ElimLin Algorithm Revisited

  • Nicolas T. Courtois
  • Pouyan Sepehrdad
  • Petr Sušil
  • Serge Vaudenay
Part of the Lecture Notes in Computer Science book series (LNCS, volume 7549)

Abstract

ElimLin is a simple algorithm for solving polynomial systems of multivariate equations over small finite fields. It was initially proposed as a single tool by Courtois to attack DES. It can reveal some hidden linear equations existing in the ideal generated by the system. We report a number of key theorems on ElimLin. Our main result is to characterize ElimLin in terms of a sequence of intersections of vector spaces. It implies that the linear space generated by ElimLin is invariant with respect to any variable ordering during elimination and substitution. This can be seen as surprising given the fact that it eliminates variables. On the contrary, monomial ordering is a crucial factor in Gröbner basis algorithms such as F4. Moreover, we prove that the result of ElimLin is invariant with respect to any affine bijective variable change. Analyzing an overdefined dense system of equations, we argue that to obtain more linear equations in the succeeding iteration in ElimLin some restrictions should be satisfied. Finally, we compare the security of LBlock and MIBS block ciphers with respect to algebraic attacks and propose several attacks on Courtois Toy Cipher version 2 (CTC2) with distinct parameters using ElimLin.

Keywords

block ciphers algebraic cryptanalysis systems of sparse polynomial equations of low degree 

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Copyright information

© Springer-Verlag Berlin Heidelberg 2012

Authors and Affiliations

  • Nicolas T. Courtois
    • 1
  • Pouyan Sepehrdad
    • 2
  • Petr Sušil
    • 2
  • Serge Vaudenay
    • 2
  1. 1.University College LondonUK
  2. 2.EPFLLausanneSwitzerland

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