Latest update.

crypto/ec/ecp_nistp521.c

@@ -1158,6 +1158,7 @@ static void point_add(felem x3, felem y3, felem z3,
     felem ftmp, ftmp2, ftmp3, ftmp4, ftmp5, ftmp6, x_out, y_out, z_out;
     largefelem tmp, tmp2;
     limb x_equal, y_equal, z1_is_zero, z2_is_zero;
+    limb points_equal;
 
     z1_is_zero = felem_is_zero(z1);
     z2_is_zero = felem_is_zero(z2);
@@ -1242,7 +1243,24 @@ static void point_add(felem x3, felem y3, felem z3,
     felem_scalar64(ftmp5, 2);
     /* ftmp5[i] < 2^61 */
 
-    if (x_equal && y_equal && !z1_is_zero && !z2_is_zero) {
+    /*
+     * The formulae are incorrect if the points are equal, in affine coordinates
+     * (X_1, Y_1) == (X_2, Y_2), so we check for this and do doubling if this
+     * happens.
+     *
+     * We use bitwise operations to avoid potential side-channels introduced by
+     * the short-circuiting behaviour of boolean operators.
+     *
+     * The special case of either point being the point at infinity (z1 and/or
+     * z2 are zero), is handled separately later on in this function, so we
+     * avoid jumping to point_double here in those special cases.
+     *
+     * Notice the comment below on the implications of this branching for timing
+     * leaks and why it is considered practically irrelevant.
+     */
+    points_equal = (x_equal & y_equal & (~z1_is_zero) & (~z2_is_zero));
+
+    if (points_equal) {
         /*
          * This is obviously not constant-time but it will almost-never happen
          * for ECDH / ECDSA. The case where it can happen is during scalar-mult