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Nestled in an old town swallowed up by the new town of Milton Keynes, sits an old mansion house that’s suffering from years of neglect and a lack of funds. In many ways, it’s similar to the hundreds of other places that occupy the British landscape, but this particular place isn’t just another crumbling country estate of the once rich and famous, it’s a site that’s valuable on a world scale.
We are, of course, talking about Bletchley Park. This was home to Station X, where the Government Code and Cypher School was moved for safety during World War II. It’s here that an incredible collection of mathematicians broke the Germans’ Enigma and Lorenz ciphers, giving the Allies incredibly detailed intelligence that helped plan and shape the war effort and ultimately end the war. In fact, the work at Bletchley is credited with cutting short the war by two years and saving millions of lives in the process.
So important was the work done that Winston Churchill referred to the Bletchley Park staff when the war had ended as, “My geese that laid the golden eggs and never cackled.”
Beyond the incredible work of breaking the Germans’ ciphers, the staff at Bletchley managed to invent the world’s first computer and create a staggering technological leap forward that continues to propel our society today.
After the war Bletchley Park was classified a secret until the 1970s. It wasn’t until 1992, after the site had passed through several hands, neglected at each turn, and the buildings were at risk of demolition, that Milton Keynes Borough Council declared the park a conservation area and saved it. Since then the park has been run by the Bletchley Park Trust whose goal has been to repair the buildings and improve the museum so that the world can see and understand the importance of the code breakers. We took a trip to the site to find out all about its history, what needs to be done and how we can all play a part in its survival.
THE START OF THE WAR
To understand the importance of Bletchley Park, it’s first necessary to understand the importance of code breaking in World War 2. From the off, the Germans decided that every communication from the lowest- to the highest-level had to be secure and impossible to decipher. For this, they decided on the Enigma machine for the majority of troops, while high command used the more complex Lorenz cipher. The Enigma was a kind of portable typewriter that turned plaintext messages into ciphertext that looked like nonsense to anyone intercepting the transmission. With all messages ciphered in such a way it was impossible, without decoding them, to tell what information was important and what was just background chatter; the Allies were essentially deaf to the Germans’ communications.
Invented by Arthur Scherbius at the end of World War I, the Engima machine had been in commercial operation since the early 1920s, when it was adopted by the German Military and modified to suit life in military service.
The machine’s beauty was that they were cheap to produce, simple to use and were statistically very hard to break if you didn’t have the initial configuration settings. Despite this, the actual workings of the Enigma machine were fairly simple.
In essence a letter typed on the keyboard produced an electrical signal that passed through the machine’s internal wiring and lit up the cipher character on the lampboard. Engima, then, is a substitution cypher where one letter is replaced with another (A for D, for example). The important factor was that reverse is also true (D is A, for example), so typing in the cyphertext would reveal the plaintext original message.
Of course, simple substitution cyphers, as this appears to be, are incredibly easy to break, as language has certain rules that can be exploited. For example, the letter ‘E’ is the most common in the English language (similar rules apply to German), so breaking a substitution cypher can start by looking for the most common letter in some text and assuming that this is ‘E’. Similar rules can help break other letters.
The Enigma machine was different and far more cunning thanks to the set of electro-mechanical rotors. These three rotors each had 26 positions that could be manually set, where each position was labelled alphabetically from A to Z on the alphabet ring (Ringstellung). Each position altered the electrical current through the machine, producing a different outcome for each position.
Too top it off, each key press moved the rotors, changing the substitution cypher with each character typed. The first rotor moved once for each key press, when a notch on it hit a pawl on the second rotor it would move to. A notch on the second rotor would hit a pawl on the third rotor and it would move as well. The effect was that hitting ‘A’ might get you ‘D’ the first key press, but the second time you hit ‘A’ you might get ‘R’, making it impossible to use purely linguistic techniques to break a code.
To decode an Enigma message on an Enigma machine the operator would need to know the original machine’s initial rotor positioning, which would be described by alphabetically listing, such as ‘ADR’.
The Germans were even more cunning that this, and made it so that each rotor was interchangeable, so could be placed in either of the rotor slots. This meant you’d have to know which rotor went where, as well as their initial position. Then, towards the end of the war Enigma machines had five rotors, of which three were chosen at any one time.
This was made even more complicated by the fact that the alphabet ring could be rotated, so you’d have to know its position before you could set the initial rotor position, otherwise the notches and pawls would be out of sync and the decrypt wouldn’t work. Interestingly, this technique has no effect on trying to break the code, as it doesn’t actually affect the cipher text: rotating the alphabet ring has no direct effect on the rotor, only on the actual starting position.
