Implement a method for determining if an element is the identity.

This corresponds to ge_isneutral() in Open Whispersystems' ed25519 library.

src/curve.rs

@@ -85,8 +85,10 @@ use core::cmp::{PartialEq, Eq};
 use constants;
 use field::FieldElement;
 use scalar::Scalar;
+use util::arrays_equal_ct;
 use util::bytes_equal_ct;
 use util::CTAssignable;
+use util::CTEq;
 use util::CTNegatable;
 
 // ------------------------------------------------------------------------
@@ -227,7 +229,7 @@ pub struct CachedPoint {
 // Constructors
 // ------------------------------------------------------------------------
 
-/// Trait for curve point types that have an identity constructor.
+/// Trait for curve point types which have an identity constructor.
 pub trait Identity {
     /// Returns the identity element of the curve.
     /// Can be used as a constructor.
@@ -292,6 +294,37 @@ impl CTAssignable for PreComputedPoint {
     }
 }
 
+// ------------------------------------------------------------------------
+// Constant-time Equality
+// ------------------------------------------------------------------------
+
+impl CTEq for ExtendedPoint {
+    fn ct_eq(&self, other: &ExtendedPoint) -> u8 {
+        arrays_equal_ct(&self.compress().0, &other.compress().0)
+    }
+}
+
+/// Trait for testing if a curve point is equivalent to the identity point.
+pub trait IsIdentity {
+    /// Return true if this element is the identity element of the curve.
+    fn is_identity(&self) -> bool;
+}
+
+/// Implement generic identity equality testing for a point representations
+/// which have constant-time equality testing and a defined identity
+/// constructor.
+impl<T> IsIdentity for T where T: CTEq + Identity {
+    fn is_identity(&self) -> bool {
+        let identity: T = T::identity();
+
+        if self.ct_eq(&identity) == 1u8 {
+            return true;
+        } else {
+            return false;
+        }
+    }
+}
+
 // ------------------------------------------------------------------------
 // Point conversions
 // ------------------------------------------------------------------------
@@ -1073,6 +1106,11 @@ mod test {
         assert!(p2.is_small_order() == false);
     }
 
+    #[test]
+    fn test_is_identity() {
+        assert!(ExtendedPoint::identity().is_identity());
+    }
+
     #[bench]
     fn bench_basepoint_mult(b: &mut Bencher) {
         b.iter(|| ExtendedPoint::basepoint_mult(&A_SCALAR));