It is quite old news that you can pick up the keystrokes from a target PC with a USB dongle, a PS/2 connector, or a bit of keystroke logging software, and it is also a few years since the majority of wireless keyboards were decrypted, meaning that it became trivial to eavesdrop on what a person was writing onscreen by looking at the radio transmissions from the keyboard. There is also a technique called “TEMPEST analysis” which records and displays what you see on your screen onto another PC.
Tempest is a sly reference to the storm of different electromagnetic (EM) radiation that comes out of a typical computer system. You see, every time an electron moves, there is an electric and magnetic feild that results. For a room temperature metal object, there is radio emmission in the millimetre range. (This is now being used for “see through your clothes” imaging cameras. I’ll post about that another day.) Warm things give off infra-red radiation. Very warm things glow.
Beyond that, to move the electrons around your PC and send signals, the electrons move back and forth, and give off some EM radio waves. These are longer wavelength, and travel through walls and air just fine, though they are very weak. The wires on your keyboard act just great as an aerial, so some people have been working on methods for reading these signals.
The videos below show this being done.
Compromising Electromagnetic Emanations of Keyboards Experiment 1/2 from Martin Vuagnoux on Vimeo.
Compromising Electromagnetic Emanations of Keyboards Experiment 2/2 from Martin Vuagnoux on Vimeo.
This isn’t something the “average person” should worry about, is what you normally hear when something like this is revealed, but who among us is average? As to the answer, well, currently there doesn’t seem to be one. Fortunately the range is short, so keep an eye out for people pointing satellite dishes at your house, and you should be fine. The other option is to make it a little harder for anyone trying. Buy a good keyboard, with a shielded cable, and make sure your PC case is done up properly. This will reduce the EM that is able to escape, and so make reading it even harder.