History of TEMPEST

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.

This teletype-encryption machine, known as the Sigaba m134c, was used alongside the Bell Telephone machine found to be leaking signals. The Bell 131-B2 used special one-time tapes to create unbreakable codes.

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.

Call it a TEMPEST in a teletype.

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.

More at: Declassified NSA Document Reveals the Secret History of TEMPEST | Threat Level | Wired.com.