public function getRequestSignature()
    {
        if (empty($this->requestSignature)) {

            $string = $this->getApiId()
                . $this->getTimeStamp()
                . $this->getNonce()
                . $this->getDataString();

            $this->requestSignature = hash_hmac('sha1', $string, $this->getService()->getApiSecret());
        }

        return $this->requestSignature;
    }