It was 1943, and an engineer with Bell Telephone was working on one of the U.S. government's most sensitive and important pieces of wartime
machinery, a Bell Telephone model 131-B2. It was a top secret encrypted teletype terminal used by the Army and Navy to transmit wartime communications
that could defy German and Japanese cryptanalysis.
Then he noticed something odd.
Far across the lab, a freestanding oscilloscope had developed a habit of spiking every time the teletype encrypted a letter. Upon closer inspection,
the spikes could actually be translated into the plain message the machine was processing. Though he likely didn't know it at the time, the engineer
had just discovered that all information processing machines send their secrets into the electromagnetic ether.
This story of how the United States first learned about the fundamental security vulnerability called "compromising emanations" is revealed for the
first time in a newly-declassified 1972 paper TEMPEST: A Signal Problem (.pdf), from the National Security Agency's secret in-house journal
Cryptologic Spectrum.
I found this story and the PDF just fascination. It reminded me of a term paper on Wim van Ek, I had to do in high school. In the late 80s, Ek
published thee first unclassified technical analysis of the security risks of emanations monitors. He was able to eavesdrop hundreds of metres using
just $15 worth of equipment and a TV.
This is a REALLY good read. Enjoy!
