Every device which uses electricity propagates an electromagnetic (EM) field. Depending upon the unit's circuitry and construction, this field can be very intense or quite weak. Through the use of specialized equipment, a determined eavesdropper could "analyze the emitted radiation generated" and eventually reconstruct it, providing the eavesdropper with an exact replica of what the device was doing at a particular point in time (Garfinkel and Spafford 1991, 392).
The U.S. Federal Communication Commission (FCC) has published EM emanation guidelines which U.S. manufacturers must comply with when creating a product for the U.S. market. These guidelines attempt to limit the amount of EM radiation an appliance can emanate. While these guidelines usually keep annoying interference away from our household receivers such as VCRs, radios, and televisions, the guidelines are insufficient when it comes to EM eavesdropping.
One of the easier appliances to eavesdrop upon is a computer's CRT tube. A computer monitor requires horizontal and vertical synchronization signals in order to properly place a picture on the screen. In addition, it also needs the signal that contains what should be painted on the screen. The horizontal and vertical sync signals transmit on low frequencies and they do not travel very far. However, the "video signals broadcast from the computer are high frequency in nature and can travel great distances just like a miniature radio transmitter" (Schwartau and Death 1991, 19). All an EM eavesdropper must do is reconstruct the horizontal and vertical sync signals and he/she can reproduce what is being displayed on a victim's computer screen. The methods for restoring these signals are widely published in magazines such as Radio Electronics and by manufacturers of TV descrambling/enhancement units.
Unfortunately, restoring video signals is not the only way the EM snoops can watch your every move. Another method entails the monitoring of the signals your keyboard creates. A keyboard and its cable acts as an antenna "amplif[ing] and radiat[ing] a unique electromagnetic signature corresponding to each individual keystroke" (Schwartau and Death 1991, 20). The EM eavesdropper could easily listen to what keys are being pressed and eventually decipher encryption keys, system passwords, or even make exact copies of what were thought to be private messages. This type of eavesdropping, when coupled with CRT video signal reconstruction, can be quite a powerful tool in an attacker's hands. Of course, these two methods are not the only possible means of EM eavesdropping, just the easiest to reconstruct. Other methods include listening to computer central processing units, random access memories, network transmissions, and secondary processors by using natural antennas such as power lines, fire suppression sprinklers, networking cables, plumbing and even door knobs.
Obviously, EM eavesdropping and EM weapons can compromise even the most ethereally secure computer system. Thankfully, these weapons are not in use by the average Joe Smith down the block. While EM eavesdropping equipment is not too expensive (less than US$5000), the attacker must possess the technical expertise needed to construct and use such a system. In terms of technical knowledge necessary, the same can be said for electromagnetic pulse transformers, however, they are even more costly than high-energy radio frequency guns. Until these weapons can be constructed and used by more than 1% of the population, the average computer user should not worry about their existence unless they fear persecution by a resource-rich entity.