def kids(self):
        """
        Return a list of key IDs. Note that this list may be shorter then
        the list of keys. The reason might be that there are some keys with
        no key ID.
        
        :return: A list of all the key IDs that exists in this bundle 
        """
        self._uptodate()
        return [key.kid for key in self._keys if key.kid != ""]