Just something I considered recently -- TEMPEST attacks are pretty well known; basically you intercept EM leaks from a computer to discover compromising information (i.e. encryption keys, what's being displayed on the screen, what's being typed, etc).
Could a three letter agency potentially create a "TEMPEST Dragnet" of sorts, similar to the NSA's Xkeyscore? Would it be possible to use, say, cell tower antennas to listen to EM emissions and record what's being displayed on thousands of peoples' computer screens, or record everyone's PGP or SSL keys, and create a Xkeyscore-like database?
Theoretically, that would provide them with near total access to peoples' computers -- whoever did it would be able to know exactly what they were doing, both online and offline, and decrypt anything they sent over the Internet. But would it actually be possible?
I'm not an expert on this... can anyone determine the feasibility of such a thing? I suppose the largest concern would be interference -- it would be difficult to tell which emissions belonged to each individual. Would it be possible to filter out or segregate emissions?