Cryptographic History of Work on the German Naval Enigma

Chapter II Research.

The Contribution of the Poles 1920 - Sept.1939.

1. Nearly all the early work done on Enigma was done by Polish cryptographers who handed over full details of the very valuable results they had obtained just before the war began.

2. The machine used from 1920 to 1931 was not the same as that used during the war. It had no stecker, only 3 differently wired wheels and 29 keys instead of 26, the modified vowels A, O and U being included. The letter X always encyphered as itself and the remaining 28 letters were encyphered in the normal way. Since there were only 28 letters encyphered each wheel had only 28 letters on its rim; for some reason O was the letter omitted and hence the Poles always referred to it as the "O bar" Enigma. A machine of this type is of course very much less secure than the steckered machine and the Poles read the traffic from 1920 to 1931 when the present machine - the "Heimsoeth and Rinke" Engima - was introduced.

3. When the German Navy started to use the new machine there were only 3 wheels in existence, only 6 stecker pairs were plugged up (compared with 8 wheels and 10 stecker pairs later on) and the "Boxing" or "Throw-on" indicator system was used. The use of only 6 instead of 10 stecker pairs was a more important difference than it may sound as it meant that when a letter was encyphered there was about a 1/4 chance of both the letter and its encode being unsteckered as against 1/20 chance with 10 stecker pairs; for this reason the machine was very much nearer the unsteckered machine than it was when 10 stecker were used and methods suitable with no stecker could be modified to apply to the early steckered Enigma in a way that was later impossible.

4. Aided by these various weaknesses and by scouring a photograph of 3 month's keys - during the whole of which period the wheel order remained unaltered! - The Poles succeeded in working out cryptographically the wiring of all 3 wheels. This was a splendid achievement and having done it they were able to read the traffic more or less currently as long as the throw-on indicator system remained in force. [With this system the letters of the encyphered indicators can be associated together so as to produce various


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