On taking square roots without quadratic nonresidues over finite fields

Abstract

We present a novel idea to compute square roots over finite fields, without being given any quadratic nonresidue, and without assuming any unproven hypothesis. The algorithm is deterministic and the proof is elementary. In some cases, the square root algorithm runs in O ~ ( log 2 ⁡ q ) \tilde {O}(\log ^2 q) bit operations over finite fields with q q elements. As an application, we construct a deterministic primality-proving algorithm, which runs in O ~ ( log 3 ⁡ N ) \tilde {O}(\log ^3 N) for some integers N N .