cross the wire
    from a client. If specified, it should be a callable which receives a
    single argument - the bytes of configuration data received across the
    network - and it should return either ``None``, to indicate that the
    passed in bytes could not be verified and should be discarded, or a
    byte string which is then passed to the configuration machinery as
    normal. Note that you can return transformed bytes, e.g. by decrypting
    the bytes passed in.
    c