protected function _determineVersion(): int
    {
        // if extensions are present
        if (count($this->_extensions)) {
            return self::VERSION_3;
        }
        // if UniqueIdentifier is present
        if (isset($this->_issuerUniqueID) || isset($this->_subjectUniqueID)) {
            return self::VERSION_2;
        }
        return self::VERSION_1;
    }