Or they may be induced on nearby conductors like signal lines, power lines, telephone lines, or water pipes and be conducted along these paths for some distance—and here we may be talking of a mile or more.
When these emissions can be intercepted and recorded, it is frequently possible to analyze them and recover the intelligence that was being processed by the source equipment. The phenomenon affects not only cipher machines but any information-processing equipment—teletypewriters, duplicating equipment, intercomms, facsimile, computers—you name it. But it has special significance for cryptomachines because it may reveal not only the plain texts of individual messages being processed but also that carefully guarded information about the internal machine processes. Thus, conceivably, the machine could be radiating information which could lead to the reconstruction of our daily changing keying variables—and from a Comsec viewpoint, that is absolutely the worst thing that can happen to us. This problem of compromising radiation we have given the covername TEMPEST.
Discovery by Bell Lab
Now, let's go back to the beginning. During World War II, the backbone systems for Army and Navy secure teletypewriter communications were one-time tapes and the primitive crypto-equipment SIGTOT. For encrypting, the Services used a Bell-telephone mixing device, called a 131-B2. When one of these mixers was being tested in a Bell laboratory, a researcher noticed, quite by accident, that each time the machine stepped, a spike appeared on an oscilloscope in a distant part of the lab. After he examined these spikes more carefully, he found that he could read the plain text of the message being enciphered by the machine!
Bell Telephone faced a dilemma. They had sold the equipment to the military with the assurance that it was secure, but it wasn't. The only thing they could do was to tell the Signal Corps about it, which they did. There they met the charter members of a club of skeptics who could not believe that these tiny pips could really be exploited under practical field conditions. They are alleged to have said something like: "Don't you realize there's a war on? We can't bring our cryptographic operations to a screeching halt based on a dubious and esoteric laboratory phenomenon. If this is really dangerous, prove it." So, the Bell engineers were placed in a building on Varick Street in New York. Across the street and about 80 feet away was Signal Corps' Varick Street cryptocenter. The engineers recorded signals for about an hour. Three or four hours later, they produced about 75% of the plain text that was being processed—a fast performance, by the way, that has rarely been equalled.
The Signal Corps was impressed by this display and directed Bell Labs to explore this phenomenon in depth and provide modifications to the 131-B2 mixer to suppress the danger. In a matter of six months or so, Bell Labs had identified three separate phenomena and suggested three basic suppression measures:
- Shielding (for radiation through space, and magnetic fields)
- Filtering (for conducted signal on power lines, signal lines, etc.)
- Masking (for either space-radiated or conducted signals, but mostly for space)
Bell Labs went ahead and modified a mixer, calling it the 131-A-1. In it they used both shielding and filtering techniques. Signal Corps took one look at it and turned thumbs down. The trouble was, to contain the offending signals, Bell had to virtually encapsulate the machine. Instead of a modification kit that could be sent to the field, the machines would have to be sent back and rehabilitated. The encapsulation caused problems of heat dissipation, made maintenance extremely difficult, and hampered operations by limiting access to the various controls.
Instead of buying this monster, the Signal Corps resorted to the only other solution they could think of. They went out and warned commanders of the problem, advised them to control a zone about 100 feet in diameter around their communications center to prevent covert interception, and let it go at that. And the cryptologic community as a whole let it go at that for the next seven years or so. The war ended; most of the people involved went back to civilian life; the files were retired, dispersed, and destroyed. The whole problem was, apparently, forgotten. Then, in 1951, the problem was, for all practical purposes, rediscovered by CIA when they were toying with the same old 131-B2 mixer. They reported having read plain text about a quarter mile down the signal line and asked if we were interested. Of course, we were. Some power line and signal line filters were built and immediately installed on these equipments and they did the job pretty well as far as conducted signals were concerned. Space radiation continued unabated, however, and the first of many "radiation" policies was issued in the form of a letter from AFSA to all Sigint activities, requiring them to:
- Control a zone 200 feet in all directions around their cryptocenters, or
- Operate at least 10 TTY devices simultaneously (the idea of masking; putting out such a profusion of signals that interception and analysis would be difficult). or
- Get a waiver based on operational necessity.
