TEMPEST

The study of unintended electromagnetic emissions from equipment, through which information can leak.

Description based on the Crypto Museum's crypto glossary.¹

According to the declassified NSA document “TEMPEST: A Signal Problem”, the problem was discovered in 1943 at Bell Labs: a researcher testing the 131-B2 mixer noticed that every step of the machine was accompanied by a spike on an oscilloscope, from which the plaintext being encrypted could be read. In a test conducted about 80 feet (roughly 24 m) from the Signal Corps cryptocentre, the engineers recovered about 75% of the plaintext from an hour's recording of the emissions within a few hours.²

After the war the matter was forgotten; according to the document, the CIA rediscovered it around 1951 with the same device, and reports of similar threats from behind the Iron Curtain go back to 1953. The problem of “compromising radiation” was given the cover name TEMPEST. The document also describes the discovery in 1964 of more than 40 microphones in the US embassy in Moscow. A related field is technical surveillance countermeasures.^2,3