Finally, the Germans had the plugboard at the front called the Steckerbrett. This allowed up to six plugs to be connected, switching two letters. So, Steckering ‘A’ and ‘U’ would mean that typing the letter ‘A’ would send the electrical impulse for ‘U’ and vice versa. The upshot was that decipher a message you really had to know the rotor order and which ones were being used, their initial position and the connections on the plugboard.
With this relatively simple machine, came a code that was incredibly hard to break and was capable of producing over one trillion different combinations. With everything encrypted this way, the Germans felt absolutely safe that their communications were completely safe from Allied eyes. Fortunately, this wasn’t to be the way.
THE POLISH INFLUENCE
Bletchley Park’s success would not have been possible if it hadn’t been for the work of the Polish mathematicians before them. On 1st September 1932 Marian Rejewski, working with Henryk Zyglaski and Jerzy Różycki, were given the task of solving the logical structure of the German military Enigma machine. The need was immense as Poland was in a desperate position and could be attacked at any time.
The mathematicians got an early break when they were given the list of cipher keys (Enigma settings) for the months of September and October. Going back through intercepted communications for this period, they managed to work out how the Enigma was wired internally, and how the rotors interacted with each other.
Next came a theme common in breaking Enigma: exploiting human error, which is now as it was then often the weakest link in any security system. To begin with the Poles used a flaw in the way that the message indicator was set.
This indicator was designed as a way for every single transmission to use a different initial rotor setting. It worked in addition to the daily key, which was set out in code books. This daily code contained the rotor order and rotors to be used, the plug board connections and, vitally, a universal initial rotor setting (the ground setting).
Using this method, Enigma operators would create a random initial rotor setting, such as ‘ABZ’, called the message indicator. They would then use the ground setting of the day to encrypt their new message indicator, which would get transmitted twice to ensure correct delivery. In our example, ‘ABZ’ could be transmitted as ‘HRDTSY’. Then, the message indicator would be then be used to encode the real message, which would be transmitted.
DOUBLE TROUBLE
This gave the Polish the information that when the message indicator was sent, the first and fourth letters (H and R), second and fifth (D and T), and third and sixth (S and Y) were the same letters.
By coming up with tables that exploited these relationships, it reduced the number of possible combinations down to 105,456. Although the codes were original broken manually by using perforated sheets, Marian Rejewski come up with an electro-mechanical device (the boma kryptologiczna), which would brute-force attack and try each of the 105,456 possible rotor combinations.
There could be several ‘right’ answers, but another mistake in Enigma made it easy to check: a letter could not have itself as its cypher text. In other words, if you typed ‘A’, you’d never get ‘A’ lighting up on the lampboard. Therefore the ciphertext could not have letters in the same position on the plaintext. Once one message had been decoded, the ground setting of the day was determined and all messages could be decrypted.
THE BRITISH TAKE OVER
Time was running out for the Polish. On 15th March 1939 the Germans took Bohemia and Moravia, and then withdrew from the German-Polish Non-Aggression Pact on 27th April 1939. With the huge threat on their doorstep, a conference was held in Warsaw on the 26th July, where the Polish revealed to the British and French that they’d broken Enigma and pledged to give each country a working reconstruction of the military Engima machine, along with details of their code-breaking techniques.
This was vital information, as the British had not been able to find a way into Enigma and were struggling to decode messages. Armed with the Polish information on code breaking and the internal wiring of the Enigma, the British were ready to continue the work.
On 15th August 1939 the Government Code and Cypher School (GC&CS) moved into Bletchley Park secretly, with the mission to decode Enigma signals. The park was chosen for several reasons, including its proximity to Oxford and Cambridge (many of the code-breakers were recruited from here), it was on the main railway line between London and Birmingham, Manchester and Glasgow, and the nearby repeater station at Fenny Stratford made it easy to lay dedicated cables for telephone and telegraph circuits.
An amazing team of mathematicians was put together, with the early work performed by Dilly Knox, John Jeffries (both who tragically died of separate illnesses before the war’s end) and, most famously, Alan Turing.
He was to play an very influential part in breaking Enigma, as since the Poles had found the flaw in the way that the message indicator was being transmitted, the Germans had changed tactics and started printing its code books without the ground setting. Instead, the cryptographic key for a day would constitute only of the wheel order (which rotors to use and their order), the ring settings (where the alphabetic ring on each rotor should be placed) and the plug settings of the plugboard.
The operator of a machine would pick a random rotor order (say, ‘XVF’) and used it to encode a second rotor order (say, ‘LPR’). The second rotor order was used to encrypt the final message. To transmit a message, first, in plaintext, the first rotor order was transmitted (‘XVF’), then the encoded second rotor order was sent. Finally, the ciphertext of the message was sent. The result was that every single message was encoded with a different rotor setting.
Turing knew from the Polish system how the Enigma was wired, but as the message indicator system had been replaced he had to find a different way of getting into the code. It’s how he did this that helped change the course of the war.
