Move is_in_correct_subgroup_assuming_on_curve() into the parent trait…

… ModelParameters

ec/src/models/mod.rs

@@ -1,4 +1,3 @@
-use crate::models::short_weierstrass_jacobian::GroupAffine;
 use ark_ff::{fields::BitIteratorBE, Field, PrimeField, SquareRootField, Zero};
 
 pub mod bls12;
@@ -9,13 +8,29 @@ pub mod mnt6;
 pub mod short_weierstrass_jacobian;
 pub mod twisted_edwards_extended;
 
+/// Required for the is_in_correct_subgroup_assuming_on_curve function.
+pub trait MultipliablePoint<R: Zero> {
+    fn mul_bits(&self, bits: impl Iterator<Item = bool>) -> R;
+}
+
 /// Model parameters for an elliptic curve.
 pub trait ModelParameters: Send + Sync + 'static {
     type BaseField: Field + SquareRootField;
     type ScalarField: PrimeField + SquareRootField + Into<<Self::ScalarField as PrimeField>::BigInt>;
 
     const COFACTOR: &'static [u64];
     const COFACTOR_INV: Self::ScalarField;
+
+    /// Requires type parameters G: the type of point passed in, and H: the type of
+    /// point that results from multiplying G by a scalar.
+    fn is_in_correct_subgroup_assuming_on_curve<G, H>(item: &G) -> bool
+    where
+        G: MultipliablePoint<H>,
+        H: Zero,
+    {
+        item.mul_bits(BitIteratorBE::new(Self::ScalarField::characteristic()))
+            .is_zero()
+    }
 }
 
 /// Model parameters for a Short Weierstrass curve.
@@ -40,14 +55,6 @@ pub trait SWModelParameters: ModelParameters {
         }
         *elem
     }
-
-    fn is_in_correct_subgroup_assuming_on_curve(item: &GroupAffine<Self>) -> bool
-    where
-        Self: Sized,
-    {
-        item.mul_bits(BitIteratorBE::new(Self::ScalarField::characteristic()))
-            .is_zero()
-    }
 }
 
 /// Model parameters for a Twisted Edwards curve.

ec/src/models/short_weierstrass_jacobian.rs

@@ -16,7 +16,9 @@ use ark_ff::{
     ToConstraintField, UniformRand,
 };
 
-use crate::{models::SWModelParameters as Parameters, AffineCurve, ProjectiveCurve};
+use crate::{
+    models::SWModelParameters as Parameters, AffineCurve, MultipliablePoint, ProjectiveCurve,
+};
 
 use num_traits::{One, Zero};
 use zeroize::Zeroize;
@@ -69,20 +71,10 @@ impl<P: Parameters> Display for GroupAffine<P> {
     }
 }
 
-impl<P: Parameters> GroupAffine<P> {
-    pub fn new(x: P::BaseField, y: P::BaseField, infinity: bool) -> Self {
-        Self { x, y, infinity }
-    }
-
-    /// Multiply `self` by the cofactor of the curve, `P::COFACTOR`.
-    pub fn scale_by_cofactor(&self) -> GroupProjective<P> {
-        let cofactor = BitIteratorBE::new(P::COFACTOR);
-        self.mul_bits(cofactor)
-    }
-
+impl<P: Parameters> MultipliablePoint<GroupProjective<P>> for GroupAffine<P> {
     /// Multiplies `self` by the scalar represented by `bits`. `bits` must be a
     /// big-endian bit-wise decomposition of the scalar.
-    pub(crate) fn mul_bits(&self, bits: impl Iterator<Item = bool>) -> GroupProjective<P> {
+    fn mul_bits(&self, bits: impl Iterator<Item = bool>) -> GroupProjective<P> {
         let mut res = GroupProjective::zero();
         // Skip leading zeros.
         for i in bits.skip_while(|b| !b) {
@@ -93,6 +85,18 @@ impl<P: Parameters> GroupAffine<P> {
         }
         res
     }
+}
+
+impl<P: Parameters> GroupAffine<P> {
+    pub fn new(x: P::BaseField, y: P::BaseField, infinity: bool) -> Self {
+        Self { x, y, infinity }
+    }
+
+    /// Multiply `self` by the cofactor of the curve, `P::COFACTOR`.
+    pub fn scale_by_cofactor(&self) -> GroupProjective<P> {
+        let cofactor = BitIteratorBE::new(P::COFACTOR);
+        self.mul_bits(cofactor)
+    }
 
