Cryptanalysis of 2r− schemes

  • Authors:
  • Jean-Charles Faugère;Ludovic Perret

  • Affiliations:
  • LIP6, Paris, France;Crypto Group, Microelectronic Laboratory, UCL, Louvain-la-Neuve, Belgium

  • Venue:
  • CRYPTO'06 Proceedings of the 26th annual international conference on Advances in Cryptology
  • Year:
  • 2006

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Abstract

In this paper, we study the security of 2R− schemes [17,18], which are the “minus variant” of two-round schemes. This variant consists in removing some of the n polynomials of the public key, and permits to thwart an attack described at Crypto'99 [25] against two-round schemes. Usually, the “minus variant” leads to a real strengthening of the considered schemes. We show here that this is actually not true for 2R− schemes. We indeed propose an efficient algorithm for decomposing 2R− schemes. For instance, we can remove up to $\left \lfloor\frac{n}{2} \right \rfloor$ equations and still be able to recover a decomposition in O(n12). We provide experimental results illustrating the efficiency of our approach. In practice, we have been able to decompose 2R− schemes in less than a handful of hours for most of the challenges proposed by the designers [18]. We believe that this result makes the principle of two-round schemes, including 2R− schemes, useless.