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On the Provable Security of the Iterated Even-Mansour Cipher against Related-Key and Chosen-Key Attacks

© INRIA Sophie Auvin - C comme Cryptographie

  • Date : 16/04/2015
  • Lieu : Amphi Rataud - ENS Paris
  • Intervenant(s) : Yannick Seurin (ANSSI)

Abstract: The iterated Even-Mansour cipher is a construction of a block cipher from r public permutations P_1,...,P_r which abstracts in a generic way the structure of key-alternating ciphers. The indistinguishability of this construction from a truly random permutation by an adversary with oracle access to the inner permutations P_1,...,P_r has been investigated in a series of recent papers. This construction has also been shown to be (fully) indifferentiable from an ideal cipher for a sufficient number of rounds (five or twelve depending on the assumptions on the key-schedule). In this paper, we extend this line of work by considering the resistance of the iterated Even-Mansour cipher to xor-induced related-key attacks (i.e., related-key attacks where the adversary is allowed to xor any constant of its choice to the secret key) and to chosen-key attacks. For xor-induced related-key attacks, we first provide a distinguishing attack for two rounds, assuming the key-schedule is linear. We then prove that for a linear key-schedule, three rounds yield a cipher which is secure against xor-induced related-key attacks up to O(2^{n/2}) queries of the adversary, whereas for a nonlinear key-schedule, one round is sufficient to obtain a similar security bound. We also show that the iterated Even-Mansour cipher with four rounds offers some form of provable resistance to chosen-key attacks, which is the minimal number of rounds to achieve this property. The main technical tool that we use to prove this result is sequential indifferentiability, a weakened variant of (full) indifferentiability introduced by Mandal et al. (TCC~2010).

Joint work with Benoît Cogliati.

Mots-clés : Séminaire Cascade Security ANSSI

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