B.2. The TEMPEST Program
In the late 1950s, the U.S. government established the TEMPEST program to attack the emanations problem. TEMPEST has become an umbrella name for the technology that contains or suppresses signal emanations from electronic equipment, and for the investigations and studies of these emanations. An unclassified government publication describes TEMPEST emanations as "unintentional, intelligence-bearing . . . signals which might disclose sensitive information transmitted, received, handled, or otherwise processed by an information processing system."
In 1974, government and industry began to work more closely together through the Industrial TEMPEST Program (ITP). ITP was founded with the following objectives:
The idea of ITP was to standardize TEMPEST requirements and technologies, and to encourage vendors to develop and test off-the-shelf TEMPEST equipment that the government could buy. The early TEMPEST products were typically standalone computer systems. Today, TEMPEST versions of most types of computer products have become available, and the actual certification efforts are supervised by the National Security Agency.
Because they're built to control electromagnetic emanations, TEMPEST products are larger, heavier, and more expensive than comparable commercial products. TEMPEST products control emanations either by shielding the signalsbuilding a container around them so they can't emanate beyond the containeror by suppressing the signalsengineering the equipment so signals don't emanate at all. (Sometimes, a product combines both methods.)
B.2.1. Faraday Screens
A shield attenuates electromagnetic signals, conducting them to ground before they can escape. A shield, which can be as small as a cable casing or as large as an entire building, is constructed in such a way that signals can't emanate outside it. This shielding to stop the flow of electromagnetic radiation is commonly called a Faraday screen.
The simplest but most expensive shield approach is to install regular computer equipment in a shielded room that provides special protection against electromagnetic leaks. Smaller shields or containment devices serve the same purpose as a shielded room; shields can be constructed for computers, workstations, peripheral devices, circuit boards, and inside wiring. Modern PCs and monitors are usually coated with radiation-dampening materials that help to prevent emanations.
The containment approach to TEMPEST security resembles the steps that were once taken to protect equipment and buildings from electromagnetic pulsewhich is a product of nuclear explosions in the atmosphere. This in fact may have contributed to the secrecy that surrounded TEMPEST. Explaining how to avoid an influx of nuclear EMP also explained some things about preventing electromagnetic leakage of information, and discussing secrets about one technology may have inadvertently given away some information about the other. In short strokes, electromagnet containment was tricky. Every cable and pipe that entered or left the screened portion of a facility required special treatment. But trying to curb the electromagnetic effects of the bomb was no easy task either.
B.2.2. Source Suppression
Some TEMPEST products use a different engineering approach. With source suppression, products are engineered in such a way that compromising signals are suppressed at the source. Sometimes this is done by adding confusing or spurious signals.
The source suppression approach can be technologically more difficult than the shield approach, but it's more foolproof. Its effectiveness doesn't depend on the proper use of the equipment by human beings, and it tends to be a more appropriate approach for products installed in an office environment.
Recent advances in microchip design have led to chips that do not radiate as much as their predecessors. Some of the best source suppression may take place in modern software. The hard edges of dark letters against white pages generate a great deal of square waves, which throw off electromagnetic radiation much more readily than gentle transitions. New techniques produce softer edges on letters, which decrease the sharp transitions and their radiations. Of course, a similar effect may be achieved by darkening the background. You may have seen this effect on some of the more hardcore cracker sites.