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**2016/947**

**Title:**

Isogeny graphs of ordinary abelian varieties

**Authors:**
Ernest Hunter Brooks, Dimitar Jetchev, Benjamin Wesolowski

**Abstract:**

Fix a prime number \ell. Graphs of isogenies of degree a power of \ell are well-understood for elliptic curves, but not for higher-dimensional abelian varieties. We study the case of absolutely simple ordinary abelian varieties over a finite field. We analyse graphs of so-called \mathfrak l-isogenies, resolving that they are (almost) volcanoes in any dimension. Specializing to the case of principally polarizable abelian surfaces, we then exploit this structure to describe graphs of a particular class of isogenies known as (\ell, \ell)-isogenies: those whose kernels are maximal isotropic subgroups of the \ell-torsion for the Weil pairing. We use these two results to write an algorithm giving a path of computable isogenies from an arbitrary absolutely simple ordinary abelian surface towards one with maximal endomorphism ring, which has immediate consequences for the CM-method in genus 2, for computing explicit isogenies, and for the random self-reducibility of the discrete logarithm problem in genus 2 cryptography.

**ePrint:**
https://eprint.iacr.org/2016/947

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