Generalized Related-Key Rectangle Attacks on Block Ciphers with Linear Key Schedule: Applications to SKINNY and GIFT

This paper gives a new generalized key-recovery model of related-key rectangle attacks on block ciphers with linear key schedules. The model is quite optimized and applicable to various block ciphers with linear key schedule. As a proof of work, we apply the new model to two very important block ciphers, i.e. SKINNY and GIFT, which are basic modules of many candidates of the Lightweight Cryptography (LWC) standardization project by NIST. For SKINNY, we reduce the complexity of the best previous 27-round related-tweakey rectangle attack on SKINNY-128-384 from $2^{331}$ to $2^{294}$. In addition, the first 28-round related-tweakey rectangle attack on SKINNY-128-384 is given, which gains one more round than before. For the case of GIFT-64, we give the first 24-round related-key rectangle attack with a time complexity $2^{91.58}$, while the best previous attack on GIFT-64 only reaches 23 rounds at most

Switching the Top Slice of the Sandwich with Extra Filling Yields a Stronger Boomerang for NLFSR-based Block Ciphers

The Boomerang attack was one of the first attempts to visualize a cipher ($E$) as a composition of two sub-ciphers ($E_0\circ E_1$) to devise and exploit two high-probability (say $p,q$) shorter trails instead of relying on a single low probability (say $s$) longer trail for differential cryptanalysis. The attack generally works whenever $p^2 \cdot q^2 > s$. However, it was later succeeded by the so-called ``sandwich attack\u27\u27 which essentially splits the cipher in three parts $E\u27_0\circ E_m \circ E\u27_1$ adding an additional middle layer ($E_m$) with distinguishing probability of $p^2\cdot r\cdot q^2$. It is primarily the generalization of a body of research in this direction that investigate what is referred to as the switching activity and capture the dependencies and potential incompatibilities of the layers that the middle layer separates. This work revisits the philosophy of the sandwich attack over multiple rounds for NLFSR-based block ciphers and introduces a new method to find high probability boomerang distinguishers. The approach formalizes boomerang attacks using only ladder, And switches. The cipher is treated as $E = E_m \circ E_1$, a specialized form of a sandwich attack which we called as the ``open-sandwich attack\u27\u27. The distinguishing probability for this attack configuration is $r \cdot q^2$. Using this innovative approach, the study successfully identifies a deterministic boomerang distinguisher for the keyed permutation of the TinyJambu cipher over 320 rounds. Additionally, a 640-round boomerang with a probability of $2^{-22}$ is presented with 95% success rate. In the related-key setting, we unveil full-round boomerangs with probabilities of $2^{-19}$, $2^{-18}$, and $2^{-12}$ for all three variants, demonstrating a 99% success rate. Similarly, for Katan-32, a more effective related-key boomerang spanning 140 rounds with a probability of $2^{-15}$ is uncovered with 70% success rate. Further, in the single-key setting, a 84-round boomerang with probability $2^{-30}$ found with success rate of 60%. This research deepens the understanding of boomerang attacks, enhancing the toolkit for cryptanalysts to develop efficient and impactful attacks on NLFSR-based block ciphers

Improved Heuristics for Short Linear Programs

In this article, we propose new heuristics for minimizing the amount of XOR gates required to compute a system of linear equations in GF(2). We first revisit the well known Boyar-Peralta strategy and argue that a proper randomization process during the selection phases can lead to great improvements. We then propose new selection criteria and explain their rationale. Our new methods outperform state-of-the-art algorithms such as Paar or Boyar-Peralta (or open synthesis tools such as Yosys) when tested on random matrices with various densities. They can be applied on matrices of reasonable sizes (up to about 32 x 32). Notably, we provide a new implementation record for the matrix underlying the MixColumns function of the AES block cipher, requiring only 94 XORs

