Like so many battles before it — and many more since — battlespace awareness would play a critical role in the Allies’ ability to counter the German submarine threat in the Atlantic.  The main challenge to achieving such awareness was the German Enigma machine.  Though Enigma was in use by more than just the Kriegsmarine, or German Navy – it was the navy’s model that was the most sophisticated and difficult to break.  Many, including the Germans, believed such a feat to be impossible.  They had developed an unbreakable cipher…or had they?The Machine

“Enigma” was the code name for a series of cipher machines developed in the 1920s by a private German firm.  Jennifer Wilcox, writing for the National Security Agency’s Center for Cryptologic History, describes the inner workings of the machine well.  “The Enigma…based its cipher capabilities on a series of wired rotor wheels and a plugboard.  Through a web of internal wiring, each of the twenty-six input contacts on the rotor were connected to a different output contact.  The wiring connections of one rotor differed from the connections on any other rotor.  Each rotor had a moveable placement notch found on an outer ring.  The Germans followed a daily list, known as the key list, to indicate where the notch would be placed.”[1]  For example, “each day, the Germans followed the key list to plug the plugboard connections, select the rotors to be placed in the machine, change the rotor notch placement, and place the rotors in” one of three positions.  In the final step, the code clerk would choose three letters which indicated “the initial rotor settings for any given message.”[1]  The initial settings were changed on a daily basis, but this final setting was changed with every message sent.  Set properly, the total number of cipher combinations possible was more than 160 trillion.[2]  Indeed, the Germans had good reason to be optimistic.  What they did not count on, however, was the cryptanalytic abilities and determination of the Allies.

Early Advances

Since the end of WWI, Poland had been hard at work attempting to break German enciphered communications.  Though they had acquired a commercial version of the Enigma, the German military had upgraded their machines rendering the Polish version useless.  Cryptanalysts set out on two primary tasks:  (1) determine “the exact wiring of the three rotors,” and (2) discover “a method for rapidly determining the rotor settings.”  Poland’s Cipher Bureau reached out to academia for the answer.



They found that answer in the early 1930s in the form of three mathematicians:  Marian Rejewski, Jerzy Rozycki, and Henryk Zygalski.  Through many painstaking months of analysis, collaboration with the French, some help from a German defector, and a little guesswork, the Poles were able to both replicate the German machine and to rapidly determine the rotor settings.  By linking two sets of three machines together, the Cipher Bureau could now read messages in less than two hours.

Unfortunately, Poland did not have the capacity to keep pace with a growing German military and the resultant increase in radio traffic.  Further complicating Polish efforts, the Germans had introduced two additional rotors.  Under this configuration, cryptanalysts had no way of determining which of the three out of five rotors would be used for message encryption.  The system would now require sixty machines to be effective.  This proved to be too much for the Poles.  In July 1939, Poland passed their advances and successes on to France and Britain.

British Successes and Challenges

Britain, taking a page right out of Poland’s play-book, immediately hired two mathematicians to improve upon previous advances.  They were Alan Turing and Gordon Welchman from Cambridge University.  In early September 1939, both men set about their task at the Government Code and Cipher School (GC&CS) in Bletchley Park.


Alan Turing


Alan Turing’s initial focus would be on developing a machine that worked using assumed text instead of indicators which the Germans had been known to change from time to time.  Cryptanalysts would play an important role in Turing’s plan as they would be employed to determine the assumed plain text.  The machine would process the assumptions through every rotor setting searching for all possible matches.  It would then discount all those which did not match.  Wilcox provides another helpful example:  “If the assumed letter was ‘G’ and the corresponding cipher letter was ‘L,’ Turing’s test register ignored any results that did not allow the electrical current to pass from ‘G’ to ‘L.’  By disproving thousands of rotor settings, those left were possible correct settings.”


Gordon Welchman

Gordon Welchman, though assigned as a traffic analysts, took it upon himself to focus efforts on the plugboard.  Wilcox writes:  “Because the plugboard uses a cable to connect one letter to another, it automatically connects the second letter back to the first.  If ‘A’ is plugged into ‘E,’ [then] ‘E’ is plugged into ‘A.’  Knowing this, Welchman designed a board that connected each letter with every other letter.  The wires created a pattern of diagonal lines.”[1]  Combining Welchman’s design with Turing’s machine would decrease the number of possible rotor settings from thousands down to single digits.  Provided with these concepts, engineers could now take on the difficult task of designing and building a machine that could theoretically break the German Enigma code.  It would be called, the Bombe.


Alan Turing Bombe Replica

The first Bombes arrived at Bletchley Park in August 1940, just as cryptanalysts were beginning to decrypt German Army and Air Force radio traffic — and proved very successful against both.  Unfortunately, the German Navy radio operators were much more disciplined than their Army and Air Force brethren.  Not only did the Kriegsmarine adhere more closely to prescribed communications security practices, they had also added three more rotors to their system bringing the total number to eight.  Both of these factors contributed significantly to Britain’s near inability to read naval radio traffic.

Capture of U-110

This inability to read Enigma enciphered messages came to an end in May 1941 when the Royal Navy captured the German submarine, U-110.  Not only did the boarding party recover an Enigma machine complete with daily settings and all eight rotors, but they also recovered code books and key lists.  This “treasure trove of secrets” allowed Bletchley Park to once again read German Navy radio traffic.

Following a subsequent increase in Allied attacks on German U-boats in the Atlantic, Admiral Donitz began to suspect a possible compromise.  Despite constant assurances from his staff to the contrary, the Admiral was determined to change the U-boat Enigma.  On February 1, 1942, the German Navy added both an additional rotor to the U-boat machines and instituted a new code.  The Allies were blind again.

*Stay tuned tomorrow for Part II of the story.




[1]  Wilcox, Jennifer.  Solving the Enigma:  History of the Cryptanalytic Bombe.  Center for Cryptologic History, National Security Agency, 2006. 

[2]  Beesly, Patrick, et. al.  ULTRA and the Battle of the Atlantic.  Naval Symposium, U.S. Naval Academy, 28 October 1977.