Cryptographic History of Work on the German Naval Enigma

The recipient would then pass the message to his commanding officer who would stecker the machine with the Offizier stecker for the day. Look up his list of settings and finding say S = PJX would set the wheels to window position PJX and decode the message.

54. Techniques were developed for breaking into Offizier messages by hand given that either the stecker or the message setting was known. With both unknown, however, a break was only possible by means of a crib; since there was only one W.O. one could afford to run cribs with only a small chance of being correct - but owning to the non-routine nature of the messages it was hard to find any correct cribs at all.

55. Once one break had been made however the whole month's traffic could be read. For suppose we broke into a message with setting J say, on April 9th. Then, knowing the stecker, we could get out by hand all other messages on April 9th giving us, say, settings A, N, Y. We could then break the stecker for any other day containing a message with setting A, J, N or Y - then we get any other settings used on these days and so on, until all the settings and all the steckers for the month were found.


56. German Naval traffic was, generally speaking sent out in 4 letter groups with the two indicator groups at the beginning and repeated at the end. Messages were normally broadcast and on fixed frequencies which changed comparatively rarely so that it was easy to remember the different frequencies and the areas and keys to which they belonged. The chief exception to this was the U-boat traffic which used a complicated W/T programme but identification of services was made easy by the use of an independent set of serial numbers for each service. We should also have had some difficulty with the Mediterranean area if the intercept stations had not given a group letter to each W/T service and appended it to the frequency when teleprinting the traffic. Fixed call signs were normally used by originators, but addressee call signs were little used except in the Mediterranean.

57. Traffic steadily increased throughout the war, from about 300 messages a day in 1940 up to 1500 - 2000 messages in 1944 and 1945; despite the decrease in the area controlled by the Germans, traffic continued to


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