Le schéma d'Even-Mansour paramétrable : preuves de sécurité à l'aide de la technique des coefficients H

Abstract : Tweakable block ciphers are a generalization of classical block ciphers which, in addition to a key and a plaintext or a ciphertext, take an additionnal parameter called a tweak. The goal of this new parameter is to bring variability to the block cipher without needing to change the key or to keep the tweak secret. The tweak should also be adversariallycontrollable without sacrificing security. In this thesis we study a particular class of block ciphers, namely key-alternating ciphers. More precisely, we study the security of the Even-Mansour scheme, which is an abstraction of these ciphers in the random permutation model, and seek to bring tweakability to this scheme while keeping strong security guarantees. To this end, we introduce a new generic construction, dubbed TEM, which replaces the round keys from the Even-Mansour construction by a value depending on both the key and the tweak, and study its security in two cases: when the tweak and key mixing is linear or highly non-linear. Our security proofs rely on the H-coefficients technique, a technique introduced by Jacques Patarin in his PhD thesis which transforms cryptographic problems into combinatorial problems in finite groups.
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Benoît-Michel Cogliati. Le schéma d'Even-Mansour paramétrable : preuves de sécurité à l'aide de la technique des coefficients H. Cryptographie et sécurité [cs.CR]. Université Paris-Saclay, 2016. Français. ⟨NNT : 2016SACLV064⟩. ⟨tel-01482669⟩

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