     /// Attempts to construct an affine point given an x-coordinate. The
     /// point is not guaranteed to be in the prime order subgroup.
@@ -137,7 +141,7 @@ impl<P: Parameters> GroupAffine<P> {
     /// Checks if `self` is in the subgroup having order that equaling that of
     /// `P::ScalarField`.
     pub fn is_in_correct_subgroup_assuming_on_curve(&self) -> bool {
-        P::is_in_correct_subgroup_assuming_on_curve(self)
+        P::is_in_correct_subgroup_assuming_on_curve::<GroupAffine<P>, GroupProjective<P>>(self)
     }
 }
 

ec/src/models/twisted_edwards_extended.rs

@@ -1,6 +1,6 @@
 use crate::{
     models::{MontgomeryModelParameters as MontgomeryParameters, TEModelParameters as Parameters},
-    AffineCurve, ProjectiveCurve,
+    AffineCurve, MultipliablePoint, ProjectiveCurve,
 };
 use ark_serialize::{
     CanonicalDeserialize, CanonicalDeserializeWithFlags, CanonicalSerialize,
@@ -29,6 +29,8 @@ use ark_ff::{
 #[cfg(feature = "parallel")]
 use rayon::prelude::*;
 
+/// Affine coordinates for a point on a twisted Edwards curve, over the
+/// base field `P::BaseField`.
 #[derive(Derivative)]
 #[derivative(
     Copy(bound = "P: Parameters"),
@@ -50,19 +52,10 @@ impl<P: Parameters> Display for GroupAffine<P> {
     }
 }
 
-impl<P: Parameters> GroupAffine<P> {
-    pub fn new(x: P::BaseField, y: P::BaseField) -> Self {
-        Self { x, y }
-    }
-
-    #[must_use]
-    pub fn scale_by_cofactor(&self) -> <Self as AffineCurve>::Projective {
-        self.mul_bits(BitIteratorBE::new(P::COFACTOR))
-    }
-
+impl<P: Parameters> MultipliablePoint<GroupProjective<P>> for GroupAffine<P> {
     /// Multiplies `self` by the scalar represented by `bits`. `bits` must be a
     /// big-endian bit-wise decomposition of the scalar.
-    pub(crate) fn mul_bits(&self, bits: impl Iterator<Item = bool>) -> GroupProjective<P> {
+    fn mul_bits(&self, bits: impl Iterator<Item = bool>) -> GroupProjective<P> {
         let mut res = GroupProjective::zero();
         for i in bits.skip_while(|b| !b) {
             res.double_in_place();
@@ -72,6 +65,17 @@ impl<P: Parameters> GroupAffine<P> {
         }
         res
     }
+}
+
+impl<P: Parameters> GroupAffine<P> {
+    pub fn new(x: P::BaseField, y: P::BaseField) -> Self {
+        Self { x, y }
+    }
+
+    #[must_use]
+    pub fn scale_by_cofactor(&self) -> <Self as AffineCurve>::Projective {
+        self.mul_bits(BitIteratorBE::new(P::COFACTOR))
+    }
 
     /// Attempts to construct an affine point given an y-coordinate. The
     /// point is not guaranteed to be in the prime order subgroup.
@@ -115,8 +119,7 @@ impl<P: Parameters> GroupAffine<P> {
     /// Checks that the current point is in the prime order subgroup given
     /// the point on the curve.
     pub fn is_in_correct_subgroup_assuming_on_curve(&self) -> bool {
-        self.mul_bits(BitIteratorBE::new(P::ScalarField::characteristic()))
-            .is_zero()
+        P::is_in_correct_subgroup_assuming_on_curve::<GroupAffine<P>, GroupProjective<P>>(self)
     }
 }
 
@@ -296,7 +299,7 @@ mod group_impl {
 
 //////////////////////////////////////////////////////////////////////////////
 
-/// `GroupProjective` implements Extended Twisted Edwards Coordinates
+/// `GroupProjective` implements Extended Twisted Edwards (Jacobian) Coordinates
 /// as described in [\[HKCD08\]](https://eprint.iacr.org/2008/522.pdf).
 ///
 /// This implementation uses the unified addition formulae from that paper (see