protected function _signParameters(array $parameters, $key) {
        $algorithm = $this->options['SignatureMethod'];
        $stringToSign = null;
        if (2 === $this->options['SignatureVersion']) {
            $stringToSign = $this->_calculateStringToSignV2($parameters);
//            var_dump($stringToSign);
        } else {
            throw new Exception("Invalid Signature Version specified");
        }
        return $this->_sign($stringToSign, $key, $algorithm);
    }