# Towards a better understanding of one-wayness: Facing linear permutations

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## Abstract

The one-wayness of linear permutations, i.e., invertible linear Boolean functions F: {0,1}n → {0, 1}n, is investigated. For linear permutations with a triangular matrix description (*tlinear* permutations), we prove that one-wayness, C(F−1)/C*(F)*, is non-trivially upperbounded by *16√n*, where C(.) denotes unrestricted circuit complexity. We also prove that this upper bound strengthens as the complexity of the inverse function increases, limiting the one-wayness of t-linear permutations with C(F−1) *=* n2/(*c* log2(*n*)) to a constant, i.e., a value that is independent of *n.* Direct implications for linear and also non-linear permutations are discussed. Moreover, and for the first time ever, a description is given about where, in the case of linear permutations, practical one-wayness would have to come from, if it exists.