pub fn keyspace_codomain_classes(
e0: &Curve2,
basis_a: (&Point2, &Point2),
ell: u64,
a: u32,
) -> Vec<(Fp2, Vec<u64>)>Expand description
Partition the keyspace by codomain j-invariant: secrets whose isogeny lands on the same curve (up to
isomorphism). These classes are exactly the Aut(E₀)-orbits — so if one secret in a class is invertible
(its codomain is the target E), every secret in the class maps to a curve ≅ E. Returns
(j, [secrets]) groups.