Turing's Treatise on the Enigma

the top row of the key board and P thus


The number was put between Y's to show that it was a number, and the whole repeated as a check. The contination of a message whose time group was 2330 would begin FORTYWEEPYYWEEPY. We could then find the position where this message started by single wheel processes, and as we already know the window position of the start, we can calculate the Ringstellung.

On the 1st May 1937 a new indicating system was introduced. The first two groups (four letters each) of the message were repeated at the end. This clearly showed that these two groups formed the indicator. The repetition also showed that no check could be expected within the first two groups themselves. This was discouraging, as the essential weakness of the boxing method was that the same thing was enciphered twice with the machine. With the new method of indicating, whatever it is, the best one can hope is that either it will enable us to 'set' the messages, or that from some information about the setting of the messages obtained from elsewhere we may be able to deduce something about the machine setting. However the first thing to be done was to find out how the indicators worked, and it was necessary therefore to try and read some messages with which the new system was being used. To do this one can use the FORTYWEEPY messges, and apply one of the methods described at the beginning of the last chapter. In this way the Poles found the keys for the 8th of May 1937, and as they found that the wheel order and the turnovers were the same as for the end of April they rightly assumed that the wheel order and Ringstellung had remained the same during the end of April and the beginning of May. This made it easier for them to


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