Papers › Adventures in Supersingularland
Adventures in Supersingularland
Sarah Arpin, Catalina Camacho-Navarro, Kristin Lauter, Joelle Lim, Kristina Nelson, Travis Scholl, Jana Sotáková
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In this paper, we study isogeny graphs of supersingular elliptic curves. Supersingular isogeny graphs were introduced as a hard problem into cryptography by Charles, Goren, and Lauter for the construction of cryptographic hash functions [CGL06]. These are large expander graphs, and the hard problem is to find an efficient algorithm for routing, or path-finding, between two vertices of the graph. We consider four aspects of supersingular isogeny graphs, study each thoroughly and, where appropriate, discuss how they relate to one another. First, we consider two related graphs that help us understand the structure: the `spine' 𝒮, which is the subgraph of 𝒢_ℓ(𝔽ₚ) given by the j-invariants in 𝔽ₚ, and the graph 𝒢_ℓ(𝔽ₚ), in which both curves and isogenies must be defined over 𝔽ₚ. We show how to pass from the latter to the former. The graph 𝒮 is relevant for cryptanalysis because routing between vertices in 𝔽ₚ is easier than in the full isogeny graph. The 𝔽ₚ-vertices are typically assumed to be randomly distributed in the graph, which is far from true. We provide an analysis of the distances of connected components of 𝒮. Next, we study the involution on 𝒢_ℓ(𝔽ₚ) that is given by the Frobenius of 𝔽ₚ and give heuristics on how often shortest paths between two conjugate j-invariants are preserved by this involution (mirror paths). We also study the related question of what proportion of conjugate j-invariants are ℓ-isogenous for ℓ= 2,3. We conclude with experimental data on the diameters of supersingular isogeny graphs when ℓ= 2 and compare this with previous results on diameters of LPS graphs and random Ramanujan graphs.
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