private function createNonceStr($length = ) {

    $chars = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";

    $str = "";

    for ($i = ; $i < $length; $i++) {

      $str .= substr($chars, mt_rand(, strlen($chars) - ), );

    }

    return $str;

  }