[Resource Topic] 2009/360: Cryptanalysis of a Generalized Unbalanced Feistel Network Structure

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Title:
Cryptanalysis of a Generalized Unbalanced Feistel Network Structure

Authors: Ruilin Li, Bing Sun, Chao Li, Longjiang Qu

Abstract:

This paper reevaluates the security of GF-NLFSR, a new kind of generalized unbalanced Feistel network structure that was proposed at ACISP 2009. We show that GF-NLFSR itself reveals a very slow diffusion rate, which could lead to several distinguishing attacks. For GF-NLFSR containing n sub-blocks, we find an n^2-round integral distinguisher by algebraic methods and further use this integral to construct an (n^2+n-2)-round impossible differential distinguisher. Compared with the original (3n-1)-round integral and (2n-1)-round impossible differential, ours are significantly better. Another contribution of this paper is to introduce a kind of non-surjective attack by analyzing a variant structure of GF-NLFSR, whose provable security against differential and linear cryptanalysis can also be provided. The advantage of the proposed non-surjective attack is that traditional non-surjective attack is only applicable to Feistel ciphers with non-surjective (non-uniform) round functions, while ours could be applied to block ciphers with bijective ones. Moreover, its data complexity is \mathcal{O}(l) with l the block length.

ePrint: https://eprint.iacr.org/2009/360

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