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Published in:
2015 | OriginalPaper | Chapter
Structural Evaluation for Generalized Feistel Structures and Applications to LBlock and TWINE
Authors : Huiling Zhang, Wenling Wu
Published in: Progress in Cryptology -- INDOCRYPT 2015
Publisher: Springer International Publishing
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Abstract
The generalized Feistel structure (GFS) is the variant of Feistel structure with \(m>2\) branches. While the GFS is widely used, the security is not well studied. In this paper, we propose a generic algorithm for searching integral distinguishers. By applying the algorithm, we prove that the low bound for the length of integral distinguishers is \(m^2+m-1\) and \(2m+1\) for Type-1 GFS and Type-2 GFS, respectively. Meanwhile, we evaluate the security of the improved Type-1 and Type-2 GFSs when the size of each branch and the algebraic degree of F-functions are specified. Our results show that the distinguishers are affected by the parameters to various levels, which will provide valuable reference for designing GFS ciphers. Although our search algorithm is generic, it can improve integral distinguishers for specific ciphers. For instance, it constructs several 16-round integral distinguishers for LBlock and TWINE, which directly extends the numbers of attacked rounds.