[P256] add a point_add function

Crypto/PubKey/ECC/P256.hs

@@ -38,9 +38,6 @@ newtype Scalar = Scalar SecureBytes
 data Point = Point !Bytes !Bytes
     deriving (Show,Eq)
 
-publicKeySize :: Int
-publicKeySize = 32
-
 scalarSize :: Int
 scalarSize = 32
 
@@ -65,6 +62,12 @@ toPoint :: Scalar -> Point
 toPoint s = withNewPoint $ \px py -> withScalar s $ \p ->
     ccryptonite_p256_basepoint_mul p px py
 
+-- | Add a point to another point
+pointAdd :: Point -> Point -> Point
+pointAdd a b = withNewPoint $ \dx dy ->
+    withPoint a $ \ax ay -> withPoint b $ \bx by ->
+        ccryptonite_p256e_point_add ax ay bx by dx dy
+
 -- | Multiply a point by a scalar
 pointMul :: Scalar -> Point -> Point
 pointMul scalar p = withNewPoint $ \dx dy ->
@@ -194,6 +197,12 @@ foreign import ccall "cryptonite_p256_base_point_mul"
     ccryptonite_p256_basepoint_mul :: Ptr P256Scalar
                                    -> Ptr P256X -> Ptr P256Y
                                    -> IO ()
+
+foreign import ccall "cryptonite_p256e_point_add"
+    ccryptonite_p256e_point_add :: Ptr P256X -> Ptr P256Y
+                                -> Ptr P256X -> Ptr P256Y
+                                -> Ptr P256X -> Ptr P256Y
+                                -> IO ()
 foreign import ccall "cryptonite_p256_point_mul"
     ccryptonite_p256_point_mul :: Ptr P256Scalar
                                -> Ptr P256X -> Ptr P256Y

cbits/p256/p256_ec.c

@@ -1277,3 +1277,29 @@ void cryptonite_p256_points_mul_vartime(
   from_montgomery(out_x, px);
   from_montgomery(out_y, py);
 }
+
+/* this function is not part of the original source
+   add 2 points together. so far untested.
+   probably vartime, as it use point_add_or_double_vartime
+ */
+void cryptonite_p256e_point_add(
+    const cryptonite_p256_int *in_x1, const cryptonite_p256_int *in_y1,
+    const cryptonite_p256_int *in_x2, const cryptonite_p256_int *in_y2,
+    cryptonite_p256_int *out_x, cryptonite_p256_int *out_y)
+{
+    felem x1, y1, z1, x2, y2, z2, px1, py1, px2, py2;
+    const cryptonite_p256_int one = P256_ONE;
+
+    to_montgomery(px1, in_x1);
+    to_montgomery(py1, in_y1);
+    to_montgomery(px2, in_x2);
+    to_montgomery(py2, in_y2);
+
+    scalar_mult(x1, y1, z1, px1, py1, &one);
+    scalar_mult(x2, y2, z2, px2, py2, &one);
+    point_add_or_double_vartime(x1, y1, z1, x1, y1, z1, x2, y2, z2);
+
+    point_to_affine(px1, py1, x1, y1, z1);
+    from_montgomery(out_x, px1);
+    from_montgomery(out_y, py1);
+}