This is a very old idea. I recall that in the 1987 book "Spycatcher" Peter Wright wrote about listening to typewriter keystrokes through a microphone bug in som

This is a very old idea. I recall that in the 1987 book "Spycatcher" Peter Wright wrote about listening to typewriter keystrokes through a microphone bug in some embessy. He was the head scientist in British intelligence and the book caused a bit of a stir at the time, being the subject of censorship attempts. I think this audio bugging of keystroakes may go back to the 1960's.