Fast Computation of Isomorphisms Between Finite Fields Using Elliptic Curves

Abstract: We propose a randomized algorithm to compute isomorphisms between finite fields using elliptic curves. To compute an isomorphism between two fields of cardinality $q^n$, our algorithm takes $$n^{1+o(1)} \log^{1+o(1)}q + \max_{\ell} \left(\ell^{n_\ell + 1+o(1)} \log^{2+o(1)} q + O(\ell \log^5q)\right)$$ time, where $\ell$ runs through primes dividing $n$ but not $q(q-1)$ and $n_\ell$ denotes the highest power of $\ell$ dividing $n$. Prior to this work, the best known run time dependence on $n$ was quadratic. Our run time dependence on $n$ is at worst quadratic but is subquadratic if $n$ has no large prime factor. In particular, the $n$ for which our run time is nearly linear in $n$ have natural density at least $3/10$. The crux of our approach is finding a point on an elliptic curve of a prescribed prime power order or equivalently finding preimages under the Lang map on elliptic curves over finite fields. We formulate this as an open problem whose resolution would solve the finite field isomorphism problem with run time nearly linear in $n$.