Cryptanalysis of Block Ciphers with New Design Strategies

Block ciphers are among the mostly widely used symmetric-key cryptographic primitives, which are fundamental building blocks in cryptographic/security systems. Most of the public-key primitives are based on hard mathematical problems such as the integer factorization in the RSA algorithm and discrete logarithm problem in the DiffieHellman. Therefore, their security are mathematically proven. In contrast, symmetric-key primitives are usually not constructed based on well-defined hard mathematical problems. Hence, in order to get some assurance in their claimed security properties, they must be studied against different types of cryptanalytic techniques. Our research is dedicated to the cryptanalysis of block ciphers. In particular, throughout this thesis, we investigate the security of some block ciphers constructed with new design strategies. These new strategies include (i) employing simple round function, and modest key schedule, (ii) using another input called tweak rather than the usual two inputs of the block ciphers, the plaintext and the key, to instantiate different permutations for the same key. This type of block ciphers is called a tweakable block cipher, (iii) employing linear and non-linear components that are energy efficient to provide low energy consumption block ciphers, (iv) employing optimal diffusion linear transformation layer while following the AES-based construction to provide faster diffusion rate, and (v) using rather weak but larger S-boxes in addition to simple linear transformation layers to provide provable security of ARX-based block ciphers against single characteristic differential and linear cryptanalysis. The results presented in this thesis can be summarized as follows: Initially, we analyze the security of two lightweight block ciphers, namely, Khudra and Piccolo against Meet-in-the-Middle (MitM) attack based on the Demirci and Selcuk approach exploiting the simple design of the key schedule and round function. Next, we investigate the security of two tweakable block ciphers, namely, Kiasu-BC and SKINNY. According to the designers, the best attack on Kiasu-BC covers 7 rounds. However, we exploited the tweak to present 8-round attack using MitM with efficient enumeration cryptanalysis. Then, we improve the previous results of the impossible differential cryptanalysis on SKINNY exploiting the tweakey schedule and linear transformation layer. Afterwards, we study the security of new low energy consumption block cipher, namely, Midori128 where we present the longest impossible differential distinguishers that cover complete 7 rounds. Then, we utilized 4 of these distinguishers to launch key recovery attack against 11 rounds of Midori128 to improve the previous results on this cipher using the impossible differential cryptanalysis. Then, using the truncated differential cryptanalysis, we are able to attack 13 rounds of Midori128 utilizing a 10-round differential distinguisher. We also analyze Kuznyechik, the standard Russian federation block cipher, against MitM with efficient enumeration cryptanalysis where we improve the previous results on Kuznyechik, using MitM attack with efficient enumeration, by presenting 6-round attack. Unlike the previous attack, our attack exploits the exact values of the coefficients of the MDS transformation that is used in the cipher. Finally, we present key recovery attacks using the multidimensional zero-correlation cryptanalysis against SPARX-128, which follows the long trail design strategy, to provide provable security of ARX-based block ciphers against single characteristic differential and linear cryptanalysis

Analysis of obligatory disclosure regarding individual’s privacy

Disclosure of personal information online has raised concerns about individuals’ privacy. In order to protect personal information users undertake measures, such as configuring privacy settings and referring to the privacy policies of the organisation’s website before engaging in a transaction. This demonstrates users’ concerns with the availability of their personal information online. Besides the individuals themselves, organisations are also exposing the personal information of their staff to the general public by publishing it on their official website. The practice of publishing employees’ information on such websites is nominally to offer better services to customers, and it is one of the steps taken to improve governmental transparency. However, there are only limited studies on individuals’ (i.e. employees’) privacy issues in the context of organisational disclosure, and their internal responses to the relevant factors. To date, far too little attention has been paid to the disclosure of personal information by organisational websites. This research addresses this phenomenon, where the issue of third-party disclosure by an entity that has a direct relationship with the individuals is investigated in the Malaysian context. For this purpose, this research introduces ‘obligatory disclosure’ as a conceptual framework for this study and adds to the knowledge of privacy-in-public in the context of public administration. The results of the study indicate that while obligatory disclosure was commonly believed to be a normal phenomenon, it creates a vulnerable environment for individuals. The study also found that employees’ concerns with privacy were influenced by the specific context. In addition, low levels of privacy concern and lack of privacy awareness regarding this phenomenon were identified. The study recommends that there is a need for a regulatory approach to protect employees’ information on organisation websites, and privacy should be incorporated as an important element of obligatory disclosure practice