Parse the SSH2_MSG_KEXGSS_GROUP message (client mode).

        :param `Message` m: The content of the SSH2_MSG_KEXGSS_GROUP message

def _parse_kexgss_group(self, m):
        """
        Parse the SSH2_MSG_KEXGSS_GROUP message (client mode).

        :param `Message` m: The content of the SSH2_MSG_KEXGSS_GROUP message
        """
        self.p = m.get_mpint()
        self.g = m.get_mpint()
        # reject if p's bit length < 1024 or > 8192
        bitlen = util.bit_length(self.p)
        if (bitlen < 1024) or (bitlen > 8192):
            raise SSHException(
                "Server-generated gex p (don't ask) is out of range "
                "({} bits)".format(bitlen)
            )
        self.transport._log(
            DEBUG, "Got server p ({} bits)".format(bitlen)
        )  # noqa
        self._generate_x()
        # now compute e = g^x mod p
        self.e = pow(self.g, self.x, self.p)
        m = Message()
        m.add_byte(c_MSG_KEXGSS_INIT)
        m.add_string(self.kexgss.ssh_init_sec_context(target=self.gss_host))
        m.add_mpint(self.e)
        self.transport._send_message(m)
        self.transport._expect_packet(
            MSG_KEXGSS_HOSTKEY,
            MSG_KEXGSS_CONTINUE,
            MSG_KEXGSS_COMPLETE,
            MSG_KEXGSS_ERROR,
        )