[Resource Topic] 2015/533: Related-Key Rectangle Attack on Round-reduced \textit{Khudra} Block Cipher

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Title:
Related-Key Rectangle Attack on Round-reduced \textit{Khudra} Block Cipher

Authors: Xiaoshuang Ma, Kexin Qiao

Abstract:

\textit{Khudra} is a block cipher proposed in the SPACE’2014 conference, whose main design goal is to achieve suitability for the increasingly popular Field Programmable Gate Array (FPGA) implementation. It is an 18-round lightweight cipher based on recursive Feistel structure, with a 64-bit block size and 80-bit key size. In this paper, we compute the minimum number of active F-functions in differential characteristics in the related-key setting, and give a more accurate measurement of the resistance of \textit{Khudra} against related-key differential cryptanalysis. We construct a related-key boomerang quartet with probability 2^{-48} for the 14-round \textit{Khudra}, which is better than the highest probability related-key boomerang quartet of the 14-round \textit{Khudra} of probability at most 2^{-72} claimed by the designers. Then we propose a related-key rectangle attack on the 16-round \textit{Khudra} without whitening key by constructing a related-key rectangle distinguisher for 12-round \textit{Khudra} with a probability of 2^{-23.82}. The attack has time complexity of 2^{78.68} memory accesses and data complexity of 2^{57.82} chosen plaintexts, and requires only four related keys. This is the best known attack on the round-reduced \textit{Khudra}.

ePrint: https://eprint.iacr.org/2015/533

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