My Visit to Bletchley Park

While traveling to London for my Building Virtual Infrastructure seminar, I made a side trip to Bletchley Park, home of British codebreaking efforts during World War II. Greg Ferro and I really enjoyed the day out, and there is much to see at Bletchley Park, as long as you are geeks like us! Since that includes just about everyone who reads this blog, I highly recommend trip!

The Ultra Secret at Bletchley Park

As tensions in Europe began escalating in the late 1930s, the British MI6’s Government Code and Cypher School (GC&CS) searched for a location outside London for their work. They settled on Bletchley Park, a Victorian manor house then in the hands of a real estate developer.

Located in northern Buckinghamshire, just outside what would become the “new town” of Milton Keynes, Bletchley Park was ideally situated on main inter-city rail and communications lines. Indeed, the Bletchley station, with service to London, is right across the street!

British Codebreaker ‘Dilly’ Knox had been working to break the German military’s Enigma code system on behalf of the Royal Navy since 1927 but, since he was a classicist rather than a mathematician, was unable to develop a practical approach. GC&CS received a “shot in the arm” in July 1939 when Polish intelligence revealed to the British that they had determined the wiring of the German military version of the Enigma and found a mathematical weakness in the machine.

The Polish Cipher Bureau had also developed a machine (which they named “Bomba” after a chocolate-covered ice cream treat) to brute-force attack German Enigma codes. Consisting of six sets of wheels, the Bomba emulated an Enigma machine and tested all possible combinations for “females”, a tell-tale sign that revealed the current Enigma settings.

This information was brought to the mathematicians brought together at Bletchley Park under Knox, including Alan Turing, Gordon Welchman, and John Jeffreys. This group had already begun work on the German Enigma codes, and Turing had devised a practical process based on “cribs” which would continue to function even after the introduction of additional code wheels.

The British Bombe and Bletchley Huts

Inspired by the Polish codebreaking approach, the British set out to develop their own brute-force codebreaking machine. Unlike the Polish Bomba, the british Bombe would support Turing’s crib-based method and could continue to function even as the Germans implemented daily changes to the Enigma wheels and “stecker” board.

The British Bombe was similar in concept to the Polish machine but far larger, with 36 “Enigma equivalents”. An early enhancement, developed by Gordon Welchman, dramatically improved the Bombe’s effectiveness by taking into account that the “stecker” connected letters in symmetrical pairs. By the end of the war, 210 Bombes had been constructed and were in constant operation.

Perhaps the greatest triumph of Bletchley Park was scalable operating procedures used to break the German code network. Previous codebreaking efforts relied on elite cryptanalysts, but the GC&CS operation broke the process down into lower-level tasks that could function independently. Intercepts were gathered and collated, cribs were created, the Bombes were operated, and the results tested by different groups at Bletchley Park and elsewhere.

Once the messages had been decrypted, they were passed to another “Hut” for translation and analysis. This operation was further scaled by separating the group handling simpler German Army and Air Force analysis from the greater challenge posed by the German Navy. Huts 4 and 8 (in the foreground of the photo above) were the center of Turing’s effort to break the evolving Naval codes.

Admiral Dönitz, commander of the German U-Boat fleet, was particularly suspicious of Enigma and worked continually to improve both the mechanics and operation of the system. This was perhaps the greatest demonstration of the effectiveness of British (and later American) codebreaking: The German Navy noted two distinct “happy times” while the Allies were unable to break their codes!

In 1942, cooperation between Bletchley Park and the United States Navy’s cryptanalysis efforts (OP-20-G) finally began in earnest. The Americans mass-produced a much-improved Bombe and supported British efforts from Washington. Soon, the Allies were able regularly to break the day’s Enigma settings before noon, receiving German military communications in near real-time!

Colossus and the Lorenz Cypher

In 1941, Allied listening posts began intercepting a decidedly different type of radio traffic. Unlike military traffic, which was sent by hand in Morse code, this stream was machine-generated. This new “Lorenz cipher” system was exposed thanks to a stunning operator error, and British cryptographers were able to deduce the entire function without ever having seen it!

A complex new machine was designed and put into use at Bletchley Park to decrypt “Tunny”, as they called the Lorenz system. So bizarre was this paper tape-driven system that the operators nicknamed it in honor of cartoonist Heath Robinson, the English equivalent of Rube Goldberg!

This machine proved too slow to handle the volume of “Tunny” traffic, so a group of engineers from the British Post Office Research Station set about designing a better solution. The fruit of their labor was Colossus, now widely considered the first programmable digital computer. By 1944, the first Colossus was in operation at Bletchley Park.

Colossus was a special-purpose computer using then-state of the art technology. Rather than mechanical wheels like the Bombes, Colossus relied on “valves” and could perform hundreds of Boolean calculations in parallel. The clock was provided by the paper tape as it ran instructions through the system, a clever trick to avoid clock skew between the storage and the computer.

Like the Bombes and all other remains of the codebreaking operation at Bletchley Park, the ten Colossus computers were destroyed after the war. In 1991, an effort was begun by The National Museum of Computing to build a replica of Colossus using surviving records, including many de-classified American documents from the NSA. The fruit of this labor can now be seen at H Block in Bletchley Park, a functional Colossus computer!

Bletchley Park itself nearly suffered the same fate as the machines it once housed. Overgrown and shrouded in secrecy, the location of all this was rescued in 1991 and turned into the museum I visited in 2012. Without that effort, this story would have nowhere to be told, and the legacy of geniuses like Alan Turing and Tommy Flowers would not properly be celebrated.

Stephen’s Stance

Bletchley Park is much more than a museum. For those interested in the history of computing, like me, or World War II buffs, it’s a must-see. And I haven’t even mentioned the Ian Fleming/James Bond connection! The reconstructed Bombe and Colossus computer are truly breathtaking, and I find myself drawn to the stories of codebreaking that took place here. I highly recommend a visit!