Bletchley Park

Bletchley Park


Bletchley Park was the headquarters of the British Military Intelligence Government Code and Cipher School during World War II. Located fifty miles north of London, on the grounds of the sprawling Victorian mansion for which it was named, Bletchley Park employed 12,000 code breakers and staff. Bletchley Park cryptologists successfully broke the major codes used by the German military and high command, creating the most advanced computing sources of the time with few resources. British cryptologists also aided United States efforts to break Japanese codes. Intelligence information gathered from Bletchley Park is credited with significantly aiding the Allied war effort and saving thousands of lives.

The beginning of Bletchley Park. Although British Military Intelligence employed code breakers during World War I, they failed to establish a permanent cryptology department in the inter-war period. In 1938, on the eve of World War II, British Military Intelligence revived the cryptology department. Drafting cryptographers from all disciplines, and heavily recruiting young men from Oxford and Cambridge, the first cryptology operations were established in London. The group's main task was to correspond with foreign code breakers in allied nations and cull information regarding their cryptology efforts against German codes.

In the summer of 1939, British Intelligence moved the cryptology department to Bletchley Park, officially dubbed Station X because it was the tenth division of the intelligence organization. A cipher school was established on the grounds to train new code breakers. As war was on the horizon, a large number of women were trained for employment at Bletchley Park. At the height of the war, three-quarters of Bletchley Park staff were women. The focus of operations at Station X shifted to active code breaking. By the outbreak of World War II in September of 1939, Bletchley Park cryptologists had already made considerable progress against some German diplomatic codes.

Early code breaking efforts. During the two years of the war, British cryptologists decoded German communications with limited success. Older codes, used for low security messages, were readily identified and broken by the Bletchley Park team. Some newer codes were broken mathematically, but decoding and translating these messages by hand proved an arduous task. By the time messages were fully understood, the information they contained was often outdated. Compounding the problem, these intercepts contained very little useful intelligence information. Since the mid-1930s, the German government had used complex cipher machines to disguise their most important communications.

The first great code breaking triumph at Bletchley Park came on August 30, 1941. A British "Y Station," one of the military listening stations that intercepted German communications, picked up a depth, a repeat transmission that used the same settings on the cipher machine. This intercept was forwarded to Bletchley Park. Cryptologists identified as "fish," the nickname for a message produced by the illusive Geheimschreiber cipher machine. Within two months, the Bletchley Park team broke the high-level German code.

To facilitate the processing of "fish" intercepts, Bletchley Park engineers borrowed an idea from plans the Polish intelligence service gave Britain before the war. They constructed a machine that aided the deciphering of intercepts, nicknamed a "bombe" because of the low, roaring noise it made while operating. The "bombe" constructed to decipher Geheimschreiber transmissions did help cryptographers to process intercepts more rapidly, but the machine required the exact synchronization of two paper tapes for printing. The tapes often broke, and the machine had to be reset. In addition, the start setting to process each intercept, the original cipher settings used by the Germans to send the message, had to be calculated by British cryptologists by hand. The process was still too complex to yield decoded intercepts ready for immediate translation to be useful to intelligence and military personnel.

Operation Ultra: breaking the German Enigma machine. Most of Germany's high-level military messages were encoded using a cipher machine called Enigma. The complex code used not only a cipher, but also an overlaying encryption to disguise the original text. The series of rotor wheels on the Enigma teleprinter gave the machine an extraordinary number of code combinations. The Germans were so confidant that the machine code was so nearly infinite in possibilities that it could never be broken. However, various intelligence services in neighboring nations had made considerable progress breaking Enigma even before the outbreak of the war. In Britain, efforts to break Enigma were known as Operation Ultra.

In the months preceding the German invasion of Poland in 1939, Polish intelligence passed on to British intelligence information on their efforts to break Enigma. Most helpful was the information Polish spies gathered on how the cipher machine operated, including sketches of the teleprinter and some of its components. With the information, Bletchley Park cryptologists found two key weak links in the Enigma code. Enigma code prohibited that any letter be encrypted as itself, and German standards of communication dictated that the same phrase

The Duke of York, foreground, reads a printout from the Colossus computer during his tour of Bletchley Park, the former British Spy Center, England. ©CORBIS SYGMA.
The Duke of York, foreground, reads a printout from the Colossus computer during his tour of Bletchley Park, the former British Spy Center, England. ©

begin all transmissions. Exploiting these two weaknesses, British cryptologists unraveled the Enigma code mathematically in late 1940.

