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:
-                return INFINITY
-            else:
-                return self.double()
 
         p = self.__curve.p()
 
+        if (self.__x - other.__x) % p == 0:
+            if (self.__y + other.__y) % p == 0:
+                return INFINITY
+            elif (self.__y - other.__y) % p == 0:
+                return self.double()
+
         l = (
             (other.__y - self.__y)
             * numbertheory.inverse_mod(other.__x - self.__x, p)