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This paper investigates related-differential structures in linear layers of AES-like ciphers, focusing on conditions under which these structures are unavoidable. It proves that non-MDS matrices always admit nontrivial related differentials, and that odd-order symmetric MDS matrices also exhibit them, thus limiting Cauchy-based constructions. The paper also provides a strengthened characterization of related differentials in circulant matrices and derives a necessary and sufficient criterion for $3 \times 3$ MDS matrices over $\mathbb{F}_{2^m}$.
Even MDS matrices, the gold standard for diffusion in cryptography, can harbor exploitable related-differential structures, undermining classical security arguments.
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.
