B.1. The Problem of Emanations
All electronic equipmenthair dryers, typewriters, telephones, microwave ovens, personal computers, laptops, and personal digital assistantsemits electrical and electromagnetic radiation through the air or through conductors. It has long been recognized that such emanations can cause interference to radio and television reception. In addition, concerns about possible health hazards associated with emanations have led to increased shielding of monitors (see the sidebar "Hazardous to Your Health?").
Hazardous to Your Health?
In the security world, fears about uncontrolled electromagnetic emissions focus on the interception and deciphering of these emissions by intruders. Of more immediate concern to most of us may be the growing evidence that emissions are physically dangerous as well. Since 1977, studies have looked at the health consequences of exposure to three types of fields: VLF (very low frequency, such as those given off by a computer's horizontal-scan frequency), ELF (extremely low frequency, such as those given off by a computer's vertical-scan frequency), and 60-Hz AC (alternating current, such as those given off by power lines and computer monitors' power transformers):
In 1979, epidemiologists Nancy Wertheimer and Ed Leeper reported on an investigation showing that children living in Denver homes located near high-current electric wires died of cancer at twice the expected rate.
In 1988, Kaiser Permanente researchers reported that of 1,583 case-controlled women who attended their clinics, women who worked with VDTs for more than 20 hours a week suffered miscarriages at a rate 80 percent higher than women performing similar work without VDTs.
In 1989, Johns Hopkins epidemiologists reported that the risk of leukemia for New York Telephone Company cable splicers, who work close to power lines, was seven times greater than that of other company workers.
In December of 1990, the Environmental Protection Agency reported that environmental studies have shown "a causal link [between power lines and] EM fields and certain forms of site-specific cancer." Large-scale studies have since been undertaken, but the results have received more attention in Europe than in the United States. Nevertheless, monitor shielding has improved, to the point that they greatly limit exposure to emissions. LCD monitors may create radio frequency (RF) energy, but they produce a tiny fraction of the magnetic flux on which CRT based monitors relied. Nevertheless, there is a good chance that the safety of VDTs and related equipment may be a recurring concern in the future, along with the safety of cell phones and other radio frequency-emitting apparatus. The topic already occupies much attention among alternative medical providers, who usually advocate long walks in tree-filled areas as a curative. The inertia of the established interests, however, has kept this topic on the fringes.
In the past, TEMPEST has focused almost exclusively on the protection of classified information. As people discover that TEMPEST-type shielding can protect people as well as data, the TEMPEST technology described in this appendix may get a new lease on life, providing human safety as well as data security.
As early as the 1950s, government and industry observers became concerned about the possibility that electronic eavesdroppers could intercept emanations to decipher them, or could obtain information about the signals used inside the equipment, and use this information to reconstruct the data being processed. They speculated that eavesdroppers could breach security even some distance from the equipment.
Studies of signal interception and decoding have borne out these speculations. It turns out that with virtually no risk of detection, eavesdroppers using relatively unsophisticated equipment can intercept and decipher signals from an electronic source. Modern listening devices allow an eavesdropper to detect emissions and reproduce data streams or video screen imagesfor example, to read the computer display screens on the desktops in a remote building. Although opinions about the ease of interception vary, in theory a modified TV could do the job if its sweep circuits were adjusted to match common computer monitor frequencies. The components needed to perform such a penetration are garden variety. Some early computer terminals broadcast signals so strong that an ordinary television set, placed beside the terminal, could broadcast everything displayed on the terminal's screen.