Paper 2020/705

On the minimal value set size of APN functions

Ingo Czerwinski

Abstract

We give a lower bound for the size of the value set of almost perfect nonlinear (APN) functions \(F\colon \mathbb{F}_2^n \to \mathbb{F}_2^n\) in explicit form and proof it with methods of linear programming. It coincides with the bound given in [CHP17]. For \(n\) even it is \(\frac{ 2^n + 2 }{3}\) and sharp as the simple example \(F(x) = x^3\) shows. The sharp lower bound for \(n\) odd has to lie between \(\frac{ 2^n + 1 }{3}\) and \(2^{n-1}\). Sharp bounds for the cases \(n = 3\) and \(n = 5\) are explicitly given.