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Isogeny volcanoes (1208.5370v3)

Published 27 Aug 2012 in math.NT and cs.CR

Abstract: The remarkable structure and computationally explicit form of isogeny graphs of elliptic curves over a finite field has made them an important tool for computational number theorists and practitioners of elliptic curve cryptography. This expository paper recounts the theory behind these graphs and examines several recently developed algorithms that realize substantial (often dramatic) performance gains by exploiting this theory.

Citations (59)

Summary

  • The paper introduces novel algorithms that exploit the hierarchical structure of isogeny volcanoes to efficiently compute elliptic curve properties.
  • It details techniques for locating the volcano floor and determining endomorphism rings, significantly reducing computational overhead.
  • The research extends applications to computing Hilbert class and modular polynomials, thereby enhancing cryptographic performance.

An Exposition on Isogeny Volcanoes in Elliptic Curve Theory

Andrew V. Sutherland's paper provides a detailed exposition on the structure and computational methodologies surrounding isogeny graphs of elliptic curves over finite fields. These isogeny volcanoes are instrumental in both the theoretical framework and practical algorithms essential for elliptic curve cryptography and computational number theory. The focus of Sutherland's work is on leveraging the distinct structure of these volcanoes to derive computational algorithms that offer substantial improvements in efficiency.

Key Concepts and Framework

The notion of an isogeny volcano is rooted in graph theory. An \ell-volcano is a type of graph with a well-defined hierarchical structure, characterized by a surface level and possibly multiple deeper levels. The vertices within these graphs are elliptic curves linked by isogenies, with edges representing \ell-isogenies specifically. The paper provides a formal definition of these structures, elucidating their mathematical properties and how they are embedded within isogeny graphs over finite fields.

An essential component of the paper is the exploration of isogenies between elliptic curves, which are morphisms that preserve the group structure of the elliptic curves. The vertices of these graphs are elliptic curves denoted by their jj-invariants, and the isogenies form the edges.

Algorithmic Applications

Sutherland demonstrates multiple algorithms that effectively exploit the structure of isogeny volcanoes to achieve computational efficiency:

  1. Finding the Floor of a Volcano: The paper outlines a strategy to locate a vertex at the deepest level of an \ell-volcano using random walks and path evaluations, optimizing the exploration of isogeny graphs without excessive computation.
  2. Determining Endomorphism Rings: The author develops a methodology to compute endomorphism rings of elliptic curves, which is critical for cryptographic applications. The strategy involves navigating the isogeny volcano via horizontal and vertical isogenies, without requiring expansive calculations for large primes dividing the index.
  3. Hilbert Class Polynomial Computation: In cryptographic contexts like the CM method, efficiently computing Hilbert class polynomials is crucial. Sutherland describes a CRT-based approach using isogeny volcanoes to enumerate class fields robustly, significantly reducing computational overhead.
  4. Modular Polynomial Calculations: The paper further extends these methodologies for computing modular polynomials, utilizing the structured setup of isogeny volcanoes to interpolate polynomial coefficients efficiently.

Theoretical and Practical Implications

The work on isogeny volcanoes by Sutherland enhances both theoretical and practical aspects of modern computational number theory and cryptography. The insights provided contribute significantly to understanding the complex structure of elliptic curve isogenies and optimizing computations related to elliptic curves.

The practical implications of this research are substantial, particularly in elliptic curve cryptography, where the efficiency of these algorithms can directly impact the performance and security of cryptographic systems.

Future Directions

Considering the advancements presented, future developments may focus on further refining these algorithms, potentially extending the applicability to more complex or larger field sizes. Additionally, there is scope for enhancing the integration of these algorithms into cryptographic protocols, further securing digital communications and data.

This paper establishes a foundational understanding of isogeny volcanoes and their applications, setting a benchmark for ongoing research in elliptic curve theory and its applications in computational mathematics.

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