Fast elliptic curve point counting using Gaussian normal basis

Abstract

In this paper we present an improved algorithm for counting points on elliptic curves over finite fields. It is mainly based on Satoh-Skjernaa-Taguchi algorithm [SST01], and uses a Gaussian Normal Basis (GNB) of small type t less than or equal to 4. In practice, about 42% (36% for prime N) of fields in cryptographic context (i.e., for p = 2 and 160 < N < 600) have such bases. They can be lifted from F-pN to Z(pN) in a natural way. From the specific properties of GNBs, efficient multiplication and the Frobenius substitution axe available. Thus a fast norm computation algorithm is derived, which runs in O(N-2mu log N) with O(N-2) space, where the time complexity of multiplying two n-bit objects is O(n(mu)). As a result, for all small characteristic p, we reduced the time complexity of the SST-algorithm from O(N2mu+0.5) to O(N2mu+ 1/mu+1) and the space complexity still fits in O(N-2). Our approach is expected to be applicable to the AGM since the exhibited improvement is not restricted to only [SST01].