Bletchley Park: The Colossal Code Cracking Computer 1

We create heroes.  We need someone with whom to identify in every story.  In World War II we have heroes in Churchill, Roosevelt, and De Gaulle.  There are countless small victories in which soldiers held a bridge, saved a piece of art, or rescued a family.  In the case of code-breaking, we have collectively decided our hero is Alan Turing.  However, the reality is so much more complex than efforts of an individual.  The reality is the building of Bletchley Park, with all its people and various machines, became a colossal, code cracking computer.

You read the last sentence correctly.  And it bears repeating.  The building of Bletchley Park was a massive computer devoted to cracking enemy coded messages in World War II.

By DeFacto (Own work) [CC BY-SA 4.0 (], via Wikimedia Commons

The Colossal Computer 

The steady hum of computers is punctuated by typewriters, the punching of cards in Hollerith machines, the collation of punch cards in sorting machines, the steady even click of women’s heels moving between the sorting machines.  Doors open and close.  Deliveries of new work are made.  Work is sorted and divided amongst the group.  New punch cards are loaded in machines and punched with the specific information contained in the work for the day.  Then compiled and sorted in the series of machines, following the arrows Mr. Freeborn had drawn on the floor in chalk in the morning.

Frederick Freeborn was responsible for the information processing, as part of decryption, at Bletchley Park.  Sophie Campbell of The Telegraph, reports in a 2014 article the area used to be referred to as “Freebornery”.

Dr. David Kenyon, Research Historian at Bletchley Park, describes of Freeborn’s workflow mechanism, “a punch card machine could only do one sorting process.  A sequence of sorts was required.  He [Freeborn] drew chalk arrows on the floor to indicate the correct sequence to extract the information.  What you are looking at is a computer program and this microcosm applies across the site.”

“Bletchley used six tons of punch cards every week.  This is approximately two million individual cards.” Kenyon relates, with respect to the sheer volume of work handled by Freeborn’s information processing section.

Almost 9,000 people worked at Bletchley Park by the end of the war.  Nearly 12,000 people over the course of the conflict.  The organization started on its course with only 150 employees, mostly code-breakers, recruited through the usual means by contacts and networks within Cambridge and Oxford.  Later, they recruited from the women’s services.  Many people were basic trained and had aptitude for the job.  The hitch in the process was “they had to guess the job, take it based on the guess,” Kenyon says of the massive recruitment efforts.

Thousands of people worked to decode messages; to gain any sort of advantage over the Nazi military commanders.  Thousands of people and machines provided daily support to the front lines by learning how troops were to advance, the future locations of battleships, and the dates of supply trains.

Multi-Lingual Code Crackers

There are many types of codes used during the war.  The Luftwaffe used a different Enigma machine than did the Kriegsmarine, used a four-rotor machine for u-boat messages.  There were also the Lorenz cyphers, book codes, hand cyphers, coded voice communication, amongst hundreds of methods of information encryption.  And the messages were being received in Italian, Japanese, and German.

Book codes are a complex form of encryption.  Essentially, there is a dictionary containing words and phrases, and it functions similar to a foreign language to English dictionary.  There are two parts, every word or phrase is associated with a group of random digits which you use as replacements in your intended message and then the numbers in the reverse part of the dictionary are for the decoder to translate the message from numeric codes.  From there, you require a second book which has random sequences of digits on each of its pages.  You select a page, add the digits from the second page to the numbers in your encoded message to come up with new numbers and send it off.  To solve the code, you must have the page number of the second book to subtract the second set of numbers used and use the reverse dictionary to decode the new numbers and, therefore, the message.

Book building is a process through which one tries to figure out the numbers, all the numbers, and it is a time-consuming process.   This method of code cracking relies on statistical analysis.

“This method was applied to solving Japanese codes, with great success,” Kenyon recounts.  “It was a separate world.  One team just worked on JN-25, Japanese naval codes.  If they successfully identified the meaning of a number they would fill it in on large rolling sheets of paper to eventually build a complete dictionary.”

However, codes in multiple languages present a different problem.  Not only do you need to decode the message well enough to understand it, you must also translate it.  Bletchley Park solved the problem by developing a Japanese language course.  “Initially, they went to the University of London and were told it would take two years to teach people Japanese.   Bletchley set up a six-month course and it was successful,” Kenyon says of the solution to the language barrier issue.

Thinking Like the Messenger

“Correctly used the [Enigma] machine is invulnerable, human factors facilitates breaking codes,” Kenyon states.  “There was one desert unit where the report was always the same: nothing happened today.”  The Enigma settings might be different, however, sending the same message allows people to easily decode it.

“Weather forecasts were often the same,” Kenyon continues.

The issue comes in with respect to the machine’s settings.  There were five rotors used with an Enigma machine and you needed to use three to encrypt a message.  “They did not understand randomness,” Kenyon states.  “There were rules; you cannot use the same setting more than once; the same rotor cannot be set in the same position on successive days.  They had so many wheel order rules, the operators would get to a point in the month where they had very few options left.  This was a good thing for the allies.”

The rules set out by the Nazi commanders allowed the allies to accurately guess the machine settings being used by various operators as time went on.

While we all need a hero, sometimes it is not possible to make one in a story.  The ground-breaking marvel of Bletchley Park is one of many stories and no specific hero.  Each of the thousands of people who worked there during World War II was responsible for a significant and collective effort which helped win the war.  The individual stories are on their website and provide insight into the roles of the people who worked there.

Header Image: From Wikimedia Commons; attribution: By DeFacto (Own work) [CC BY-SA 4.0 (], via Wikimedia Commons

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About Jennifer Cornick

Freelance journalist and blogger for various publications in Vienna. When I am not writing, I can generally be found with a book (or anything with words on it - even cereal boxes).

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