How IBM Selectric Keyloggers Spied on US Diplomats During Cold War

A National Security Agency memo that recently resurfaced a few years after it was first published contains a detailed analysis of what very possibly was the world's first keylogger

The electromechanical implants were nothing short of an engineering marvel. The highly miniaturized series of circuits were stuffed into a metal bar that ran the length of the typewriter, making them invisible to the naked eye. The implant, which could only be seen using X-ray equipment, recorded the precise location of the little ball Selectric typewriters used to imprint a character on paper.