> Naively I would have thought every keystroke sounds the same Even if the keystroke sounds were identical at source, the acoustic environment between the key

> Naively I would have thought every keystroke sounds the same Even if the keystroke sounds were identical at source, the acoustic environment between the key and mics (key position, shape of the surrounding keyboard area, position of hands, screen etc.) adds its own unique reverb signature too. Speaking of mics - a lot of laptops have stereo mics. That's enough to start locating the acoustic source spatially by phase, in addition to the other characteristics.