Expose point addition

uECC.c

@@ -858,6 +858,52 @@ static void XYcZ_addC(uECC_word_t * X1,
  uECC_vli_set(X1, t7, num_words);
 }
 
+/* Compute P + Q */
+static void EccPoint_add(uECC_word_t * result,
+ const uECC_word_t * P,
+ const uECC_word_t * Q,
+ uECC_Curve curve) {
+
+ // Variables for x, y and z coordinates for P and Q
+ uECC_word_t Px[uECC_MAX_WORDS];
+ uECC_word_t Py[uECC_MAX_WORDS];
+ uECC_word_t Qx[uECC_MAX_WORDS];
+ uECC_word_t Qy[uECC_MAX_WORDS];
+ uECC_word_t z[uECC_MAX_WORDS]
+ wordcount_t num_words = curve->num_words;
+
+ /* Get P = (Px, Py) and Q = (Qx, Qy) */
+ uECC_vli_set(Px, P, num_words);
+ uECC_vli_set(Py, P + num_words, num_words);
+ uECC_vli_set(Qx, Q, num_words);
+ uECC_vli_set(Qy, Q + num_words, num_words);
+
+ // If P = Q, the formula is not the same.
+ if (1 == uECC_vli_equal(P, Q, 2*curve->num_words)) {
+ // Perform point doubling by coordinate
+ // Before call: (Qx , Qy ) = Q and (Px , Py ) = P
+ // After call: (Qx', Qy') = 2Q and (Px', Py') = P'
+ XYcZ_initial_double(Qx, Qy, Px, Py, 0, curve);
+ }
+ else {
+ /* Compute the final 1/Z value */
+ uECC_vli_modSub(z, Qx, Px, curve->p, num_words); /* (Qx - Px) or (x2 - x1) */
+ uECC_vli_modInv(z, z, curve->p, num_words); /* 1 / (x2 - x1) */
+
+ // Perform point addition by coordinate
+ // Before call: (Px , Py ) = P and (Qx , Qy ) = Q
+ // After call: (Px', Py') = P' and (Qx', Qy') = P + Q
+ XYcZ_add(Px, Py, Qx, Qy, curve);
+
+ // Remove the extra (x2-x1)² and (x2-x1)³ of Qx and Qy respectively.
+ apply_z(Qx, Qy, z, curve);
+ }
+
+ // Store result in the return variable
+ uECC_vli_set(result, Qx, num_words);
+ uECC_vli_set(result + num_words, Qy, num_words);
+}
+
 /* result may overlap point. */
 static void EccPoint_mult(uECC_word_t * result,
  const uECC_word_t * point,
@@ -1654,6 +1700,15 @@ void uECC_vli_mmod_fast(uECC_word_t *result, uECC_word_t *product, uECC_Curve cu
 #endif
 }
 
+void uECC_point_add(uECC_word_t * result,
+ const uECC_word_t * P,
+ const uECC_word_t * Q,
+ uECC_Curve curve) {
+
+ // If P = Q, EccPoint_add will take care of applying the right formula for point doubling
+ EccPoint_add(result, P, Q, curve);
+}
+
 void uECC_point_mult(uECC_word_t *result,
  const uECC_word_t *point,
  const uECC_word_t *scalar,

uECC_vli.h

@@ -149,6 +149,14 @@ const uECC_word_t *uECC_curve_b(uECC_Curve curve);
 
 int uECC_valid_point(const uECC_word_t *point, uECC_Curve curve);
 
+/* Addition two points P and Q. result = P + Q
+ Points are represented by the X coordinate followed by the Y coordinate
+ in the same array, both coordinates are curve->num_words long. */
+void uECC_point_add(uECC_word_t * result,
+ const uECC_word_t * P,
+ const uECC_word_t * Q,
+ uECC_Curve curve);
+
 /* Multiplies a point by a scalar. Points are represented by the X coordinate followed by
  the Y coordinate in the same array, both coordinates are curve->num_words long. Note
  that scalar must be curve->num_n_words long (NOT curve->num_words). */