My understanding is that a Snowden-leaked 2008 NSA internal catalog contains airgap-hopping exploits by the dozen, and that the existence of successful attacks on air gapped networks (like Stuxnet) are documented and not controversial.
This understanding comes in large measure from a casual reading of Bruce Schneier's blog. I am not an security expert and my "you don't understand what you're talking about" reflexes are firing.
But moving to areas where I know more, I think e.g. if I tried writing a program to take as input the sounds of someone typing and output the letters they typed, I'd have a decent chance of success.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
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