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2016 | OriginalPaper | Chapter

Cryptanalysis of Feistel Networks with Secret Round Functions

Authors : Alex Biryukov, Gaëtan Leurent, Léo Perrin

Published in: Selected Areas in Cryptography – SAC 2015

Publisher: Springer International Publishing

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Abstract

Generic distinguishers against Feistel Network with up to 5 rounds exist in the regular setting and up to 6 rounds in a multi-key setting. We present new cryptanalyses against Feistel Networks with 5, 6 and 7 rounds which are not simply distinguishers but actually recover completely the unknown Feistel functions.
When an exclusive-or is used to combine the output of the round function with the other branch, we use the so-called yoyo game which we improved using a heuristic based on particular cycle structures. The complexity of a complete recovery is equivalent to \(\text {O}(2^{2n})\) encryptions where n is the branch size. This attack can be used against 6- and 7-round Feistel Networks in time respectively \(\text {O}(2^{n2^{n-1}+2n})\) and \(\text {O}(2^{n2^{n}+2n})\). However when modular addition is used, this attack does not work. In this case, we use an optimized guess-and-determine strategy to attack 5 rounds with complexity \(\text {O}(2^{n2^{3n/4}})\).
Our results are, to the best of our knowledge, the first recovery attacks against generic 5-, 6- and 7-round Feistel Networks.

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Footnotes
1
However, such a yoyo game cannot be played against a \(\boxplus \)-Feistel, as explained in Sect. 3.4. It only works in characteristic 2.
 
2
It can also recover \(S_{4}\) in an identical fashion but this part is omitted for the sake of clarity.
 
3
We experimentally found that a lower bound of 0.1 is more than sufficient even for n as small as 4.
 
4
CPU: Intel core i7-3770 (3.40 GHz); 8 Gb of RAM. The program was compiled with g++ and optimization flag -O3.
 
5
A random permutation of a space of size N is expected to have about \(\log _{e}(N)\) cycles.
 
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Metadata
Title
Cryptanalysis of Feistel Networks with Secret Round Functions
Authors
Alex Biryukov
Gaëtan Leurent
Léo Perrin
Copyright Year
2016
Publisher
Springer International Publishing
DOI
https://doi.org/10.1007/978-3-319-31301-6_6

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