[Resource Topic] 2017/042: Indifferentiability of Iterated Even-Mansour Ciphers with Non-Idealized Key-Schedules: Five Rounds are Necessary and Sufficient

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Title:
Indifferentiability of Iterated Even-Mansour Ciphers with Non-Idealized Key-Schedules: Five Rounds are Necessary and Sufficient

Authors: Yuanxi Dai, Yannick Seurin, John Steinberger, Aishwarya Thiruvengadam

Abstract:

We prove that the 5-round iterated Even-Mansour (IEM) construction (which captures the high-level structure of the class of key-alternating ciphers) with a non-idealized key-schedule (such as the trivial key-schedule, where all round keys are equal) is indifferentiable from an ideal cipher. In a separate result, we also prove that five rounds are necessary by describing an attack against the corresponding 4-round construction. This closes the gap regarding the exact number of rounds for which the IEM construction with a non-idealized key-schedule is indifferentiable from an ideal cipher, which was previously only known to lie between four and twelve.

ePrint: https://eprint.iacr.org/2017/042

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