I never thought of it as simply cryptography. Now I can definately understand how this could work. Think about how simple it would be to put one of the thousands of inexpensive and easy to get ahold of miniature audio transmiters and how easy it would be to stick it behind someone's monitor, or under their keyboard. If they can decrypt 256bit encrypted data ill bet they can figure out what you'r typing. Cuz the spacebar is REDICULOUSLY obvious you have your word separator right up front so that makes it much simpler. And I'm sure every keyboard in the world has certain keys that have quirks and make a very different noise from the rest, just knowing that is a large step, I can think of lots of things like that that in combination with anti-cryptography tactics could most definately make it possible.