Turing, Welchman and Bletchley Park

In the past year I read several books about the events that took place at  Bletchley Park during World War II. It all started when the movie “Imitation Game” came out and the biography of Alan Turing was republished. I had read Turing’s biography back in the 80s, but I was happy to read it again, especially since an electronic version was available. The book is  called “Alan Turing: The Enigma” and it  was written by a British mathematician Andrew Hodges.

Broadly, Alan Turing’s life can be divided into three phases: his life before the war, his work on the Enigma at Bletchley Park during the war, and his work on computers in the late 40s and 50s. His major achievement of the first part of his life was the 1936 publication of a paper titled: “On Computable Numbers, With An Application To The Entschedeidungsproblem.  The paper presented the idea of an abstract machine (now we call these Turing Machines) that could  mechanically  solve any problem for which an algorithm  could be devised. Essentially this paper invented and described in detail a programmable computer, as we know them today. The paper went further, also showing that there are some problems with are “not computable” – that is impossible to  solve with a programmed machine.

Alan Turing was on his way to an academic career when World War II broke out. He was recruited to help in breaking the German communication that has been encrypted with the Enigma machine. The center for all this work was Bletchley Park, and this is where Alan Turing spent his war years. The initial breakthrough of deciphering the Enigma was accomplished by Polish mathematicians before the war. Turing and the Bletchley Park cryptographers continued on this work, eventually creating an industrial size organization that would break the daily keys of large number of a German communication networks.  Turing in particular worked on breaking the Naval Enigma, including designing number of machines that were used to speed up the computations needed to find the daily keys.

In October of last year I got a chance to visit Bletchley Park, which is now a museum, and while browsing though the museum book store I found a book about Gordon Welchman. The book was “Gordon Welchman: Bletchley Park’s Architect of Ultra Intelligence“, by Joel Greenberg. Welchman was a contemporary of Turing, and he arrived at Bletchley around the same time as Turing and like Turing worked on breaking of the Enigma.

When he first arrived at Bletchley, Gordon Welchman pioneered the discipline of traffic analysis. That is learning as much as possible from the way messages were transmitted in various German networks, without actually decrypting the contents. One thing that made this possible was that each Enigma message had a header, transmitted in the clear, that contain destination and source addresses (these were of course coded indicators that changed everyday). Still surprising amount can be deduced from this information.

Welchman was also the one to organize how the intercepts were obtained and how they were handled once at Bletchley. In particular he set up and managed the work of Hut 6, which mostly dealt with analyzing and breaking the German Army and Air Force traffic.

Unlike Turing, after the war Welchman immigrated to the U.S. where he worked on development of computers and communications networks. He became a US citizen, obtained a security clearance and worked at MITRE on government projects.

Now, when World War II ended the British and the American governments decided to keep what was done at Bletchley secret. Only in the 1980s some of what went on at Bletchley has started to become public. Some of this material was in fact used by Turing’s biographer in his book.

Gordon Welchman was close to retirement by then, and seeing how Bletchley secrets were slowly being revealed he decided to write a book about his experiences. The book is call “The Hut Six Story” and it describes a lot of technical details of how Enigma traffic was broken.  Once Welchman announced his plans to publish “The Hut Six Story”, over 40 years after the war, both the British and American Secret services tried to stop it. Eventually the book was published in the early 90s, but as a result Gordon Welchman lost his security clearance and was no longer able to work in the Defense Industry.

The most interesting parts in “The Hut Six Story” are the description of some of the methods used to break the daily Enigma keys. Many of these methods depended on finding a “crib” – that is guessing correctly what a part of a message may be – and then testing different key settings to eliminate the ones that are not possible. The presence of cribs in the Enigma messages were mostly due to sloppiness of the German Enigma operators. For example, sending a daily message with same text (like: “nothing to report”).

These three books gave me a very in depth look at what happened at Bletchley Park in World War II, and I f you are interested in this topic consider reading them.