ensure that the points are really the same before doubling

Author: tomato42

src/ecdsa/ellipticcurve.py

@@ -1185,14 +1185,15 @@ def __add__(self, other):
         if self == INFINITY:
             return other
         assert self.__curve == other.__curve
-        if self.__x == other.__x:
-            if (self.__y + other.__y) % self.__curve.p() == 0:
+
+        p = self.__curve.p()
+
+        if (self.__x - other.__x) % p == 0:
+            if (self.__y + other.__y) % p == 0:
                 return INFINITY
             else:
                 return self.double()
 
-        p = self.__curve.p()
-
         l = (
             (other.__y - self.__y)
             * numbertheory.inverse_mod(other.__x - self.__x, p)

src/ecdsa/test_ellipticcurve.py

@@ -146,6 +146,16 @@ def setUpClass(cls):
         cls.c192 = CurveFp(p, -3, b)
         cls.p192 = Point(cls.c192, Gx, Gy, r)
 
+    def test_points_with_different_curves(self):
+        with self.assertRaises(AssertionError):
+            self.g_23 + self.p192
+
+    def test_add_point_to_negative(self):
+        self.assertIs(self.g_23 + (-self.g_23), INFINITY)
+
+    def test_add_point_to_explicit_negative(self):
+        self.assertIs(self.g_23 + Point(self.c_23, 13, -7), INFINITY)
+
     def test_p192(self):
         # Checking against some sample computations presented
         # in X9.62: