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International Association for Cryptologic Research (IACR)
Abstract
We show generic attacks on unbalanced Feistel ciphers based on the
meet-in-the-middle technique. We analyze two general classes of unbalanced Feistel
structures, namely contracting Feistels and expanding Feistels. In both of the cases,
we consider the practical scenario where the round functions are keyless and known
to the adversary. In the case of contracting Feistels with 4 branches, we show attacks
on 16 rounds when the key length k (in bits) is as large as the block length n (in bits),
and up to 24 rounds when k = 2n. In the case of expanding Feistels, we consider two
scenarios: one, where different nonlinear functions without particular structures are
used in the round function, and a more practical one, where a single nonlinear is used
but different linear functions are introduced in the state update. In the former case,
we propose generic attacks on 13 rounds when k = n, and up to 21 rounds when
k = 2n. In the latter case, 16 rounds can be attacked for k = n, and 24 rounds for
k = 2n
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