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Luby-Rackoff Ciphers from Weak Round Functions?

  • Ueli Maurer
  • Yvonne Anne Oswald
  • Krzysztof Pietrzak
  • Johan Sjödin
Part of the Lecture Notes in Computer Science book series (LNCS, volume 4004)

Abstract

The Feistel-network is a popular structure underlying many block-ciphers where the cipher is constructed from many simpler rounds, each defined by some function which is derived from the secret key.

Luby and Rackoff showed that the three-round Feistel-network – each round instantiated with a pseudorandom function secure against adaptive chosen plaintext attacks (CPA) – is a CPA secure pseudorandom permutation, thus giving some confidence in the soundness of using a Feistel-network to design block-ciphers.

But the round functions used in actual block-ciphers are – for efficiency reasons – far from being pseudorandom. We investigate the security of the Feistel-network against CPA distinguishers when the only security guarantee we have for the round functions is that they are secure against non-adaptive chosen plaintext attacks (nCPA). We show that in the information-theoretic setting, four rounds with nCPA secure round functions are sufficient (and necessary) to get a CPA secure permutation. Unfortunately, this result does not translate into the more interesting pseudorandom setting. In fact, under the so-called Inverse Decisional Diffie-Hellman assumption the Feistel-network with four rounds, each instantiated with a nCPA secure pseudorandom function, is in general not a CPA secure pseudorandom permutation.

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Copyright information

© Springer-Verlag Berlin Heidelberg 2006

Authors and Affiliations

  • Ueli Maurer
    • 1
  • Yvonne Anne Oswald
    • 1
  • Krzysztof Pietrzak
    • 2
  • Johan Sjödin
    • 1
  1. 1.Department of Computer ScienceETH ZurichZurichSwitzerland
  2. 2.Département d’informatique, Ecole Normale SupérieureParisFrance

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