Self-dual bent sequences for complex Hadamard matrices

Abstract

A new notion of bent sequence related to Hadamard matrices was introduced recently, motivated by a security application (Solé et al. 2021). In this paper we introduce the analogous notion for complex Hadamard matrices, and we study the self-dual class in length at most 90. We use three competing methods of generation: Brute force, Linear Algebra and Groebner bases. Regular complex Hadamard matrices and Bush-type complex Hadamard matrices provide many examples. We introduce the strong automorphism group of complex Hadamard matrices, which acts on their associated self-dual bent sequences. We give an efficient algorithm to compute that group. We also answer the question which complex Hadamard matrices can be uniquely reconstructed from the off-diagonal elements, define a related concept of mixed-skew Hadamard matrix, and show the existence of mixed-skew Hadamard matrices of small orders.