protected function _HMAC_SHA1($key, $data, $raw_output = FALSE)
    {
        if (strlen($key) > 64) {
            $key = sha1($key, TRUE);
        }

        if (strlen($key) < 64) {
            $key = str_pad($key, 64, chr(0));
        }

        $k_ipad = substr($key, 0, 64) ^ str_repeat(chr(0x36), 64);
        $k_opad = substr($key, 0, 64) ^ str_repeat(chr(0x5C), 64);

        $inner  = pack('H40', sha1($k_ipad . $data));
        $digest = sha1($k_opad . $inner, $raw_output);

         return $digest;
     }