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Paper: Security of Even-Mansour Ciphers under Key-Dependent Messages

Authors:
Pooya Farshim , ENS, CNRS & INRIA, PSL Research University, Paris
Louiza Khati , ENS, CNRS & INRIA, PSL Research University, Paris; Oppida
Damien Vergnaud , ENS, CNRS & INRIA, PSL Research University, Paris
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DOI: 10.13154/tosc.v2017.i2.84-104
URL: https://tosc.iacr.org/index.php/ToSC/article/view/639
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Abstract: The iterated Even–Mansour (EM) ciphers form the basis of many blockcipher designs. Several results have established their security in the CPA/CCA models, under related-key attacks, and in the indifferentiability framework. In this work, we study the Even–Mansour ciphers under key-dependent message (KDM) attacks. KDM security is particularly relevant for blockciphers since non-expanding mechanisms are convenient in setting such as full disk encryption (where various forms of key-dependency might exist). We formalize the folklore result that the ideal cipher is KDM secure. We then show that EM ciphers meet varying levels of KDM security depending on the number of rounds and permutations used. One-round EM achieves some form of KDM security, but this excludes security against offsets of keys. With two rounds we obtain KDM security against offsets, and using different round permutations we achieve KDM security against all permutation-independent claw-free functions. As a contribution of independent interest, we present a modular framework that can facilitate the security treatment of symmetric constructions in models that allow for correlated inputs.
BibTeX
@article{tosc-2017-28492,
  title={Security of Even-Mansour Ciphers under Key-Dependent Messages},
  journal={IACR Trans. Symmetric Cryptol.},
  publisher={Ruhr-Universität Bochum},
  volume={2017, Issue 2},
  pages={84-104},
  url={https://tosc.iacr.org/index.php/ToSC/article/view/639},
  doi={10.13154/tosc.v2017.i2.84-104},
  author={Pooya Farshim and Louiza Khati and Damien Vergnaud},
  year=2017
}