Connecting your computer to the Internet gives would-be spies an obvious entry point to your machine.
But other ways exist to snoop.
Because even computers that aren't connected to the Internet broadcast their activity in the form of electromagnetic radiation.
Basically your computer is full of transistors.
And they're switching current from high to low depending if it is a zero or a one of the bit that they're trying to execute."
Alenka Zajic, an electrical engineer at Georgia Tech.
"When you do that you're creating a voltage fluctuation and current fluctuation.
And that basically creates electromagnetic field."
By hooking an antenna and receiver up to a laptop, Zajic and her colleagues were able to log the keystrokes of a computer in the next room, by measuring exceedingly tiny fluctuations in the computer's radiation.
The same technique can reveal which programs you're using, too.
Every one of them has a different signature in electromagnetic fields.
So I can tell which application you opened by looking into the spectrum.
The researchers quantified the signal available to eavesdroppers in a recent paper,presented at the IEEE/ACM International Symposium on Microarchitecture in the U.K.
Of course, real spies at the NSA and the CIA probably already know about this trick, she says.
But by alerting developers to the problem, it might be possible to mask these electromagnetic leaks.
And keep your computer's activity to yourself.