Benes and butterfly schemes revisited

  • Authors:
  • Jacques Patarin;Audrey Montreuil

  • Affiliations:
  • Université de Versailles, Versailles, France;Université de Versailles, Versailles, France

  • Venue:
  • ICISC'05 Proceedings of the 8th international conference on Information Security and Cryptology
  • Year:
  • 2005

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Abstract

In [1], W. Aiello and R. Venkatesan have shown how to construct pseudo-random functions of 2n bits →2n bits from pseudo-random functions of n bits →n bits. They claimed that their construction, called “Benes”, reaches the optimal bound (m≪2n) of security against adversaries with unlimited computing power but limited by m queries in an Adaptive Chosen Plaintext Attack (CPA-2). However a complete proof of this result is not given in [1] since one of the assertions of [1] is wrong. Due to this, the proof given in [1] is valid for most attacks, but not for all the possible Chosen Plaintext Attacks. In this paper we will in a way fix this problem since for all ε0, we will prove CPA-2 security when m≪2n(1−ε). However we will also see that the probability to distinguish Benes functions from random functions is sometime larger than the term in $\frac{m^2}{2^{2n}}$ given in [1]. One of the key idea in our proof will be to notice that, when m≫2$^{\rm 2{\it n}/3}$ and m≪2n, for large number of variables linked with some critical equalities, the average number of solutions may be large (i.e. ≫1) while, at the same time, the probability to have at least one such critical equalities is negligible (i.e. ≪1). (An extended version of this paper is available from the authors)