Add affine_xy, affine_x, affine_y functions to CurveVar

CHANGELOG.md

@@ -37,6 +37,7 @@
     - Add `UInt::{from_bytes_le, from_bytes_be, to_bytes_be}`.
 - [\#143](https://github.com/arkworks-rs/r1cs-std/pull/143) Add `AllocVar::new_variable_with_inferred_mode`.
 - [\#144](https://github.com/arkworks-rs/r1cs-std/pull/144) Add `ToConstraintFieldGadget` bounds to `CurveVar` and `FieldVar`
+- [\#190](https://github.com/arkworks-rs/r1cs-std/pull/190) Add `affine_xy, affine_x, affine_y` functions to `CurveVar`
 
 ### Improvements
 

src/groups/curves/short_weierstrass/mod.rs

@@ -371,7 +371,7 @@ where
     }
 }
 
-impl<P, F> CurveVar<SWProjective<P>, BasePrimeField<P>> for ProjectiveVar<P, F>
+impl<P, F> CurveVar<SWProjective<P>, BasePrimeField<P>, F> for ProjectiveVar<P, F>
 where
     P: SWCurveConfig,
     F: FieldVar<P::BaseField, BasePrimeField<P>>,
@@ -573,6 +573,11 @@ where
         *self += Self::constant(base).scalar_mul_le(bits.iter())?;
         Ok(())
     }
+
+    fn affine_xy(&self) -> Result<(F, F), SynthesisError> {
+        let self_affine = self.to_affine()?;
+        Ok((self_affine.x, self_affine.y))
+    }
 }
 
 impl<P, F> ToConstraintFieldGadget<BasePrimeField<P>> for ProjectiveVar<P, F>

src/groups/curves/twisted_edwards/mod.rs

@@ -395,7 +395,7 @@ where
     }
 }
 
-impl<P, F> CurveVar<TEProjective<P>, BasePrimeField<P>> for AffineVar<P, F>
+impl<P, F> CurveVar<TEProjective<P>, BasePrimeField<P>, F> for AffineVar<P, F>
 where
     P: TECurveConfig,
     F: FieldVar<P::BaseField, BasePrimeField<P>>
@@ -547,6 +547,10 @@ where
 
         Ok(())
     }
+
+    fn affine_xy(&self) -> Result<(F, F), SynthesisError> {
+        Ok((self.x.clone(), self.y.clone()))
+    }
 }
 
 impl<P, F> AllocVar<TEProjective<P>, BasePrimeField<P>> for AffineVar<P, F>

src/groups/mod.rs

@@ -29,7 +29,7 @@ pub trait GroupOpsBounds<'a, G, T: 'a>:
 
 /// A variable that represents a curve point for
 /// the curve `C`.
-pub trait CurveVar<C: CurveGroup, ConstraintF: PrimeField>:
+pub trait CurveVar<C: CurveGroup, ConstraintF: PrimeField, F: FieldVar<C::BaseField, ConstraintF>>:
     'static
     + Sized
     + Clone
@@ -68,6 +68,19 @@ pub trait CurveVar<C: CurveGroup, ConstraintF: PrimeField>:
     /// This *should not* allocate any variables.
     fn constant(other: C) -> Self;
 
+    /// Returns the x and y coordinates in Affine representation.
+    fn affine_xy(&self) -> Result<(F, F), SynthesisError>;
+
+    /// Returns the x coordinate in Affine representation.
+    fn affine_x(&self) -> Result<F, SynthesisError> {
+        self.affine_xy().map(|(x, _)| x)
+    }
+
+    /// Returns the y coordinate in Affine representation.
+    fn affine_y(&self) -> Result<F, SynthesisError> {
+        self.affine_xy().map(|(_, y)| y)
+    }
+
     /// Allocates a variable in the subgroup without checking if it's in the
     /// prime-order subgroup.
     fn new_variable_omit_prime_order_check(

src/pairing/bls12/mod.rs

@@ -59,7 +59,10 @@ impl<P: Bls12Config> PairingVar<P> {
     }
 }
 
-impl<P: Bls12Config> PG<Bls12<P>> for PairingVar<P> {
+impl<P: Bls12Config>
+    PG<Bls12<P>, FpVar<<P as Bls12Config>::Fp>, Fp2Var<<P as Bls12Config>::Fp2Config>>
+    for PairingVar<P>
+{
     type G1Var = G1Var<P>;
     type G2Var = G2Var<P>;
     type G1PreparedVar = G1PreparedVar<P>;

src/pairing/mnt4/mod.rs

@@ -196,7 +196,9 @@ impl<P: MNT4Config> PairingVar<P> {
     }
 }
 
-impl<P: MNT4Config> PG<MNT4<P>> for PairingVar<P> {
+impl<P: MNT4Config> PG<MNT4<P>, FpVar<<P as MNT4Config>::Fp>, Fp2Var<<P as MNT4Config>::Fp2Config>>
+    for PairingVar<P>
+{
     type G1Var = G1Var<P>;
     type G2Var = G2Var<P>;
     type G1PreparedVar = G1PreparedVar<P>;

src/pairing/mnt6/mod.rs

@@ -191,7 +191,9 @@ impl<P: MNT6Config> PairingVar<P> {
     }
 }
 
-impl<P: MNT6Config> PG<MNT6<P>> for PairingVar<P> {
+impl<P: MNT6Config> PG<MNT6<P>, FpVar<<P as MNT6Config>::Fp>, Fp3Var<<P as MNT6Config>::Fp3Config>>
+    for PairingVar<P>
+{
     type G1Var = G1Var<P>;
     type G2Var = G2Var<P>;
     type G1PreparedVar = G1PreparedVar<P>;

src/pairing/mod.rs

@@ -14,14 +14,19 @@ type BasePrimeField<E> = <<E as Pairing>::BaseField as ark_ff::Field>::BasePrime
 
 /// Specifies the constraints for computing a pairing in the yybilinear group
 /// `E`.
-pub trait PairingVar<E: Pairing> {
+pub trait PairingVar<
+    E: Pairing,
+    F1: FieldVar<<E::G1 as ark_ec::CurveGroup>::BaseField, E::BaseField>,
+    F2: FieldVar<<E::G2 as ark_ec::CurveGroup>::BaseField, E::BaseField>,
+>
+{
     /// An variable representing an element of `G1`.
     /// This is the R1CS equivalent of `E::G1Projective`.
-    type G1Var: CurveVar<E::G1, BasePrimeField<E>>;
+    type G1Var: CurveVar<E::G1, BasePrimeField<E>, F1>;
 
     /// An variable representing an element of `G2`.
     /// This is the R1CS equivalent of `E::G2Projective`.
-    type G2Var: CurveVar<E::G2, BasePrimeField<E>>;
+    type G2Var: CurveVar<E::G2, BasePrimeField<E>, F2>;
 
     /// An variable representing an element of `GT`.
     /// This is the R1CS equivalent of `E::GT`.