Paper 2016/539 Original: eprint.iacr.org/2016/539 ↗

Cryptanalysis of a Theorem: Decomposing the Only Known Solution to the Big APN Problem (Full Version)

Abstract

The existence of Almost Perfect Non-linear (APN) permutations operating on an even number of bits has been a long standing open question until Dillon et al., who work for the NSA, provided an example on 6 bits in 2009. In this paper, we apply methods intended to reverse-engineer S-Boxes with unknown structure to this permutation and find a simple decomposition relying on the cube function over $GF(2^3)$. More precisely, we show that it is a particular case of a permutation structure we introduce, the butterfly. Such butterflies are $2n$-bit mappings with two CCZ-equivalent representations: one is a quadratic non-bijective function and one is a degree $n+1$ permutation. We show that these structures always have differential uniformity at most 4 when $n$ is odd. A particular case of this structure is actually a 3-round Feistel Network with similar differential and linear properties. These functions also share an excellent non-linearity for $n=3,5,7$. Furthermore, we deduce a bitsliced implementation and significantly reduce the hardware cost of a 6-bit APN permutation using this decomposition, thus simplifying the use of such a permutation as building block for a cryptographic primitive.