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Alan Turing in America – Cryptography

David Zitarelli (Temple University)

The play Breaking the Code and the movie The Imitation Game popularized Alan Turing’s decisive role in deciphering codes produced by the German encrypting/decrypting machines.  In fact, his interest in cryptography may have been sparked in Princeton when he set out to build the binary-multiplier machine.  In any event, Turing spent the war years at Bletchley Park, a country mansion housing Britain’s leading cryptanalysts.  He was assigned to work with naval communications and, using information supplied by Polish mathematicians, was intimately involved with building machines that could decipher messages sent from the German military.  That project enabled the British Admiralty to defeat German submarine maneuvers aimed at strangling Britain’s shipping routes in the Atlantic.

Not only did Turing pursue his studies on logic and ideas on computing after arriving back in Cambridge, England, but he also published two conventional papers.  However, all of this activity came to a grinding halt shortly after Great Britain declared war on Germany in 1939, whereupon Turing was asked by a British governmental office for help with breaking codes.  German scientists at the Lorenz engineering company, adept at mathematics and its applications, had built a machine called Enigma that automated the processes of coding and decoding messages.  Enigma required the work of only three people: an operator who typed the coded message, a wireless operator who tapped out the enciphered message in Morse code, and a receiving operator who typed the message and handed it to the intended target.  The German Luftwaffe sent thousands of such messages a day, some with top-level plans of action but others of a more mundane nature such as weather reports.

Figure 5. Slate statue of Alan Turing at Bletchley Park with the best-known image of Turing on the wall to the right.  The photographer reported, "Unfortunately, you can't see his coffee cup under his desk."  (Source: Photograph by Jon Callas, San Jose, California, USA. Licensed under CC BY 2.0 via Wikimedia Commons.)         

Alan Turing worked in a unit at Bletchley Park called Hut 8, where he was one of four senior codebreakers.  This team soon broke the German code using mathematical and statistical ideas to build their own machine that enabled them to decipher German messages within hours of their transmission.  The British machine was called Bombe, after the name given by a Polish group of cryptanalysts who had first broken the German code.

In response, the German navy developed a highly secure form of Enigma for coordinating efforts by U-boats to locate and then attack convoys carrying food, oil, and other raw materials from the U.S. and Canada.  The resulting embargo was literally starving Britain and some inhabitants began to clamor for surrender.  However, the team-of-four attacked this problem with a vengeance and by June 1941 had decrypted this more difficult code as well.  Jack Copeland, director of the Turing Archive for the History of Computing in New Zealand, described the result of this breakthrough aptly: “The effect was immediate; convoy reroutings based on intelligence from Hut 8 were so successful that the North Atlantic U-boats did not sight a single convoy for 23 days following Turing’s first break” [5, pp. 1540-1541].

Shortly, the team at Bletchley became so overwhelmed with the volume of traffic that they appealed directly to Prime Minister Winston Churchill for permission to hire additional personnel.  The wartime leader immediately instructed his chief of staff, “Make sure they have all they want on extreme priority and report to me that this had been done” [Idem, p. 1540].  With all due speed and diligence a committee was formed to scour universities throughout the country for the most able mathematicians.  One of those recruited to Bletchley Park from Oxford University was 18-year-old Peter Hilton (1923-2010).  When he joined Turing’s group in January 1942 he had completed only a little more than a year of undergraduate studies but, under Turing’s tutelage, Hilton aided the war effort immensely.  Using what he called “cribs” (educated guesses for words), he was able to break a code even harder than the Naval Enigma, one called “Offizer” because the messages were so sensitive that only German officers were permitted to send and receive them instead of the normal operators.

Cat-and-mouse moves and counter-moves by the Lorenz engineering group in Germany and the British office at Bletchley Park continued.  The German firm produced Schlüsselzusatz SZ40, a state-of-the-art, 12-wheel cipher machine for encrypting messages.  The British group worked on this problem from June 1941 to January 1942, when William Tutte (1917-2002), using a method invented by Alan Turing, singlehandedly reconstructed the SZ40 without ever having seen it.  In February 1944 British engineers and mathematicians built the world’s first electrical digital computer, called the Colossus, for automating decryption and thus obviating the need for work to be done by hand.  By the end of the year ten colossi were operational.  One of these machines played a decisive role in the U.S. war effort by providing detailed knowledge of Germany’s preparations in advance of the Allied invasion at Normandy in 1944 that came to be called D-Day.

David Zitarelli (Temple University), "Alan Turing in America – Cryptography," Convergence (January 2015)