Cryptographic History of Work on the German Naval Enigma


14. The main event in December was the breaking of Squash (Kurier). Kurier was at first simply a method of very high speed transmission of short signals on a frequency deviating by a given amount from that on which traffic was normally sent. On December 9th a system was introduced of sending out hourly 8 letter messages in Shark indicating the deviation by a repeated bigram plus four dummy letter e.g. if AB was the bigram the message might be J A B A B X V L   J, X, V, L being dummy letters. The indicating system was found to be based on a simple usage of the K book and we were able to use these curious signals to break Shark "paired days" and once to break the first day of a pair.

15. Bounce was broken in December and settled down to re-encode into Porpoise and Bonito. Like a good many keys after starting in a small way it grew a great deal and spread from North Italy to the Danube in the East and Frankfort in the North, carrying information about river operations planned against the Remagen and other bridges; for a short time after the Remagen bridgehead there was a Bounce "flap" so that it rather surprisingly had its hour of fame, late though it arrived on the scene.

January 1945.

16. In January Sucker settled down to re-use the Dolphin November keys; it did this for February and March as well and steadily re-encoded into Dolphin the whole time. This was extremely valuable as Dolphin by now had become very difficult and we might otherwise have lost it.

17. The only other event in January was the breaking of two Shark Sonderschluessel - the special keys for individual U-boats first introduced in November 1944. This was an R.E. from Limpet and represented our sole successes until Op. 20 G broke Sonder 161 in April from a Sucker R.E. This was the biggest of all the Sonder keys and further progress was made showing (1) that W.O. and stecker changed only 3 times a month (2) that keys repeated from month to month. By virtue


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