def fill_missing(self):
        """
        We do not want TLSServerKeyExchange.build() to overload and recompute
        things every time it is called. This method can be called specifically
        to have things filled in a smart fashion.

        XXX We should account for the point_format (before 'point' filling).
        """
        s = self.tls_session

        if self.curve_type is None:
            self.curve_type = _tls_ec_curve_types["named_curve"]

        if self.named_curve is None:
            curve = ec.SECP256R1()
            s.server_kx_privkey = ec.generate_private_key(curve,
                                                          default_backend())
            self.named_curve = next((cid for cid, name in six.iteritems(_tls_named_curves)  # noqa: E501
                                     if name == curve.name), 0)
        else:
            curve_name = _tls_named_curves.get(self.named_curve)
            if curve_name is None:
                # this fallback is arguable
                curve = ec.SECP256R1()
            else:
                curve_cls = ec._CURVE_TYPES.get(curve_name)
                if curve_cls is None:
                    # this fallback is arguable
                    curve = ec.SECP256R1()
                else:
                    curve = curve_cls()
            s.server_kx_privkey = ec.generate_private_key(curve,
                                                          default_backend())

        if self.point is None:
            pubkey = s.server_kx_privkey.public_key()
            try:
                # cryptography >= 2.5
                self.point = pubkey.public_bytes(
                    serialization.Encoding.X962,
                    serialization.PublicFormat.UncompressedPoint
                )
            except TypeError:
                # older versions
                self.key_exchange = pubkey.public_numbers().encode_point()
        # else, we assume that the user wrote the server_kx_privkey by himself
        if self.pointlen is None:
            self.pointlen = len(self.point)

        if not s.client_kx_ecdh_params:
            s.client_kx_ecdh_params = curve