public function getSPkey()
    {
        $key = null;
        if (isset($this->_sp['privateKey']) && !empty($this->_sp['privateKey'])) {
            $key = $this->_sp['privateKey'];
        } else {
            $keyFile = $this->_paths['cert'].'sp.key';

            if (file_exists($keyFile)) {
                $key = file_get_contents($keyFile);
            }
        }
        return $key;
    }