## Cryptology ePrint Archive: Report 2014/151

Security Analysis of Key-Alternating Feistel Ciphers

Rodolphe Lampe and Yannick Seurin

Abstract: We study the security of \emph{key-alternating Feistel} ciphers, a class of key-alternating ciphers with a Feistel structure. Alternatively, this may be viewed as the study of Feistel ciphers where the pseudorandom round functions are of the form $F_i(x\oplus k_i)$, where $k_i$ is the (secret) round key and $F_i$ is a \emph{public} random function that the adversary is allowed to query in a black-box way. Interestingly, our results can be seen as a generalization of traditional results \emph{à la} Luby-Rackoff in the sense that we can derive results for this model by simply letting the number of queries of the adversary to the public random functions $F_i$ be zero in our general bounds. We make an extensive use of the coupling technique. In particular (and as a result of independent interest), we improve the analysis of the coupling probability for balanced Feistel schemes previously carried out by Hoang and Rogaway (CRYPTO 2010).

Category / Keywords: secret-key cryptography / block cipher, key-alternating cipher, Feistel cipher, coupling, provable security

Original Publication (with minor differences): IACR-FSE-2014

Date: received 28 Feb 2014, last revised 28 Feb 2014

Contact author: rodolphe lampe at gmail com, yannick seurin@m4x org

Available format(s): PDF | BibTeX Citation

Note: An abridged version appears in the proceedings of FSE 2014. This is the full version.

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