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Explicit isogenies in quadratic time in any characteristic
Published online by Cambridge University Press: 26 August 2016
Abstract
Consider two ordinary elliptic curves $E,E^{\prime }$ defined over a finite field $\mathbb{F}_{q}$, and suppose that there exists an isogeny $\unicode[STIX]{x1D713}$ between $E$ and $E^{\prime }$. We propose an algorithm that determines $\unicode[STIX]{x1D713}$ from the knowledge of $E$, $E^{\prime }$ and of its degree $r$, by using the structure of the $\ell$-torsion of the curves (where $\ell$ is a prime different from the characteristic $p$ of the base field). Our approach is inspired by a previous algorithm due to Couveignes, which involved computations using the $p$-torsion on the curves. The most refined version of that algorithm, due to De Feo, has a complexity of $\tilde{O} (r^{2})p^{O(1)}$ base field operations. On the other hand, the cost of our algorithm is $\tilde{O} (r^{2})\log (q)^{O(1)}$, for a large class of inputs; this makes it an interesting alternative for the medium- and large-characteristic cases.
MSC classification
- Type
- Research Article
- Information
- LMS Journal of Computation and Mathematics , Volume 19 , Special Issue A: Algorithmic Number Theory Symposium XII , 2016 , pp. 267 - 282
- Copyright
- © The Author(s) 2016
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