Abstract
Inspired by the works of Nyberg and Knudsen, the wide trail strategy suggests to ensure that the number of active S-boxes in a differential characteristic or a linear approximation is sufficiently high, thus, offering security against differential and linear attacks. Many cipher designers are relying on this strategy, and most new designs include analysis based on counting the number of active S-boxes.
Unfortunately, this analysis is not always accurate and needs to be performed in a very delicate manner. To counter the common approach, we give an example of a 4-round Feistel construction with a very large number of active S-boxes that is expected to resist differential and linear cryptanalysis. However, we show that S-box counting arguments are insufficient in cases where one can find many differential characteristics with the same input and output difference. Namely, we show for a “provably” secure 128-bit block, 4-round Feistel with at least 36 active AES S-boxes, that one can construct differential characteristics with probability \(2^{-118}\) much higher than the bound of \(2^{-216}\). Even if we compare this 4-round Feistel construction to a random permutation we obtain a 10x factor in the probability of the characteristic.
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Notes
- 1.
There are many other works that combine multiple linear approximations, starting as early as [20].
- 2.
We alert the reader that here \(\delta \) is treated as a probability, rather than the number of pairs that satisfy the differential transition, as common in difference distribution tables.
- 3.
We note that, although the wide trail papers uses the term trail, we use the original term: characteristic.
- 4.
Using standard randomness assumptions about the distribution of the \(\beta _{i,j}\)’s values, the probability of a given \(\alpha _i\) to have such a \(\beta _{i,j}\) is about \(2^{-61}\). As there are \(2^{64}-1\) possible \(\alpha \)’s the probability that all of them have no adequate \(\beta \) is \((1-2^{-61})^{2^{64}} \approx (1/e)^8 = 0.03\%\). Moreover, when this is the case, one has almost for sure two values \(\alpha ,\beta \) for which \(DDT^k[\alpha ,\beta ]=32\) and \(DDT^{k'}[\beta ,\alpha ]=30\).
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Acknowledgements
The authors thank the anonymous reviewers for their comments and insights. We also want to thank Gregor Leander for his comments on this work during the Dagstuhl Seminar on Symmetric Cryptography which significantly improved this paper. This research was supported in part by the Israel Ministry of Science and Technology, the Center for Cyber, Law, and Policy in conjunction with the Israel National Cyber Bureau in the Prime Minister’s Office, by the Israeli Science Foundation through grants No. 880/18 and 3380/19, and by the Department of Science and Technology, Govt of India, grant 73/2018, and the Idit Doctoral Fellowship Program at the University of Haifa.
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Dunkelman, O., Kumar, A., Lambooij, E., Sanadhya, S.K. (2020). Counting Active S-Boxes is not Enough. In: Bhargavan, K., Oswald, E., Prabhakaran, M. (eds) Progress in Cryptology – INDOCRYPT 2020. INDOCRYPT 2020. Lecture Notes in Computer Science(), vol 12578. Springer, Cham. https://doi.org/10.1007/978-3-030-65277-7_15
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