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Linearity within the SMS4 Block Cipher

  • Muhammad Reza Z’aba
  • Leonie Simpson
  • Ed Dawson
  • Kenneth Wong
Part of the Lecture Notes in Computer Science book series (LNCS, volume 6151)

Abstract

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.

Keywords

SMS4 block cipher round function fixed point encryption key scheduling algorithm linearity cryptanalysis 

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Copyright information

© Springer-Verlag Berlin Heidelberg 2010

Authors and Affiliations

  • Muhammad Reza Z’aba
    • 1
  • Leonie Simpson
    • 1
  • Ed Dawson
    • 1
  • Kenneth Wong
    • 1
  1. 1.Information Security InstituteQueensland University of TechnologyBrisbaneAustralia

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