private function _generateInitialResponse($authcid, $authzid)
    {
        $gs2_cbind_flag = 'n,'; // TODO: support channel binding.
        $this->gs2_header = $gs2_cbind_flag . (!empty($authzid)? 'a=' . $authzid : '') . ',';

        // I must generate a client nonce and "save" it for later comparison on second response.
        $this->cnonce = $this->_getCnonce();
        // XXX: in the future, when mandatory and/or optional extensions are defined in any updated RFC,
        // this message can be updated.
        $this->first_message_bare = 'n=' . $authcid . ',r=' . $this->cnonce;
        return $this->gs2_header . $this->first_message_bare;
    }