How the Bletchley Park code-breaking station came into existence

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.