Replace (x, y) tuple with Coordinates struct

The previous `CurveAffine::get_xy` method returned the coordinates as
`CtOption<(C::Base, C::Base)>`. However, `ConditionallySelectable` is
not implemented for any tuple or array types, making it impossible to
use any of the useful `CtOption` methods like `and_then`. We replace it
with `CurveAffine::coordinates -> CtOption<Coordinates<Self>>` and
`impl ConditionallySelectable for Coordinates` to enable operating over
coordinates in constant time.

src/arithmetic/curves.rs

@@ -99,8 +99,10 @@ pub trait CurveAffine:
     /// The projective form of the curve
     type CurveExt: CurveExt<AffineExt = Self, ScalarExt = <Self as CurveAffine>::ScalarExt>;
 
-    /// Gets the $(x, y)$ coordinates of this point.
-    fn get_xy(&self) -> CtOption<(Self::Base, Self::Base)>;
+    /// Gets the coordinates of this point.
+    ///
+    /// Returns None if this is the identity.
+    fn coordinates(&self) -> CtOption<Coordinates<Self>>;
 
     /// Obtains a point given $(x, y)$, failing if it is not on the
     /// curve.
@@ -131,3 +133,49 @@ pub trait CurveAffine:
     /// Returns the curve constant $b$.
     fn b() -> Self::Base;
 }
+
+/// The affine coordinates of a point on an elliptic curve.
+#[derive(Clone, Copy, Debug, Default)]
+pub struct Coordinates<C: CurveAffine> {
+    pub(crate) x: C::Base,
+    pub(crate) y: C::Base,
+}
+
+impl<C: CurveAffine> Coordinates<C> {
+    /// Returns the x-coordinate.
+    ///
+    /// Equivalent to `Coordinates::u`.
+    pub fn x(&self) -> &C::Base {
+        &self.x
+    }
+
+    /// Returns the y-coordinate.
+    ///
+    /// Equivalent to `Coordinates::v`.
+    pub fn y(&self) -> &C::Base {
+        &self.y
+    }
+
+    /// Returns the u-coordinate.
+    ///
+    /// Equivalent to `Coordinates::x`.
+    pub fn u(&self) -> &C::Base {
+        &self.x
+    }
+
+    /// Returns the v-coordinate.
+    ///
+    /// Equivalent to `Coordinates::y`.
+    pub fn v(&self) -> &C::Base {
+        &self.y
+    }
+}
+
+impl<C: CurveAffine> ConditionallySelectable for Coordinates<C> {
+    fn conditional_select(a: &Self, b: &Self, choice: Choice) -> Self {
+        Coordinates {
+            x: C::Base::conditional_select(&a.x, &b.x, choice),
+            y: C::Base::conditional_select(&a.y, &b.y, choice),
+        }
+    }
+}

src/curves.rs

@@ -15,7 +15,7 @@ use rand::RngCore;
 use subtle::{Choice, ConditionallySelectable, ConstantTimeEq, CtOption};
 
 use super::{Fp, Fq};
-use crate::arithmetic::{CurveAffine, CurveExt, FieldExt, Group};
+use crate::arithmetic::{Coordinates, CurveAffine, CurveExt, FieldExt, Group};
 
 macro_rules! new_curve_impl {
     (($($privacy:tt)*), $name:ident, $name_affine:ident, $iso:ident, $base:ident, $scalar:ident,
@@ -653,8 +653,8 @@ macro_rules! new_curve_impl {
                     | self.infinity
             }
 
-            fn get_xy(&self) -> CtOption<(Self::Base, Self::Base)> {
-                CtOption::new((self.x, self.y), !self.is_identity())
+            fn coordinates(&self) -> CtOption<Coordinates<Self>> {
+                CtOption::new(Coordinates { x: self.x, y: self.y }, !self.is_identity())
             }
 
             fn from_xy(x: Self::Base, y: Self::Base) -> CtOption<Self> {