From eb8b8c19a81a52d9cf705d90a597a78cdaf2b6f6 Mon Sep 17 00:00:00 2001
From: ChangLiu <lc6chang@gmail.com>
Date: Sat, 12 Feb 2022 21:47:30 +0800
Subject: [PATCH] Fix doubling points on the x axis bug, add unittest (#18)

---
 ecc/curve.py        | 21 +++++----------------
 tests/test_curve.py |  8 +++++++-
 2 files changed, 12 insertions(+), 17 deletions(-)

diff --git a/ecc/curve.py b/ecc/curve.py
index a4d9f58..8558a8f 100644
--- a/ecc/curve.py
+++ b/ecc/curve.py
@@ -100,10 +100,10 @@ def add_point(self, P: Point, Q: Point) -> Point:
         elif Q.is_at_infinity():
             return P
 
-        if P == Q:
-            return self._double_point(P)
         if P == -Q:
             return self.INF
+        if P == Q:
+            return self._double_point(P)
 
         return self._add_point(P, Q)
 
@@ -111,14 +111,6 @@ def add_point(self, P: Point, Q: Point) -> Point:
     def _add_point(self, P: Point, Q: Point) -> Point:
         pass
 
-    def double_point(self, P: Point) -> Point:
-        if not self.is_on_curve(P):
-            raise ValueError("The point is not on the curve.")
-        if P.is_at_infinity():
-            return self.INF
-
-        return self._double_point(P)
-
     @abstractmethod
     def _double_point(self, P: Point) -> Point:
         pass
@@ -134,17 +126,14 @@ def mul_point(self, d: int, P: Point) -> Point:
         if d == 0:
             return self.INF
 
-        res = None
+        res = self.INF
         is_negative_scalar = d < 0
         d = -d if is_negative_scalar else d
         tmp = P
         while d:
             if d & 0x1 == 1:
-                if res:
-                    res = self.add_point(res, tmp)
-                else:
-                    res = tmp
-            tmp = self.double_point(tmp)
+                res = self.add_point(res, tmp)
+            tmp = self.add_point(tmp, tmp)
             d >>= 1
         if is_negative_scalar:
             return -res
diff --git a/tests/test_curve.py b/tests/test_curve.py
index a7931e4..9eb2ad4 100644
--- a/tests/test_curve.py
+++ b/tests/test_curve.py
@@ -1,7 +1,7 @@
 import unittest
 
 from ecc.curve import (
-    P256, secp256k1, Curve25519, M383, E222, E382
+    P256, secp256k1, Curve25519, M383, E222, E382, Point
 )
 
 CURVES = [P256, secp256k1, Curve25519, M383, E222, E382]
@@ -24,3 +24,9 @@ def test_operator(self):
                 self.assertEqual(curve.INF + curve.INF, curve.INF)
                 self.assertEqual(0 * P, curve.INF)
                 self.assertEqual(1000 * curve.INF, curve.INF)
+
+    def test_double_points_y_equals_to_0(self):
+        P = Point(x=0, y=0, curve=Curve25519)
+        self.assertEqual(P + P, Curve25519.INF)
+        self.assertEqual(2 * P, Curve25519.INF)
+        self.assertEqual(-2 * P, Curve25519.INF)