Abstract:For a prime $p{\,>\,}3$ and a supersingular elliptic curve $E$ defined over $\mathbb{F}_{p^2}$ with ${j(E)\notin\{0,1728\}}$, consider an endomorphism $\alpha$ of $E$ represented as a composition of $L$ isogenies of degree at most $d$. We prove that the trace of $\alpha$ may be computed in $O(n^4(\log n)^2 + dLn^3)$ bit operations, where $n{\,=\,}\log(p)$, using a generalization of the SEA algorithm for computing the trace of the Frobenius endomorphism of an ordinary elliptic curve. When $L\in O(\log p)$ and $d\in O(1)$, this complexity matches the heuristic complexity of the SEA algorithm. Our theorem is unconditional, unlike the complexity analysis of the SEA algorithm, since the kernel of an arbitrary isogeny of a supersingular elliptic curve is defined over an extension of constant degree, independent of $p$. We also provide practical speedups, including a fast algorithm to compute the trace of $\alpha$ modulo $p$.
From: Travis Morrison [view email]
[v1]
Mon, 27 Jan 2025 18:56:07 UTC (107 KB)
[v2]
Thu, 11 Jun 2026 19:05:38 UTC (122 KB)
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