NTL – Saad Quader

242_roots_unity_9 — 9th roots of unity. Source: Kyle Miller’s article.

To this day, no method of finding a generator of $Z_p^*$ is known to be more efficient than essentially trying 2, then 3, and so on. Who cares? Well, the difficulty of breaking a certain public key cryptosystem (due to El Gamal) depends on the difficulty of working with generators of $Z_p^*$ . — Keith Conrad

An $n$ th root of unity in a finite field $F$ is an element $r \in F$ satisfying $r^n=1$ , where $n$ is an integer. If $n$ is the smallest positive integer with this property, $r$ is called a primitive $n$ th root of unity. If $r$ is a primitive $n$ th root of unity, then all elements in the set $\mu_n = \{1, r, r^2, \cdots, r^{n-1}\}$ are also roots of unity. Actually, the set $\mu_n$ form a cyclic group of order $n$ under multiplication, with generator $r$ .

Problem: Suppose you are given a finite field $F=GF(2^d)$ of degree $d$ , and you are promised that there indeed exists a primitive $p$ th root of unity $r\in F$ for $p$ prime. Find $r$ , and in particular, produce a C++code that finds it.

In what follows, we talk about how to find such a root and provide my C++ code; the code uses the awesome NTL library.

Continue reading “Finding a Primitive p-th Root of Unity in a Finite Field, with C++ Code” →