## Cryptology ePrint Archive: Report 2016/625

On the Design Rationale of SIMON Block Cipher: Integral Attacks and Impossible Differential Attacks against SIMON Variants

Kota Kondo and Yu Sasaki and Tetsu Iwata

Abstract: SIMON is a lightweight block cipher designed by NSA in 2013. NSA presented the specification and the implementation efficiency, but they did not provide detailed security analysis nor the design rationale. The original SIMON has rotation constants of $(1,8,2)$, and K\"{o}lbl {\it et al}.~regarded the constants as a parameter $(a,b,c)$, and analyzed the security of SIMON block cipher variants against differential and linear attacks for all the choices of $(a,b,c)$. This paper complements the result of K\"{o}lbl {\it et al}.~by considering integral and impossible differential attacks. First, we search the number of rounds of integral distinguishers by using a supercomputer. Our search algorithm follows the previous approach by Wang {\it et al}., however, we introduce a new choice of the set of plaintexts satisfying the integral property. We show that the new choice indeed extends the number of rounds for several parameters. We also search the number of rounds of impossible differential characteristics based on the miss-in-the-middle approach. Finally, we make a comparison of all parameters from our results and the observations by K\"{o}lbl {\it et al}. Interesting observations are obtained, for instance we find that the optimal parameters with respect to the resistance against differential attacks are not stronger than the original parameter with respect to integral and impossible differential attacks. We also obtain a parameter that is better than the original parameter with respect to security against these four attacks.

Category / Keywords: secret-key cryptography / SIMON, Lightweight block cipher, Integral attack, Impossible differential attack, Design rationale, Rotation constant

Original Publication (with minor differences): ACNS 2016
DOI:
10.1007/978-3-319-39555-5_28