technical surveillance countermeasures

Technical methods of detecting and avoiding listening and surveillance devices.

Description based on Crypto Museum; more generally, measures against surveillance are known as countermeasures (CM).¹

The most famous example: according to the CIA, in 1952 a technical sweep of the US ambassador's residence in Moscow found a listening device hidden in a wooden carving of the Great Seal, presented by the Soviets in 1945. It was a passive bug with no power source, activated by a signal from outside. According to a declassified NSA document, more than 40 microphones were found in the US embassy in Moscow in 1964.^2,3

A separate issue is protection against unintended electromagnetic emanations from equipment (TEMPEST), from which – as Bell Labs demonstrated in 1943 – enciphered text can be recovered.³