Alan Turing · Chapter 8

Delilah and the Bell Labs Tea-Time Encounter

Portable Speech Scrambling at Hanslope Park and Meeting Claude Shannon in Manhattan

Boarding the fast troopship RMS Queen Elizabeth on the Clyde in November 1942, Alan Turing sailed unescorted across the U-boat-patrolled North Atlantic to New York. Four months earlier, in July 1942 at Bletchley Park, he had invented a hand cryptanalytic method called Turingery. That method deduced the cam settings of the twelve-wheel German Lorenz SZ40/42 teleprinter cipher (Tunny). Meanwhile, Hut 8's break into four-rotor Atlantic U-boat traffic was finally underway. Now the British Admiralty and Foreign Office needed their chief cryptological theorist in the United States. His mission was to coordinate Bombe production with United States Navy cryptanalysts in Washington and inspect the top-secret transatlantic voice scrambler being built in Manhattan.

From January through March 1943, Turing held a security pass to Bell Telephone Laboratories at 463 West Street overlooking the Hudson River. There he audited the fifty-five-ton speech encryption terminal known to the Americans as Project X-61753 or SIGSALY, and to the British as the Green Hornet. Filling forty steel racks and drawing thirty kilowatts of power, SIGSALY achieved unbreakable security by slicing voice into quantized vocoder bands. It then added pure thermal noise stamped onto synchronized sixteen-inch vinyl phonograph records.

Every afternoon at teatime in the Bell Labs cafeteria, Turing sat across a table from Bell Labs mathematician Claude Shannon. Strict wartime compartmentalization forbade Turing from mentioning Bletchley Park's Enigma Bombes or Banburismus. Equally, Shannon could not discuss his own mathematical cryptanalysis memos. Instead, the two founders of modern computing and information theory spent their afternoons discussing Universal Turing Machines and symbolic logic. Turing showed Shannon his 1936 paper On Computable Numbers, and the two men debated whether an electronic brain could imitate the human cortex. During one conversation, Turing remarked that his goal was not merely to program a machine to play chess. Rather, he wanted to feed a learning machine cultural ideas so it could converse like a person.

Sailing back to Britain aboard a convoy escort in March 1943, Turing pondered a hard engineering flaw in Bell Labs's SIGSALY. Forty racks of equipment and crates of vinyl noise turntables worked for stationary bunkers in Washington and London. However, army commanders and warships could never carry a fifty-five-ton machine into the field. Later in 1943, Turing transferred from Bletchley Park to the Secret Intelligence Service Radio Security Service compound at Hanslope Park in Buckinghamshire. Joined in 1944 by Royal Electrical and Mechanical Engineers officer Donald Bayley and mathematician Robin Gandy, Turing built a table-top digital speech scrambler code-named Delilah.

Working with soldering irons and surplus radio parts in a brick hut at Hanslope Park, Turing and Bayley compressed voice encryption into four compact subsystems totaling roughly thirty vacuum valves:

When Turing and Donald Bayley demonstrated the completed Delilah unit at Hanslope Park in late 1944 by encrypting and cleanly recovering a gramophone recording of Winston Churchill's voice across a desk, the entire scrambler occupied less space than four shoeboxes.