Welcome to the resource topic for 2014/786
Title:
On the Indifferentiability of Key-Alternating Feistel Ciphers with No Key Derivation
Authors: Chun Guo, Dongdai Lin
Abstract:Feistel constructions have been shown to be indifferentiable from random permutations at STOC 2011. Whereas how to properly mix the keys into an un-keyed Feistel construction without appealing to domain separation technique to obtain a block cipher which is provably secure against known-key and chosen-key attacks (or to obtain an ideal cipher) remains an open problem. We study this, particularly the basic structure of NSA’s SIMON family of block ciphers. SIMON family takes a construction which has the subkey xored into a halve of the state at each round. More clearly, at the i-th round, the state is updated according to $$(x_i,x_{i-1})\mapsto(x_{i-1}\oplus F_i(x_i)\oplus k_i,x_i)$$ For such key-alternating Feistel ciphers, we show that 21 rounds are sufficient to achieve indifferentiability from ideal ciphers with 2n-bit blocks and n-bit keys, assuming the n-to-n-bit round functions F_1,\ldots,F_{21} to be random and public and an identical user-provided n-bit key to be applied at each round. This gives an answer to the question mentioned before, which is the first to our knowledge.
ePrint: https://eprint.iacr.org/2014/786
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