Article at IEEE Spectrum about a new book that looks at how the Soviets embedded a transmitter into the USA embassy typewriters.
[A] recent book authored by Eric Haseltine, titled The Spy in Moscow Station (Macmillan, 2019), is a true story of bug hunting that should be of interest to all. It recounts a lengthy struggle by Charles Gandy, an electrical engineer at the United States’ National Security Agency, to uncover an elaborate and ingenious scheme by Soviet engineers to intercept communications in the American embassy in Moscow.
After tens of thousands of fruitless X-rays, a technician noticed a small coil of wire inside the on/off switch of an IBM Selectric typewriter. Gandy believed that this coil was acting as a step-down transformer to supply lower-voltage power to something within the typewriter. Eventually he uncovered a series of modifications that had been concealed so expertly that they had previously defied detection.
Other components of the typewriters, such as springs and screws, had been repurposed to deliver power to the hidden circuits and to act as antennas. Keystroke information was stored and sent in encrypted burst transmissions that hopped across multiple frequencies.