Linearity within the SMS4 Block Cipher

Z'aba, Muhammad Reza, Simpson, Leonie, Dawson, Edward, & Wong, Kenneth Koon-Ho (2010) Linearity within the SMS4 Block Cipher. In Lecture Notes in Computer Science : Information Security and Cryptology (ICISC 2009), Springer-Verlag, Beijing, China, pp. 248-265.

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We present several new observations on the SMS4 block cipher, and discuss their cryptographic significance. The crucial observation is the existence of fixed points and also of simple linear relationships between the bits of the input and output words for each component of the round functions for some input words. This implies that the non-linear function T of SMS4 does not appear random and that the linear transformation provides poor diffusion. Furthermore, the branch number of the linear transformation in the key scheduling algorithm is shown to be less than optimal. The main security implication of these observations is that the round function is not always non-linear. Due to this linearity, it is possible to reduce the number of effective rounds of SMS4 by four. We also investigate the susceptibility of SMS4 to further cryptanalysis. Finally, we demonstrate a successful differential attack on a slightly modified variant of SMS4. These findings raise serious questions on the security provided by SMS4.

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ID Code: 39134
Item Type: Conference Paper
Refereed: Yes
Keywords: SMS4, block cipher, round function, fixed point, encryption, key scheduling algorithm, linearity, cryptanalysis
DOI: 10.1007/978-3-642-16342-5_18
ISBN: 978-3-642-16341-8
ISSN: 0302-9743
Subjects: Australian and New Zealand Standard Research Classification > INFORMATION AND COMPUTING SCIENCES (080000) > DATA FORMAT (080400) > Data Encryption (080402)
Divisions: Past > QUT Faculties & Divisions > Faculty of Science and Technology
Past > Institutes > Information Security Institute
Copyright Owner: Copyright 2010 Springer-Verlag.
Deposited On: 13 Dec 2010 02:53
Last Modified: 18 Jul 2014 05:00

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