Acoustic cryptanalysis

Someone on the PGP users mailing list posted a link to a proof-of-concept of
acoustic cryptanalysis.

I can remember listening to my Apple II doing computations and having some idea what it was doing by the sounds it made. If I wasn't listening to the CPU, I was listening to the disks. There were things that I did so often, that I knew just what they sounded like and what to listen for to know when it was finished.

What these folks are doing is using the sounds the CPU makes to attack the security of the system. It seems obvious now that I read it, but I wouldn't have thought of it before. Very interesting stuff.