)

Yoyo Tricks with AES

In this paper we present new fundamental properties of SPNs. These properties turn out to be particularly useful in the adaptive chosen ciphertext/plaintext setting and we show this by introducing for the first time key-independent yoyo-distinguishers for 3- to 5-rounds of AES. All of our distinguishers beat previous records and require respectively $3, 4$ and $2^{25.8}$ data and essentially zero computation except for observing differences. In addition, we present the first key-independent distinguisher for 6-rounds AES based on yoyos that preserve impossible zero differences in plaintexts and ciphertexts. This distinguisher requires an impractical amount of $2^{122.83}$ plaintext/ciphertext pairs and essentially no computation apart from observing the corresponding differences. We then present a very favorable key-recovery attack on 5-rounds of AES that requires only $2^{11.3}$ data complexity and $2^{31}$ computational complexity, which as far as we know is also a new record. All our attacks are in the adaptively chosen plaintext/ciphertext scenario. Our distinguishers for AES stem from new and fundamental properties of generic SPNs, including generic SAS and SASAS, that can be used to preserve zero differences under the action of exchanging values between existing ciphertext and plaintext pairs. We provide a simple distinguisher for 2 generic SP-rounds that requires only 4 adaptively chosen ciphertexts and no computation on the adversaries side. We then describe a generic and deterministic yoyo-game for 3 generic SP-rounds which preserves zero differences in the middle but which we are not capable of exploiting in the generic setting

On possibility of using convolutional neural networks for creating universal attacks on iterative block ciphers

Исследуется возможность применения свёрточных нейронных сетей к задаче анализа стойкости итеративных блочных шифров. Предлагается новый подход к построению атак-различителей на основе свёрточной нейронной сети, обученной различать графические эквиваленты шифртекстов, полученных в режиме шифрования CTR (счётчика) после разного числа раундов, в том числе после такого, которое обеспечивает удовлетворительные статистические свойства шифртекста. По аналогии со статистическими тестами, предложенный подход позволяет создавать различители без необходимости проведения аналитического исследования каждого шифра, что даёт возможность строить универсальные различители сразу для серии шифров. Предлагается несколько схем построения универсальных атак-различителей, которые, как демонстрируется экспериментально, в ряде случаев позволяют выявлять отклонения от случайности на меньших выборках и при большем числе раундов, чем ранее известные статистические тесты

Multiset-Algebraic Cryptanalysis of Reduced Kuznyechik, Khazad, and secret SPNs

We devise the first closed formula for the number of rounds of a blockcipher with secret components so that these components can be revealed using multiset, algebraic-degree, or division-integral properties, which in this case are equivalent. Using the new result, we attack 7 (out of 9) rounds of Kuznyechik, the recent Russian blockcipher standard, thus halving its security margin. With the same technique we attack 6 (out of 8) rounds of Khazad, the legacy 64-bit blockcipher. Finally, we show how to cryptanalyze and find a decomposition of generic SPN construction for which the inner-components are secret. All the attacks are the best to date

On Recovering Affine Encodings in White-Box Implementations

Ever since the first candidate white-box implementations by Chow et al. in 2002, producing a secure white-box implementation of AES has remained an enduring challenge. Following the footsteps of the original proposal by Chow et al., other constructions were later built around the same framework. In this framework, the round function of the cipher is encoded by composing it with non-linear and affine layers known as encodings. However, all such attempts were broken by a series of increasingly efficient attacks that are able to peel off these encodings, eventually uncovering the underlying round function, and with it the secret key. These attacks, however, were generally ad-hoc and did not enjoy a wide applicability. As our main contribution, we propose a generic and efficient algorithm to recover affine encodings, for any Substitution-Permutation-Network (SPN) cipher, such as AES, and any form of affine encoding. For AES parameters, namely 128-bit blocks split into 16 parallel 8-bit S-boxes, affine encodings are recovered with a time complexity estimated at $2^{32}$ basic operations, independently of how the encodings are built. This algorithm is directly applicable to a large class of schemes. We illustrate this on a recent proposal due to Baek, Cheon and Hong, which was not previously analyzed. While Baek et al. evaluate the security of their scheme to 110 bits, a direct application of our generic algorithm is able to break the scheme with an estimated time complexity of only $2^{35}$ basic operations. As a second contribution, we show a different approach to cryptanalyzing the Baek et al. scheme, which reduces the analysis to a standalone combinatorial problem, ultimately achieving key recovery in time complexity $2^{31}$. We also provide an implementation of the attack, which is able to recover the secret key in about 12 seconds on a standard desktop computer

Extended Generalized Feistel Networks using Matrix Representation

International audienceWhile Generalized Feistel Networks have been widely studied in the literature as a building block of a block cipher, we propose in this paper a unified vision to easily represent them through a matrix representation. We then propose a new class of such schemes called Extended Generalized Feistel Networks well suited for cryptographic applications. We instantiate those proposals into two particular constructions and we finally analyze their security