I heard about a device that had fewer keys say 5 only, one for each finger, but to use it you had to insert your hand into a slot in the machine so it’s complet

I heard about a device that had fewer keys say 5 only, one for each finger, but to use it you had to insert your hand into a slot in the machine so it’s completely invisible from the outside what keys you’re pressing. Presumably in theory they could use one slot for each hand to get to the full 10 digit range.