Paper 2026/1361 Original: eprint.iacr.org/2026/1361 ↗

k-idempotent Bent functions in Partial Spread class

Abstract

Idempotent functions are an important class of Boolean functions, related to cryptography and coding theory.} This paper investigates $k$-idempotent functions, a natural generalization of {idempotent} functions. Based on two classes of Dillon-type bent functions proposed in {the paper ``New characterizations of Dillon-like hyperbent functions via Dickson polynomials", by Z. Tu, C. Li, X. Zeng, T. Helleseth and N. Li, 02 October 2025, Volume 38, article number 35, in Journal of Cryptology}, we establish the existence of $k$-idempotent bent functions {(that are not $l$-idempotents for $l<k$)} from the finite field $\mathbb{F}_{2^n}$ to $\mathbb{F}_2$ in the partial spread class. More precisely, for any divisor $k$ of $n$ with $k\ge3$, it is demonstrated that there are $k$-idempotent bent functions. For $k=1,2$, complete characterizations on the $k$-idempotent bent conditions are provided via the $2$-adic orders of integers. The key problem reduces to solving a specific linear equation and {computing certain values of the trace function associated with its solutions}, which is closely connected to computing the Walsh transform of the Gold functions. By reformulating the target equation into a system of two linear equations and applying trace map properties, the trace is explicitly computed and thus the $k$-idempotent property is verified.