Three researchers have developed a method of
monitoring keystrokes by listening to the sound of the keyboard. Impressively (and perhaps frighteningly), their method does not require one to type a training text first; just drop a microphone into earshot of the keys, let it listen, and within 15 minutes or so of typing, the algorithms will have guessed which sounds correspond to which keys, by using the statistic properties of the English language and a machine-learning technique. In retrospect, it seems quite obvious (not counting the details); it would be surprising if intelligence agencies hadn't had something of this sort for some time.