Considered an attack, a type of surveillance available against Cathode Ray Tube (CRT) monitors to view what is being displayed on the screen. Can be done with fairly simple equipment. Information about what is on the screen can be gathered from radiated or conducted electromagnetic emissions. Radiated signals from monitors can be detected in the VHF region, these signals may pass though walls but are considerably attenuated. Low frequency magnetic fields from the line and scan coils may be detected close to the monitor. Liquid Crystal Display (LCD) monitors are not vulnerable to this attack as they do not use large magnetic fields and high voltages in parts of the circuitry carring picture information.
Tempest (aka Van Eck freaking) is harder than it used to be due to higher refresh rates and resolutions of modern monitors and EMC regulations. Computers have proliferated, there is usually at least ten other monitors in close promixity to the one you want to watch with the signals mixing together.
The technique has been known since at least 1984.
Keypress information does not normally leak in detectable ways.
TEMPEST is a codeword of the US government for a number of standards in which equipment has to be provided with to evade unintentional signals to a minimum. In practice, the word is used as a description for various methods of espionage using electromagnetic "leakradiation."
- The Complete, Unofficial TEMPEST Information Page
- An archive with TEMPEST documents
- [http://www.cl.cam.ac.uk/TechReports/UCAM-CL-TR-577.pdf Markus Kuhn's PHD thesis on the eavesdropping risks of computer