Analyzing Linear Layers in Related-Differential Cryptanalysis
This work provides theoretical foundations for understanding related-differential vulnerabilities in linear layers, which is important for cipher designers seeking to avoid such weaknesses.
The authors systematically investigate conditions under which linear layers in AES-like ciphers avoid or exhibit related-differential structure. They prove that non-MDS matrices always admit such differentials, odd-order symmetric MDS matrices do as well, and circulant matrices of order n with n not congruent to ±2 mod 12 also unavoidably have them, providing explicit criteria for 3×3 MDS matrices.
In AES-like ciphers, diffusion layers are commonly instantiated using MDS matrices, since their optimal branch number yields strong diffusion guarantees and underpins classical resistance arguments against differential and linear cryptanalysis. However, Daemen and Rijmen (2009) showed that linear layers may still exhibit related-differential structure beyond what the MDS criterion captures, and Bardeh and Rijmen (2022) demonstrated that this phenomenon can be exploited in attacks on reduced-round AES. In this work, we systematically investigate the conditions under which linear layers avoid or exhibit these differentials, identifying matrix classes for which such structure is unavoidable. We first prove that every non-MDS matrix admits a nontrivial pair of related differentials, showing that the MDS property is necessary for avoiding them. We then establish that every odd-order symmetric MDS matrix admits related differentials, which rules out broad families of Cauchy-based constructions. We also substantially strengthen the circulant case by proving that related differentials are unavoidable for every circulant matrix of order $n$ with $n \not\equiv \pm 2 \pmod{12}$. Finally, we revisit the characterization of $3 \times 3$ MDS matrices over $\mathbb{F}_{2^m}$ for the absence of related differentials, and derive an explicit necessary and sufficient criterion in terms of $15$ polynomial constraints.