Authors: Pooya Farshim, Louiza Khati, Damien Vergnaud
Abstract: The iterated Even-Mansour (EM) ciphers form the basis of many block cipher 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 block ciphers 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 such as RKA or KDM that allow for correlated inputs.
Authors: Louiza Khati, Nicky Mouha, Damien Vergnaud
Abstract: We revisit the problem of Full Disk Encryption (FDE), which refers to the encryption of each sector of a disk volume. In the context of FDE, it is assumed that there is no space to store additional data, such as an IV (Initialization Vector) or a MAC (Message Authentication Code) value. We formally define the security notions in this model against chosen-plaintext and chosen-ciphertext attacks. Then, we classify various FDE modes of operation according to their security in this setting, in the presence of various restrictions on the queries of the adversary. We will find that our approach leads to new insights for both theory and practice. Moreover, we introduce the notion of a diversifier, which does not require additional storage, but allows the plaintext of a particular sector to be encrypted to different ciphertexts. We show how a 2-bit diversifier can be implemented in the EagleTree simulator for solid state drives (SSDs), while decreasing the total number of Input/Output Operations Per Second (IOPS) by only 4%.