# On two-to-one mappings over finite fields

## TL;DR

This paper systematically studies two-to-one mappings over finite fields, characterizing them via Walsh transforms, presenting various constructions, and exploring their applications in cryptography and related areas.

## Contribution

It provides a comprehensive characterization, new constructions, and applications of two-to-one mappings over finite fields, advancing understanding in cryptographic function design.

## Key findings

- Characterization of 2-to-1 mappings via Walsh transforms
- New constructions including AGW-like criterion and polynomial forms
- Applications to bent functions, semi-bent functions, and permutation polynomials

## Abstract

Two-to-one ($2$-to-$1$) mappings over finite fields play an important role in symmetric cryptography. In particular they allow to design APN functions, bent functions and semi-bent functions. In this paper we provide a systematic study of two-to-one mappings that are defined over finite fields. We characterize such mappings by means of the Walsh transforms. We also present several constructions, including an AGW-like criterion, constructions with the form of $x^rh(x^{(q-1)/d})$, those from permutation polynomials, from linear translators and from APN functions. Then we present $2$-to-$1$ polynomial mappings in classical classes of polynomials: linearized polynomials and monomials, low degree polynomials, Dickson polynomials and Muller-Cohen-Matthews polynomials, etc. Lastly, we show applications of $2$-to-$1$ mappings over finite fields for constructions of bent Boolean and vectorial bent functions, semi-bent functions, planar functions and permutation polynomials. In all those respects, we shall review what is known and provide several new results.