From fcd399c2065e7f3cd8388be798b52e41da7f3314 Mon Sep 17 00:00:00 2001
From: Tony Arcieri <bascule@gmail.com>
Date: Fri, 12 Sep 2025 16:51:21 -0600
Subject: [PATCH] elliptic-curve: change `Curve::ORDER` to be `Odd`

Previously it was `NonZero`.

This associated constant is for the curve's prime order subgroup, which
will always be odd.

There are many modular algorithms which require the modulus is odd, and
bounding the `ORDER` on `Odd` ensures those algorithms will be
compatible with the scalar field.

The `primefield` crate and the params we are computing for every field
of every curve are also `Odd` (i.e. the `modulus` of `MontyParams`).
---
 elliptic-curve/src/dev.rs              |  4 ++--
 elliptic-curve/src/lib.rs              |  9 +++------
 elliptic-curve/src/scalar/primitive.rs | 12 ++++++------
 3 files changed, 11 insertions(+), 14 deletions(-)

diff --git a/elliptic-curve/src/dev.rs b/elliptic-curve/src/dev.rs
index e8016cc01..f5c871a77 100644
--- a/elliptic-curve/src/dev.rs
+++ b/elliptic-curve/src/dev.rs
@@ -6,7 +6,7 @@
 use crate::{
     BatchNormalize, Curve, CurveArithmetic, CurveGroup, FieldBytesEncoding, PrimeCurve,
     array::typenum::U32,
-    bigint::{Limb, NonZero, U256},
+    bigint::{Limb, Odd, U256},
     error::{Error, Result},
     ops::{Invert, LinearCombination, Reduce, ShrAssign},
     point::{AffineCoordinates, NonIdentity},
@@ -70,7 +70,7 @@ impl Curve for MockCurve {
     type FieldBytesSize = U32;
     type Uint = U256;
 
-    const ORDER: NonZero<U256> = NonZero::<U256>::from_be_hex(
+    const ORDER: Odd<U256> = Odd::<U256>::from_be_hex(
         "ffffffff00000000ffffffffffffffffbce6faada7179e84f3b9cac2fc632551",
     );
 }
diff --git a/elliptic-curve/src/lib.rs b/elliptic-curve/src/lib.rs
index a473e09ad..a5b2e1aa2 100644
--- a/elliptic-curve/src/lib.rs
+++ b/elliptic-curve/src/lib.rs
@@ -133,7 +133,7 @@ pub use {
 #[cfg(feature = "pkcs8")]
 pub use pkcs8;
 
-use bigint::NonZero;
+use bigint::Odd;
 use core::{
     fmt::Debug,
     ops::{Add, ShrAssign},
@@ -174,11 +174,8 @@ pub trait Curve: 'static + Copy + Clone + Debug + Default + Eq + Ord + Send + Sy
         + FieldBytesEncoding<Self>
         + ShrAssign<usize>;
 
-    /// Order of this elliptic curve, i.e. number of elements in the scalar
-    /// field.
-    // TODO(tarcieri): make `Odd`? the prime order subgroup should always have an odd number of
-    // elements, even if there is a cofactor
-    const ORDER: NonZero<Self::Uint>;
+    /// Order of this curve's prime order subgroup, i.e. number of elements in the scalar field.
+    const ORDER: Odd<Self::Uint>;
 }
 
 /// Marker trait for elliptic curves with prime order.
diff --git a/elliptic-curve/src/scalar/primitive.rs b/elliptic-curve/src/scalar/primitive.rs
index adfcf1cec..92d00bd87 100644
--- a/elliptic-curve/src/scalar/primitive.rs
+++ b/elliptic-curve/src/scalar/primitive.rs
@@ -3,7 +3,7 @@
 use crate::{
     Curve, Error, FieldBytes, FieldBytesEncoding, Result,
     array::Array,
-    bigint::{Limb, NonZero, prelude::*},
+    bigint::{Limb, Odd, prelude::*},
     scalar::FromUintUnchecked,
     scalar::IsHigh,
 };
@@ -62,12 +62,12 @@ where
     };
 
     /// Scalar modulus.
-    pub const MODULUS: NonZero<C::Uint> = C::ORDER;
+    pub const MODULUS: Odd<C::Uint> = C::ORDER;
 
     /// Generate a random [`ScalarPrimitive`].
     pub fn random<R: CryptoRng + ?Sized>(rng: &mut R) -> Self {
         Self {
-            inner: C::Uint::random_mod(rng, &Self::MODULUS),
+            inner: C::Uint::random_mod(rng, Self::MODULUS.as_nz_ref()),
         }
     }
 
@@ -254,7 +254,7 @@ where
 
     fn add(self, other: &Self) -> Self {
         Self {
-            inner: self.inner.add_mod(&other.inner, &Self::MODULUS),
+            inner: self.inner.add_mod(&other.inner, Self::MODULUS.as_nz_ref()),
         }
     }
 }
@@ -296,7 +296,7 @@ where
 
     fn sub(self, other: &Self) -> Self {
         Self {
-            inner: self.inner.sub_mod(&other.inner, &Self::MODULUS),
+            inner: self.inner.sub_mod(&other.inner, Self::MODULUS.as_nz_ref()),
         }
     }
 }
@@ -327,7 +327,7 @@ where
 
     fn neg(self) -> Self {
         Self {
-            inner: self.inner.neg_mod(&Self::MODULUS),
+            inner: self.inner.neg_mod(Self::MODULUS.as_nz_ref()),
         }
     }
 }