Nonadjacent Radix-τ Expansions of Integers in Euclidean Imaginary Quadratic Number Fields

Abstract In his seminal papers, Koblitz proposed curves for cryptographic use. For fast operations on these curves, these papers also initiated a study of the radix- $\tau $ expansion of integers in the number fields $\mathbb{Q}\left( \sqrt{-3} \right)$ and $\mathbb{Q}\left( \sqrt{-7} \right)$ . The (window) nonadjacent form of $\tau $ -expansion of integers in $\mathbb{Q}\left( \sqrt{-7} \right)$ was first investigated by Solinas. For integers in $\mathbb{Q}\left( \sqrt{-3} \right)$ , the nonadjacent form and the window nonadjacent form of the $\tau $ -expansion were studied. These are used for efficient point multiplications on Koblitz curves. In this paper, we complete the picture by producing the (window) nonadjacent radix- $\tau $ expansions for integers in all Euclidean imaginary quadratic number fields.