'Get channel binding data for current connection.  Raise ValueError
if the requested `cb_type` is not supported.  Return bytes of the data
or None if the data is not available (e.g. before the handshake).'

def get_channel_binding(self, cb_type='tls-unique'):

    if (cb_type not in CHANNEL_BINDING_TYPES):
        raise ValueError('Unsupported    channel    binding    type')
    if (cb_type != 'tls-unique'):
        raise NotImplementedError('{0}    channel    binding    type    not    implemented'.format(cb_type))
    if (self._sslobj is None):
        return None
    return self._sslobj.tls_unique_cb()
