public static function create($length = 6, $num = 32) {
    $card['list'] = array();
    for($i = 0; $i < $num; $i++) {
      $card['list'][$i]   = phpsecRand::str($length);
      $card['usable'][$i] = true;
    }

    $card = self::save($card);

    return $card['id'];
  }