Even though cryptologists could read portions of Enigma transmissions, they encountered the same delay of accessing intercepted information as they had with other codes. Another bombe was constructed that could process Enigma codes, expediting code breaking. However, cryptologists and engineers at Bletchley Park realized that another mechanical solution was needed to fully exploit German intercepts. To this end, two Bletchley Park engineers invented Colossus, the first electronic, programmable machine in 1943. Colossus not only decoded messages, but also broke through the overlaying cipher, producing a ready to translate copy of the intercept in the original German. With Colossus, Bletchley Park could decipher German communications before the intended recipients. Translated intercepts were immediately passed on to intelligence and military officials, making Bletchley Park central to the Allied war effort.

Security at Bletchley Park. Concerned that the German military and government would change encryption devises if they knew of the operation, operations at Bletchley Park were shrouded in absolute secrecy. Details of Operation Ultra and other specific code breaking missions were fully known by only four people. A special intelligence protocol was established to funnel information into and out of Bletchley Park. No one link in the chain of information knew more than two other people involved in the operation.

In order to guard Bletchley Park secrets in the event of a German invasion or bombing campaign of Britain, Bletchley Park's extensive archives of every decoded intercept and the accompanying original intercept were photographed and catalogued at the Bodleian Library at Oxford University. Code breaking equipment was supposed to be entirely disassembled, put on a nearby train to Liverpool, and then ferried to the United States if Bletchley Park were in danger of falling into enemy hands. The tight security surrounding Bletchley Park was remarkably successful. The operation was one of the few government and military outposts that was not compromised by German spies.

Legacy of Bletchley Park. The work of cryptologists and engineers at Bletchley Park is often credited with shortening the duration of the war in Europe by an estimated two to three years. Bletchley Park intelligence aided military strategy, the shipment of necessary troops and supplies, and turned the tide of the war in favor of the Allies.

German U-boats controlled the seas until Bletchley Park decoded intercepts provided military leaders and shipping interests with up-to-date fleet positions and mission reports. Ultra intelligence aided the sinking of the German destroyer, Bismarck , a great moral victor for the British Navy.

On land, Station X intelligence helped Allied forces plan their invasions of North Africa, Italy, and France. During the D-Day offensive and the subsequent Allied march across France, military field command received daily intelligence updates based on information garnered by Bletchley Park code breaking efforts.

Bletchley Park also intercepted the first dispatches relating to German prisoner of war and concentration camps. Other intercepts decoded by Bletchley Park provided Allied military leaders with the first evidence of the Holocaust.

After the war, the Bletchley Park was abandoned and the staff sworn to secrecy regarding their wartime employment. All of the deciphering equipment, including replica teleprinters, bombes, and even Colossus, were disassembled and archived or simply destroyed. By March of 1946, no trace of Station X operations remained on the grounds of Bletchley Park, with the exception of the hastily constructed outbuildings, known as huts, which housed offices and staff. British Military Intelligence, known after the war as MI-6, did not dissolve the Government Cipher School or cryptology department. The department was moved to MI-6 headquarters in London, and then to Cheltenham in 1952 where its main mission was the decoding of Soviet Cold War-era communications.

Although its contribution to the war effort was highly significant, the exploits of Bletchley Park were not fully known until details regarding Operation Ultra and Station X were finally declassified in 1989. The continued secrecy of Bletchley Park allowed American engineers in 1945 to take credit for the invention of the world's first computer, ENIAC, built two years after Colossus. No member of the Bletchley Park staff betrayed the secrets of Station X until the government opened its files to the public.



Hinsley, F. H. British Intelligence in the Second World War. Cambridge: Cambridge University Press, 1988.

Hinsley, F. H. and Alan Stripp, eds. Codebreakers: The Inside Story of Bletchley Park. Oxford: Oxford University Press, 2001.

Smith, Michael. Station X: Decoding Nazi Secrets. London: TV Books, 2000.


Codes and Ciphers
Codes, Fast and Scalable Scientific Computation
Colossus I
FISH (German Geheimschreiber Cipher machine
Operation Magic
OSS (United States Office of Strategic Services
Poland, Intelligence and Security
Ultra, Operation
United Kingdom, Intelligence and security
World War II

User Contributions:

Comment about this article, ask questions, or add new information about this topic:

Bletchley Park forum