The Ham Nomads

11 Sep 2014. <GB-ENG> Bletchley, Mil Key —

The short version (executive summary):

Today was fun — we went to Bletchley Park, home of the allied codebreakers during World War II. We saw some of the earliest computers, went through the huts where the codebreaking took place, played with interactive demonstrations of codebreaking, and had such a thoroughly enthralling time that we decided to come back tomorrow to allow us to really take it all in!

The long version:

Today was a fun day, particularly for the computer nerd Warren. We went to Bletchley Park, the site of the codebreaking effort during World War II — and also the place where Alan Turing did so much, not only in codebreaking, but also in the development of computing as it is known today. The Americans may claim ENIAC (1946) as the first digital computer, but Colossus was doing its work (under the cloak of secrecy) here at Bletchley in 1943! Although Churchill mandated that all the computers were to be dismantled at the end of the war, for security reasons, enough expertise remains from that time to this day for the National Museum of Computing to have reconstructed a working copy of Colossus Mark II, but also of its predecessor, named by the Bletchley community as 'Heath Robinson', as well as the Tunny machine which finally processed the output of these others.

An Enigma Machine

At Bletchley, they broke the codes for the 3-rotor German Enigma machines, and somewhat later, for the 4-rotor German U-boat Enigma machines. They also deciphered the later German Lorenz cipher, the Italian system based on the Swedish Hagelin C38 machine, and some 55 different Japanese systems. Once the day's keys had been deciphered — the Germans changed Enigma keys and settings on a daily basis — the deciphering of the day's messages could be done in virtually real time. But this determining of the keys was a problem, until Alan Turing and Gordon Welchman developed a machine, the Bombe, to speed up the task.

The Bombe (front)

The Bombe (back)

Modified Typex

This machine effectively replicated up to 36 different Enigma machines, and ran using brute force techniques until a possible setting was found. It could run all possible combinations of these 36 machines in about 12 minutes. This setting would be tested using separate checking machines, while the Bombe would be allowed to continue to find more "stop points". This would still be futile, but for the fact that the codebreakers, working with the raw data and the knowledge (and expectation) of common German words and phrases being encoded, would have provided "best guesses" as to many of the plug-board settings of the Enigma machines, which would be plugged into the Bombe, and which vastly shortened the task. It turned out that Bletchley routinely broke the keys most days, often in the early hours of the morning. Once the key was found, the day's messages were an open book.  The rotor and plug settings were set up on a modified Typex Machine (the British 5-rotor version of Enigma!) and the codes were typed in, coming out in plain-text German at the other end.

A Lorenz Machine

Tunny

The later Lorenz machine was a different kettle of fish — it ran on 12 rotors, not all with the same number of circuits through them — they ranged from 19 to 61, and they did not necessarily step with each key depression. Bill Tutte was able to deduce from samples of encrypted output the logical structure of the Lorenz Machine, and engineers from the Post Office were able to construct a machine logically identical. This was the Tunny machine, which was used from then on to decrypt German Lorenz messages — but first it had to be told the starting rotor settings.

'Heath Robinson'

Why 'Heath Robinson'?!!

Colossus Rebuild

Apparently a German error, sending (almost) the same message twice with the same settings on the Lorenz machine gave Colonel John Tiltman sufficient information to go away and come back in ten days with the code broken. Without this error on the Germans' part, the job would have been horrendous. But now they knew the system behind the rotor settings, they were able to design machines to do most of the work. The first, 'Heath Robinson', worked, but somewhat unreliably. Then the Post Office engineers came up with Colossus Mark I, to be followed up with Colossus Mark II. With these, the first genuine electronic computers, they were able to routinely find the settings within 6 hours, and from then on all the Lorenz messages were an open book! Lorenz was particularly important, as is was used exclusively by the German High command, so the messages contained high level military information. Also, because the Germans considered it 'unbreakable', the settings were changed far less often — perhaps on a monthly basis rather than a daily one., which made things much easier for the codebreakers.

Alan Turing's Office

Decoding Room

The secrecy of Bletchley Park was paramount. Once a message had been decoded (usually in Hut 6), it was passed across to Hut 3 using a rather ingenious high-tech method — the messages were placed on a tray, a chute was opened between the two huts, and the tray pushed through with a broomstick; once the message had been picked up in Hut 3, the tray was retrieved by means of a string that had been attached to it! In Hut 3, the message was rewritten to remove any sign of German origin, and instead making it look as if it had come from a spy somewhere in Germany. This was the key of working at Bletchley — no-one in one section knew anything of what was being done in another. Nothing was ever allowed to betray the work of Bletchley. Any intelligence obtained via Bletchley had to wait for action until a credible conventional espionage source could be set up to explain the data and, moreover, only the number of intercepts that could be statistically explained by such sources were actually acted on — this deception was codenamed 'Ultra'. When we were in Coventry, we saw might have been one of the results of this policy — it was commonly thought that Bletchley had decoded the German communications in ample time to warn the city of the imminent bombing raid, but to have warned the city would have revealed that the codes had been broken, so the city was devastated, with the loss of about 600 lives (this was later shown to be untrue, as the decoding was only achieved a very short time before the actual bombing raid) — but the work of Bletchley was never betrayed, and it is commonly considered that its continued work shortened the war by at least two years.

We heard some interesting stories about the secrecy of Bletchley. Apparently a 50-year reunion was held of Bletchley workers, and a husband and wife, married shortly after the war, both came along separately, neither knowing of their partner's involvement until they met up with each other at the reunion.

A ticket to Bletchley Park entitles you to unlimited reentry for 12 months, and we were finding the amount of input today rather overwhelming, so have decided to remain in the area and return tomorrow. (As tomorrow's day reads much the same as today's, we have merged and blended the two.)

Distance driven — today, 19 miles ( 31 km ); to date, 7,440 miles ( 11,974 km )