Page:TEMPEST A Signal Problem.djvu/3

The Sigint community conformed as best it could; and, in some instances, general-service communicators adopted similar rules. The figure of 200 feet, by the way, was quite arbitrary. It had not been determined because we had hard evidence that, beyond that distance, interception was impractical; rather, it was the largest security zone we believed the majority of stations could reasonably maintain, and we knew that, with instrumentation then available, exploitation at that range would, at best, be exceedingly difficult.

At the same time that we were trying to cope with the 131-B2 mixer, we began to examine every other cipher machine. Everything tested radiated, and radiated rather prolifically. With rotor machines, the voltage on their power lines tended to fluctuate as a function of the number of rotors moving, and so a fourth phenomenon, called power line modulation, was discovered.

Progress in examining the machines and developing suppression measures was very slow. By 1955, however, a number of possible techniques for suppressing the phenomena had been tried. Filtering techniques were refined somewhat; teletypewriter devices were modified so that all relays operated at once and only a single spike was produced with each character, instead of five smaller spikes, representing each baud, but the size of the spike hanged with each character produced, and the analysts could still read it quickly. A "balanced" ten-wire system was tried which would cause each radiated signal to appear identical, but to achieve and maintain such balanced proved impractical. Hydraulic techniques—to replace the electrical—were tried and abandoned, and experiments were made with different types of batteries and motor generators, in attempts to lick the power-line problem. None was very successful.

During this period, the business of discovering new TEMPEST threats, or refining techniques and instrumentation for detecting, recording, and analyzing these signals, progressed more swiftly than the asrt of suppressing them. Perhaps the attack is more exciting than the defense—something more glamorous about finding a way to read one of these signals than going through the drudgery necessary to suppress that whacking great spike first seen in 1943. At any rate, when they turned over the next rock, they found the acoustic problem under it. Phenomenon No. 5.

Acoustics

We found that most acoustics emanations are difficult to exploit if the microphonic device is outside of the room containing the source equipment; even a piece of paper inserted between, say, an offending keyboard and a pick-up device, is usually enough to prevent sufficiently accurate recordings to permit exploitation. Shotgun microphones—the kind used to pick up a quarterback's signals in a huddle—and large parabolic antennae are effective at hundreds of feet—if there is a direct shot at the equipment. The acoustic threat is, therefore, confined to those installations where the covert interceptor can get some kind of microphone—such as an ordinary telephone that has been bugged or left off the hook—in the same room with the information-processing device. We also discovered that, when the room is "sound-proofed" with ordinary acoustic tile, the job of exploitation is easier because the sound-proofing cuts down reflected and reverberating sound, providing clearer signals. A disturbing discovery was that ordinary microphones, probably planted to pick up conversations in a cryptocenter, could detect machine sounds with enough fidelity to permit exploitation. And such microphones were discovered in Prague, Budapest, Warsaw and, of course, Moscow.

Seismics

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