Tutte was given the key that had been retrieved by Tiltman (see Tiltman Break). A number of codebreakers had tried and failed to find a pattern in the key. If a pattern was found this would determine the way in which the key was generated and thus how the message was enciphered.
At cryptographical school, Tutte learnt that patterns could be determined by writing a key over an appropriate length and looking for repetitions in the rows. Tutte worked for months trying different lengths and eventually found a repetition on a length of 574, but thought it unlikely that a cipher machine would have a wheel with 574 starting positions.
Since 41 was a prime factor of 574 (i.e. 574 can be divided exactly by 41), Tutte wrote the key out over a length of 41 enabling him to deduce that the first chi wheel had 41 starting positions. Example 1 shows some of the key written out on a length of 41 with a repetition highlighted.
Tutte’s next deduction was that the first Psi wheel had 43 starting positions. After these two successes, the rest of the research section joined in the attack and soon the entire structure of the Lorenz machine was known without ever having seen one. The wheel start positions (known as wheel settings) changed for each message, but the wheel patterns (the way in which the pins around the wheels were set- either operative or inoperative) changed monthly.
With further work, Tutte was able to develop a statistical method to determine the chi wheel settings (start positions). This statistical was used by Tommy Flowers to design Colossus, the world’s first computer.
Alan Turing (on loan from Hut 8) developed a method for determining the wheel patterns (pin settings).