SilcAsyncOperation
    silc_connauth_responder(SilcConnAuth connauth,
                            SilcConnAuthGetAuthData get_auth_data,
                            SilcConnAuthCompletion completion,
                            void *context);

    Starts the connection authentication protocol as responder.  The
    `get_auth_data' is called to retrieve the authentication data for
    this connection.  The `completion' will be called after the protocol
    has completed.

    This returns SilcAsyncOperation context which can be used to abort
    the protocol before it is completed.  Returns NULL on error.