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1.9 Tempest

One security concern is having intruders tap into a wire, giving them the ability to eavesdrop and possibly modify or inject messages. Another security concern is electronic emanation, whereby through the magic of physics, the movement of electrons can be measured from a surprising distance away. This means that intruders can sometimes eavesdrop without even needing to physically access the link. The U.S. military Tempest program measures how far away an intruder must be before eavesdropping is impossible. That distance is known as the device's control zone. The control zone is the region that must be physically guarded to keep out intruders that might be attempting to eavesdrop. A well-shielded device will have a smaller control zone. I1 remember being told in 1979 of a tape drive that had a control zone over two miles. Unfortunately, most control zone information is classified, and I2 couldn't get me1 to be very specific about them, other than that they're usually expressed in metric. Since it is necessary to keep intruders away from the control zone, it's certainly better to have something with a control zone on the order of a few inches rather than a few miles (oh yeah, kilometers).

CIA eavesdroppers could not intercept the radio transmissions used by Somali warlord Mohammed Farah Aidid; his radios, intelligence officials explained, were too "low tech."
—Douglas Waller & Evan Thomas, Newsweek, October 10, 1994, page 32

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