The machine contained three wheels with letters of the alphabet in order printed on them. These were set in any position, and linked by an electrical circuit. When a letter was pressed, providing the recipient had the same setting, they could decode the message.
Contrary to German belief, the machine had flaws.
Firstly, no letter could ever represent itself, and this helped Bletchley Park when decoding messages. Also, if the message was shorter than 26 letters, only the first wheel moved. Lastly, the machine was not random, there were 159,000,000,000,000,000,000 combinations.
The German operators unwittingly contributed to the code breaking.
The rotors never stayed in the same position for more than 2 days, which aided the decoding.
The ‘Double Indicator’, a codebook issued to operators, which specified the day’s settings for the machine, was not always adhered to. Some operators chose their own setting, and sent the three letters twice. This repetition gave the code breakers another way of cracking the code. The operators often used familiar letters such as HIT – for Hitler.
Alan Turing and John Herivel played crucial parts in breaking the Enigma code. Turing’s idea was that a machine could carry out calculations if fed information on a strip of paper. He put this into practice at Bletchley Park. This led to the construction of ‘Bombes’ - machines that sped up the decoding process by going through the possible combinations of settings on the Enigma.
In 1940, believing that the Polish information was not completely accurate, he questioned them and found that he was correct.
Herivel’s idea was to imagine yourself as a German operator and he discovered that they were careless with their choice of letters (i.e. HIT for Hitler).
‘Crib’ was a correctly deciphered part of a message, which gave them clues, such as the 6am weather broadcast.
In 1941, 2 U-boats were captured, and parts of the ‘Dolphin’ Enigma machine, the Navy’s version, were found. This allowed the Dolphin code to be cracked.
In 1941, aboard the U-570, the ‘Shark’, a 4-wheeled Enigma machine was captured, enabling Bletchley Park to crack the code that was to be used on all U boats.
‘Fish’, the machine that Hitler used, was coded differently to the others, and did not use Morse code. This was a problem, as no one knew what it looked like. After 2 months, Bletchley Park managed to construct their own ‘Fish’ and were able to decipher very important messages.
