Abstract
This article provides a rigorous mathematical treatment of generalized (GNI) and closed-loop invariants (CLI), which extend the standard notion of nonlinear invariants used in the cryptanalysis of block ciphers. We first introduce the concept of an active cycle set, which is useful for defining standard invariants of concatenated S-boxes. We also present an algorithm for finding the cycle decomposition of a substitution layer provided the knowledge of the cycle decomposition of the constituent S-boxes. Employing the cycle decomposition of a bijective S-box, we precisely characterize the cardinality of its generalized and CLIs. We demonstrate that quadratic invariants (especially useful for mounting practical attacks in cases when the linear layer is an orthogonal matrix) might not exist for many S-boxes used in practice, whereas there are many quadratic invariants of generalized type. For generalized invariants, we draw an important conclusion that these invariants are not affine invariant, and therefore for two affine permutations A1,A2 over \({\mathbb {F}_{2}^{m}}\) the set of generalized invariants of S is not necessarily the same as for A1 ∘ S ∘ A2. In the context of closed-loop invariants, it is shown that the inverse mapping S(x) = x− 1 over \(\mathbb {F}_{2^{4}}\) admits quadratic CLIs that additionally possess linear structures, whereas for m > 4 there are no quadratic CLIs of S(x) = x− 1 over \(\mathbb {F}_{2^{m}}\). Moreover, we identify the existence of both standard and closed-loop invariants for the so-called MiMC (Minimal Multiplicative Complexity) [1] design, which uses an S-box layer based on the permutation S(x) = x3 over \(\mathbb {F}_{2^{m}}\) (m odd). We present a method to specify these invariants even when m is prime, for which the authors [1] claimed resistance against a type of invariant attacks—subfield attacks.


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Notes
For 0 ≤ i ≤ 2m − 2, the q-cyclotomic class modulo 2m − 1 of i is defined as the set \(\mathcal {C}(q,i)=\{ q^{j}i : 0\leq j\leq t_{i}-1 \}\) where ti is the least positive integer such that \(q^{t_{i}}i\equiv i \text { mod } (2^{m}-1)\).
φ denotes Euler’s totient function.
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Acknowledgements
The authors would like to thank the National Natural Science Foundation of China, Guangxi Science and Technology Foundation, Guangxi Natural Science Foundation and the Slovenian Research Agency for their financial support on this work.
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The second author is supported in part by the National Natural Science Foundation of China (61872103), in part by Guangxi Science and Technology Foundation (Guike AB18281019), in part by Guangxi Natural Science Foundation (2019GXNSFGA245004). The third author is partly supported by the Slovenian Research Agency (research program P1-0404 and research projects J1-1694, J1-9108 and N1-0159).
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The concept of active cycle sets, the algorithm in Section 2.2 and the idea for Section 4.3 were provided by the second author. Section 4.2 is mainly due to the third author as well as the idea of studying properties of generalized invariants using the cycle structure of permutations. Main results and examples in Sections 3, 4.1 and 4.3 and Theorem 6 are due to the first author. All authors contributed to the writing of the main manuscript text and its review.
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Rodríguez, R., Wei, Y. & Pasalic, E. A theoretical analysis of generalized invariants of bijective S-boxes. Cryptogr. Commun. 15, 487–512 (2023). https://doi.org/10.1007/s12095-022-00615-1
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DOI: https://doi.org/10.1007/s12095-022-00615-1
Keywords
- Block ciphers
- Generalized nonlinear invariants
- Permutation cycles
- Closed-loop invariants
- Linear structures
- Distinguishing attacks
- SP networks