International Association
for Cryptologic Research

Fix odd primes $p, \ell$ with $p \equiv 3 \mod 4$ and $\ell \neq p$. Consider the rational quaternion algebra ramified at $p$ and $\infty$ with a fixed maximal order $\mathcal{O}_{1728}$. We give explicit formulae for bases of all cyclic norm $\ell^n$ ideals of $\mathcal{O}_{1728}$ and their right orders, in Hermite Normal Form (HNF). Further, in the case $\ell \mid p+1$, or more generally, $-p$ is a square modulo $\ell$, we derive a parametrization of these bases along paths of the $\ell$-ideal graph, generalising the results of [1]. With such orders appearing as the endomorphism rings of supersingular elliptic curves defined over $\overline{\mathbb{F}_{p}}$, we note several potential applications to isogeny-based cryptography including fast ideal sampling algorithms. We also demonstrate how our findings may lead to further structural observations, by using them to prove a result on the successive minima of endomorphism rings of supersingular curves defined over $\mathbb{F}_p$.

[1] = https://eprint.iacr.org/2025/033