Turing's Treatise on the Enigma

Methods of reading the individual messages

With the system of indication that has been used since May 1937 we are not able to read all the messages as soon as we have read one. A few may be read by single wheel processes, starting from a short crib, but we cannot hope to read the whole traffic in this way. Also, when we have found the Grundstellung, and if there is plenty of traffic, we may be able to make use of some bigrammes which occured in messages already read. These methods are not enough by themselves. In the 1937 traffic there was no 'not probable', and we had planned a method for finding the right starting position, making use of the fact that the correct decode would probably have more letters E in it than any of the others. It was intended to have a long punched paper roll, the punching showing the effect of enciphering E in the various positions. This paper was to move under a series of about 200 brushes whose position was determined by the letters of the enciphered message. The number of brushes which poked through the holes at any moment was the number of letters E in the decode of the message, the window position being determined by the position of the roll. All positions giving more than a certain number of letters E were to be recorded and these positions independently tested. This machine was called 'the rack'.

It was never necessary to make a rack because when the 1938 messages were read it was found that the word EINS occured very frequently. We therefore made a catalogue of the encoded values of EINS at every possible starting position, and arranged the encoded values in alphabetical order. The unanalysed catalogue was made by enciphering first E at every possible position, then I, N and S. This was done with the automatic typewriting enigmas. The values of I were stuck below the values of E with a stagger: the values of N and S were underneath these again, with suitable staggers. The result was that the effect of enciphering EINS appeared in vertical columns.


< previous

next >