Abstract
In the middle of the Second World War, the US Army Signal Corp suddenly realised they had a problem with their encrypted communications. The Bell Telephone machine, used to encode signals using the army's rotor-based key generator, would create a spike on an osciloscope in a remote part of the lab every time the machine changed its output. Worse, the spikes depended on the signal passing through, making it possible to recover the plain text being fed to the encryption machine. Officers were at first hesitant to halt the use of the system. But they allowed an experiment that placed Bell engineers 25m away, on the other side of the street in New York from the office that housed one of the Signal Corp's cryptocentres. They listened for an hour and, within four hours, demonstrated they were able to recover three-quarters of the messages being sent out. Bell spent the next six months fixing the machinery to stop it spewing out high levels of electromagnetic interference (EMI).
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