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As advertised I spent most of the day at the Museum of Computing in Bletchley Park. I felt very much at home as all the guides seemed to be ex BT engineers, there were lots of them and each gave a very detailed explanation of the different machines. The only sad thing was apart from me, a group of 6 Italian computer Phd students and two German couples I did not spot any other visitors. It is a great museum (all be it very technical) so please visit it otherwise it will close.
The first decoding machines were electro-mechanical, the British (thanks to some Polish work done before the war) knew how the Enigma coding machines worked (one pictured) but to decode anything you had to know how the machines were set up on a particular day (they changed the settings daily). It was possible to work this out with paper and pencil but it took so long (weeks or months) the messages were of little use once they were decoded. It was realised that the machines always changed every letter, so if the sender typed the letter "T" it would never be sent in the code as a T. So stage one was to pick a sentence that you expect to be fairly common such as weather forecast (WETTERVORHERSAGE in German) and compare this with the seemingly random letters being sent. At the top of the page you can see weatherforecast being compared with a stream of letters and in the top example the third letter T is sent as a T so this can't be the combination. Move it along five letters and there are no "same letters" so this might be the setting.
Obviously there is more to than that but if you can compare streams of messages with likely phrases very quickly then you can soon rule out 95% of the combinations and concentrate on the few.
The machine shown opposite, called a bombe for a reason no one can remember, had lots of rotating wheels with connections that mimic the Enigma machine. Each wheel was rotated in turn testing all the likely combinations of words until it hit one which seemed to work. Once you knew the settings for the Enigma machines that day another machine was used to translate all the messages.
Enigma was used for all low level, day to day communications but the high command used a different, more complex system called Lorenz which the British had no idea how it worked. Eventually a genius mathematician called Max Newman, despite never having seen a machine, worked out how it was configured. The snag was the encoding was so complex it took weeks of brain power to decode the set up. Electro mechanical machines were developed but they kept breaking and were not up to the job. They were so unreliable they were called "Robinsons" after the cartoonist Heath Robinson.
A young GPO boffin called Tommy Flowers thought he could solve the problem with an electronic machine. This was rubbished by the Bletchley Park managers but his GPO boss took a chance and sponsored Tommy to build a machine in the Post Office labs. It worked and soon ten machines were in use in Bletchley. This was the very first electronic computer in the world, it was called Colossus. It used valves as transistors had not yet been invented.
After the war the Russians, who were unaware that the codes had been broken, adopted the same German machines to encode their messages. For that reason the Colossus computer remained a state secret until the 70's as up until the mid 60's GCHQ were still using Colossus to read Russian messages. Consequently Tommy got very little recognition.
After very well informed and detailed descriptions of the code breaking computers the rest of the museum shows the evolution of computers up to the present day. I was particularly taken with the 1965 magnetic storage disk shown, it is about 4 feet diameter and can store 4 Mbytes of data on each side, my phones sim card which is smaller than a 5p coin has 8,000 times more capacity.
So a very enjoyable day out, I highly recommend it.
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