Locally-APN Binomials with Low Boomerang Uniformity in Odd Characteristic

TL;DR

This paper extends the class of functions over finite fields that are locally-APN with low boomerang uniformity, providing new conditions and analyzing their differential and boomerang spectra.

Contribution

It generalizes previous results by establishing new conditions for locally-APN functions with boomerang uniformity at most 2 and studies their spectra.

Findings

Functions $F_r$ are locally-APN with boomerang uniformity ≤ 2 under new conditions.

Differential spectra of $F_3$ and $F_{(2q-1)/3}$ are characterized.

Boomerang spectrum of $F_2$ when $p=3$ is analyzed.

Abstract

Recently, several studies have shown that when $q\equiv3\pmod{4}$, for certain choices of $r$, the function $F_r(x)=x^r+x^{r+\frac{q-1}{2}}$ defined over $\Fq$ is locally-APN and has boomerang uniformity at most~$2$. In this paper, we extend these results by showing that if there is at most one $x\in \Fq$ with $\chi(x)=\chi(x+1)=1$ satisfying $(x+1)^r - x^r = b$ for all $b\in \Fqmul$ and $\gcd(r,q-1)\mid 2$, then $F_r$ is locally-APN with boomerang uniformity at most $2$. Moreover, we study the differential spectra of $F_3$ and $F_{\frac{2q-1}{3}}$, and the boomerang spectrum of $F_2$ when $p=3$.