x1 is a key fitting/releasing/opening/unlocking lock x2, and having relevant properties x3.
Also x2 fastener, mechanism; code key (= termifckiku, kiktermifra); x3 is dependent on the type of key, but are those form properties of the key that enable it to serve the function of opening the lock - in the case of a metal key to a padlock, for example, this would be the shaft and teeth. See also stela.