A note on the differential spectrum of a class of locally APN functions

TL;DR

This paper investigates the differential spectrum of specific cryptographic functions over finite fields, focusing on cases where u equals ±1, and relates their properties to quadratic character sums.

Contribution

It provides new insights into the differential spectrum of functions f_u for u=±1, extending previous work on their differential uniformity.

Findings

Differential spectrum of f_{±1} expressed via quadratic character sums.

Properties of the differential spectrum of cryptographic functions analyzed.

Differential uniformity bounds for these functions are refined.

Abstract

Let $\gf_{p^n}$ denote the finite field containing $p^n$ elements, where $n$ is a positive integer and $p$ is a prime. The function $f_u(x)=x^{\frac{p^n+3}{2}}+ux^2$ over $\gf_{p^n}[x]$ with $u\in\gf_{p^n}\setminus\{0,\pm1\}$ was recently studied by Budaghyan and Pal in \cite{Budaghyan2024ArithmetizationorientedAP}, whose differential uniformity is at most $5$ when $p^n\equiv3~(mod~4)$. In this paper, we study the differential uniformity and the differential spectrum of $f_u$ for $u=\pm1$. We first give some properties of the differential spectrum of any cryptographic function. Moreover, by solving some systems of equations over finite fields, we express the differential spectrum of $f_{\pm1}$ in terms of the quadratic character sums.