tests/mocks/libraries/encrypt.php

<?php

class Mock_Libraries_Encrypt extends CI_Encrypt {

  // Overide inaccesible protected method
  public function __call($method, $params)
  {
    if (is_callable(array($this, '_'.$method)))
    {
      return call_user_func_array(array($this, '_'.$method), $params);
    }

    throw new BadMethodCallException('Method '.$method.' was not found');
  }
}