National Security Agency Declassifies Report on TEMPEST

On April 29, 2008, the National Security Agency declassified a report titled, “TEMPEST: A Signal Problem—The story of the discovery of various compromising radiations from communications and Comseq equipment.” The story begins during World War II when a researcher noticed that every time a Bell Telephone encrypting machine encrypted a letter, a spike appeared on an oscilloscope in a distant part of the lab. Upon examination of the spikes, the researcher found that he could determine the plain text of the message being encrypted by the machine. When alarmed Bell engineers reported this to the military, they were met with skepticism and a challenge to prove the threat. Working across the street from an Army Signal Corps cryptography center in New York City, Bell engineers were able to interpret with about 75 percent accuracy the information that those using the encrypting machine were unwittingly transmitting. Learn more about the birth of TEMPEST technology by reading this recently released document online.