Implement constant time equality check for scalars.

src/scalar.rs

@@ -29,6 +29,7 @@
 //! between two scalars, the `UnpackedScalar` struct is stored as
 //! limbs.
 
+use core::cmp::{Eq, PartialEq};
 use core::ops::{Index, IndexMut};
 use core::ops::{Neg};
 
@@ -39,6 +40,7 @@ use rand::Rng;
 use constants;
 use field::{load3, load4};
 use util::CTAssignable;
+use util::arrays_equal_ct;
 
 /// The `Scalar` struct represents an element in ℤ/lℤ, where
 ///
@@ -48,6 +50,25 @@ use util::CTAssignable;
 #[derive(Copy, Clone)]
 pub struct Scalar(pub [u8; 32]);
 
+// XXX make CTEq traits
+impl Eq for Scalar{}
+impl PartialEq for Scalar {
+    /// Test equality between two `Scalar`s in constant time.
+    ///
+    /// Returns
+    ///
+    /// True if they are equal, and false otherwise.
+    fn eq(&self, other: &Self) -> bool {
+        let equal: u8 = arrays_equal_ct(&self.0, &other.0);
+
+        if equal == 1 {
+            return true;
+        } else {
+            return false;
+        }
+    }
+}
+
 impl Index<usize> for Scalar {
     type Output = u8;