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**2017/449**

**Title:**

Differentially 4-Uniform Permutations with the Best Known Nonlinearity from Butterflies

**Authors:**
Shihui Fu, Xiutao Feng, Baofeng Wu

**Abstract:**

Many block ciphers use permutations defined over the finite field \mathbb{F}_{2^{2k}} with low differential uniformity, high nonlinearity, and high algebraic degree to provide confusion. Due to the lack of knowledge about the existence of almost perfect nonlinear (APN) permutations over \mathbb{F}_{2^{2k}}, which have lowest possible differential uniformity, when k>3, constructions of differentially 4-uniform permutations are usually considered. However, it is also very difficult to construct such permutations together with high nonlinearity; there are very few known families of such functions, which can have the best known nonlinearity and a high algebraic degree. At Cryptoâ€™16, Perrin et al. introduced a structure named butterfly, which leads to permutations over \mathbb{F}_{2^{2k}} with differential uniformity at most 4 and very high algebraic degree when k is odd. It is posed as an open problem in Perrin et al.'s paper and solved by Canteaut et al. that the nonlinearity is equal to 2^{2k-1}-2^k. In this paper, we extend Perrin et al.'s work and study the functions constructed from butterflies with exponent e=2^i+1. It turns out that these functions over \mathbb{F}_{2^{2k}} with odd k have differential uniformity at most 4 and algebraic degree k+1. Moreover, we prove that for any integer i and odd k such that \gcd(i,k)=1, the nonlinearity equality holds, which also gives another solution to the open problem proposed by Perrin et al. This greatly expands the list of differentially 4-uniform permutations with good nonlinearity and hence provides more candidates for the design of block ciphers.

**ePrint:**
https://eprint.iacr.org/2017/449

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