For several years at the beginning of the war the Allies couldn’t find a flaw in the Enigma code. They had captured several Enigma machines, as well as obtained the occasional code sheet – but they needed to crack the cipher. Luckily, a few poor choices on the part of the Nazi’s allowed the cryptographers at Bletchley park to discover a weakness in the Enigma code.
In order to ensure a very strong encryption, the Nazi’s added a plug board to their military Enigma machines – an old telephone style mapping between letter pairs, essentially making two letters swap before being sent into the rotors for encryption. This added a crazy amount of mathematical redundancy to the Enigma (catapulting the possible initial configurations well into the trillions) however it also allowed for an important weakness to be discovered.
Because of the way Enigma was designed, it was impossible for a character to be encoded (go through the encryption process) into itself – a certain character always had to result in a certain different other character.
Now, this flaw allowed – through a lot of trial and error – the deduction of some some plug positions – and from those plug positions you could extrapolate the positions of others, and assuming you didn’t find any problems (such as two plugs pointing to the same letter, in which case you’d try another guess) you could effectively brute-force the plug positions, without actually having to try all of the possible options. However, the Allies needed to decrypt the German’s messages every day, so they had to build a device to automate this process for them.
Enter the Bombe. Designed by Alan Turing, the Bombe took the form of emulating several hundred Enigma rotors, as well as functioning as a logical electrical circuit to automate the deductions needed to rule out flawed possible attempts. By 1941, the British had built 12 operational Bombe’s, but by 1944 had over 150 of them, cracking a variety of German messages. The Nazi’s were so kind enough to broadcast a weather report every morning encrypted by the Enigma code – a broadcast that was done in the same format daily, which the British could then crack and reveal the Enigma settings used for that day.
Eventually the German’s switched to a variety of different Enigma machines, such a 4 or 5 rotor machines as well as double-encrypting messages, however the British code breakers often caught up with them fairly quickly. The British themselves took the concept of the Enigma cipher and improved upon it – such as fixing the issue of letters not able to be encrypted into themselves. The German’s had attempted to crack it, but thought it impossible as it was even more sophisticated than Enigma itself.