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Fix handling of prime/composite-order curves within short-weierstrass

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This commit is contained in:
Luke Parker
2025-08-28 22:31:33 -04:00
parent da190759a9
commit 45bd376c08
4 changed files with 25 additions and 11 deletions
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13
crypto/short-weierstrass/src/lib.rs
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@@ -15,10 +15,6 @@ mod projective;
pub use projective::Projective;
/// An elliptic curve represented in short Weierstrass form, with equation `y^2 = x^3 + A x + B`.
///
/// This elliptic curve is expected to be of prime order. If a generator of the elliptic curve has
/// a composite order, the elliptic curve is defined solely as its largest odd-prime-order
/// subgroup, further considered the entire group/elliptic curve.
pub trait ShortWeierstrass: 'static + Sized + Debug {
/// The field the elliptic curve is defined over.
type FieldElement: Zeroize + PrimeField;
@@ -26,9 +22,9 @@ pub trait ShortWeierstrass: 'static + Sized + Debug {
const A: Self::FieldElement;
/// The constant `B` from the curve equation.
const B: Self::FieldElement;
/// A generator of this elliptic curve.
/// A generator of this elliptic curve's largest prime-order subgroup.
const GENERATOR: Affine<Self>;
/// The scalar type.
/// The scalar type for the elliptic curve's largest prime-order subgroup.
///
/// This may be omitted by specifying `()`.
type Scalar;
@@ -45,4 +41,9 @@ pub trait ShortWeierstrass: 'static + Sized + Debug {
///
/// This is expected to return the `x` coordinate and if the `y` coordinate is odd.
fn decode_compressed(bytes: &Self::Repr) -> (<Self::FieldElement as PrimeField>::Repr, Choice);
/// If the point is outside the largest prime-order subgroup and isn't the identity point.
///
/// This may immediately return `Choice::new(0)` for curves of prime order.
fn has_torsion_element(point: Projective<Self>) -> Choice;
}