I remembered that some researchers had done this before (http://www.freedom-to-tinker.com/?p=893) and it was a lot easier than replacing the keyboard. The basic idea is that all keybaords have unique sounds, and if you can interpret those sounds, then something like 90% of the keypresses can be determined by just simply listening (and if you use lasers, you don't even need to enter the room, as long as it has a window, and you have line of sight.
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I remembered that some researchers had done this before (http://www.freedom-to-tinker.com/?p=893) and it was a lot easier than replacing the keyboard. The basic idea is that all keybaords have unique sounds, and if you can interpret those sounds, then something like 90% of the keypresses can be determined by just simply listening (and if you use lasers, you don't even need to enter the room, as long as it has a window, and you have line of sight.