How to read third party cookie to access password protected pages

The check is handled by post_password_required(), which seems to be incredibly inconvenient to override.

The only way I can think of is early during load:

  1. Check if there is a valid WP cookie (if there we are done).
  2. Check if there is valid external cookie (if not we are done).
  3. If there is then generate a WP cookie value and stuff it into $_COOKIE global in expected place.