## Cryptology ePrint Archive: Report 2007/449

Generic Attacks on Unbalanced Feistel Schemes with Expanding Functions

Jacques Patarin and Valérie Nachef and Côme Berbain

Abstract: \begin​{abstract} Unbalanced Feistel schemes with expanding functions are used to construct pseudo-random permutations from $kn$ bits to $kn$ bits by using random functions from $n$ bits to $(k-1)n$ bits. At each round, all the bits except $n$ bits are changed by using a function that depends only on these $n$ bits. C.S.Jutla \cite{Jut} investigated such schemes, which he denotes by $F^d_k$, where $d$ is the number of rounds. In this paper, we describe novel Known Plaintext Attacks (KPA) and Non Adaptive Chosen Plaintext Attacks (CPA-1) against these schemes. With these attacks we will often be able to improve the result of C.S.Jutla. We also give precise formulas for the complexity of our attacks in $d$, $k$ and $n$. \end{abstract}

Category / Keywords: block ciphers

Publication Info: extended version of a paper published at Asiacrypt 2007

Date: received 1 Dec 2007, last revised 5 Apr 2009

Contact author: valerie nachef at u-cergy fr

Available format(s): PDF | BibTeX Citation

Note: There are new attacks in Section 6.

Short URL: ia.cr/2007/449

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