Forbidden isogenies - Massachusetts Institute of Technology

Forbidden isogenies

Bradley W. Brock1 Everett W. Howe 2

1IDA Center for Communications Research, Princeton 2Unaffiliated mathematician

14th Algorithmic Number Theory Symposium Rump session 1 July 2020

Email: however@alumni.caltech.edu

Web site:

Twitter: @howe

Brock and Howe

Forbidden isogenies

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Graphs of Richelot isogenies

Richelot graphs

Vertices: Principally polarized abelian surfaces over Fq Edges: Richelot isogenies from one PPAS to another

One might choose to restrict to subgraphs: Supersingular abelian surfaces Superspecial abelian surfaces Jacobians ...

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A few papers that discuss algorithms based on Richelot graphs

Wouter Castryck, Thomas Decru, Benjamin Smith: Hash functions from superspecial genus-2 curves using Richelot isogenies

Craig Costello, Benjamin Smith: The supersingular isogeny problem in genus 2 and beyond

E. V. Flynn, Yan Bo Ti: Genus two isogeny cryptography

Toshiyuki Katsura, Katsuyuki Takashima: Counting Richelot isogenies between superspecial abelian surfaces

Katsuyuki Takashima: Efficient algorithms for isogeny sequences and their cryptographic applications

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What do these graphs even look like?

Brock and Howe

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What do these graphs even look like?

Volcanos?

Mount Ngauruhoe, by Flickr user russellstreet

Brock and Howe

Forbidden isogenies

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What do these graphs even look like?

Volcanos?

Expanders?

Mount Ngauruhoe, by Flickr user russellstreet

Olympic athlete John Grimek

Brock and Howe

Forbidden isogenies

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What do these graphs even look like?

Volcanos?

Expanders?

Mount Ngauruhoe, by Flickr user russellstreet

Olympic athlete John Grimek

Are they connected? Are there short paths? What's the diameter?

Brock and Howe

Forbidden isogenies

4 of 11

What do these graphs even look like?

Volcanos?

Expanders?

Mount Ngauruhoe, by Flickr user russellstreet

Olympic athlete John Grimek

Are they connected? Are there short paths? What's the diameter?

Why are we stuck using these confusing graphs?

Brock and Howe

Forbidden isogenies

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