Netinfo Security ›› 2024, Vol. 24 ›› Issue (12): 1845-1854.doi: 10.3969/j.issn.1671-1122.2024.12.004

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The Spectral Invariant Subspace of Word-Based Block Ciphers

CUI Ting, ZHOU Yidong(), CHEN Shiwei, ZHANG Yi   

  1. Department of Cryptogram Engineering, Information Engineering University of PLA, Zhengzhou 450001, China
  • Received:2024-10-12 Online:2024-12-10 Published:2025-01-10

Abstract:

This paper combined the idea of invariant subspace attacks with linear cryptanalysis, and proposed a spectral invariant subspace analysis method. This approach leveraged the property of spectral invariant subspaces to distinguish a block cipher by examining whether a pair of input/output linear masks resides within the same non-trivial subspace. Firstly, it demonstrated that if an S-box satisfied the spectral invariant subspace property, it was linearly equivalent to several smaller S-boxes operating in parallel. Secondly, an efficient algorithm for searching spectral invariant subspaces of S-boxes was presented, which proved effective for commonly used sizes of S-boxes. Furthermore, if the S-boxes employed in a word-based block cipher shared the same spectral invariant subspace, then it followed that the entire cipher possesses this characteristic as well. By utilizing this property, an infinite-round distinguisher with probability 1 for the target cipher was constructed. This paper offered new insights into the relationship between S-boxes and block cipher security and provided valuable guidance for designing new block ciphers. As application, an infinite-round distinguisher with probability 1 specifically for variant Midori128 was developed.

Key words: linear cryptanalysis, word-based block ciphers, spectral invariant subspace, Midori128

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