From 0d8f33153c144632e97b5a49cefe1e66a292463a Mon Sep 17 00:00:00 2001
From: Isis Lovecruft <isis@patternsinthevoid.net>
Date: Tue, 20 Apr 2021 23:58:29 +0000
Subject: [PATCH] Implement Identity for MontgomeryPoint.

This can be used in checks for non-contributory behaviour in protocols
where that is a concern.
---
 src/montgomery.rs | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/src/montgomery.rs b/src/montgomery.rs
index de0f8b7..867496c 100644
--- a/src/montgomery.rs
+++ b/src/montgomery.rs
@@ -94,6 +94,13 @@ impl PartialEq for MontgomeryPoint {
 
 impl Eq for MontgomeryPoint {}
 
+impl Identity for MontgomeryPoint {
+    /// Return the group identity element, which has order 4.
+    fn identity() -> MontgomeryPoint {
+        MontgomeryPoint([0u8; 32])
+    }
+}
+
 impl Zeroize for MontgomeryPoint {
     fn zeroize(&mut self) {
         self.0.zeroize();
@@ -351,6 +358,14 @@ mod test {
 
     use rand_core::OsRng;
 
+    #[test]
+    fn identity_in_different_coordinates() {
+        let id_projective = ProjectivePoint::identity();
+        let id_montgomery = id_projective.to_affine();
+
+        assert!(id_montgomery == MontgomeryPoint::identity());
+    }
+
     #[test]
     #[cfg(feature = "serde")]
     fn serde_bincode_basepoint_roundtrip() {