I've got an even better solution: Picture a piece of metal, cut in a specific way as to allow metal "tumblers" inside a small cylinder to turn, engaging and disengaging the locks and/or ignition, whereas other pieces of metal, cut differently, would not allow any motion. I know, it sounds far out there, but we should give it a shot.

That doesn't sound very secure at all. I've heard there are little known techniques called "lockpicking" and "rakes" that make such technology practically useless.