Affine equivalence of cubic homogeneous rotation symmetric Boolean functions

TL;DR

This paper investigates the affine equivalence of cubic homogeneous rotation symmetric Boolean functions, introducing new concepts and providing classifications for certain cases, advancing understanding in cryptographic function analysis.

Contribution

It introduces the concept of patterns to analyze affine equivalence classes and characterizes these classes for prime and power-of-3 variables, extending prior quadratic case results.

Findings

Determined the structure of the group G_n acting on cubic functions.

Classified affine equivalence classes for prime and power-of-3 variables.

Verified the conjecture for n < 22.

Abstract

Homogeneous rotation symmetric Boolean functions have been extensively studied in recent years because of their applications in cryptography. Little is known about the basic question of when two such functions are affine equivalent. The simplest case of quadratic rotation symmetric functions which are generated by cyclic permutations of the variables in a single monomial was only settled in 2009. This paper studies the much more complicated cubic case for such functions. A new concept of \emph{patterns} is introduced, by means of which the structure of the smallest group G_n, whose action on the set of all such cubic functions in $n$ variables gives the affine equivalence classes for these functions under permutation of the variables, is determined. We conjecture that the equivalence classes are the same if all nonsingular affine transformations, not just permutations, are allowed. This conjecture is verified if n < 22. Our method gives much more information about the equivalence classes; for example, in this paper we give a complete description of the equivalence classes when n is a prime or a power of 3.