Cryptographic History of Work on the German Naval Enigma

wheel key carrying some high command traffic in the Mediterranean; it was never of much importance and we only broke three pairs of days in all.

30. Bonito, a 4 wheel key, carried much more interesting material. Like Seahorse it had originally been thought to be on the unsteckered machine and the traffic went to Twinn's section, who passed it to us to try on the Naval machine. After a certain amount of trouble April 27th was broken early in May, the traffic having been sufficiently heavy (40 messages) for the alphabets to be partially recovered, using the same sort of technique as that employed on Porpoise.

31. At first traffic was quite small and concerned with experimental work on midget submarines in North Italy but later it became the operational key for all "Small Battle Units" - Midget Submarines, Saboteurs and similar things and people. The volume of traffic steadily increased from 5 - 10 a day in April 1944 to 100 a day in Spring 1945; with this increase went a corresponding increase in Naval Section's interest in the key and in our ability to break it - 23 days in July and all August were read, the throw-on system making breaking much easier with increased traffic volume. On September 1st the throw-on indicating system was abandoned and only 8 days in all were broken in the next month and a half, cribs being bad and the only available R.E's very difficult. On October 19th we had a typical stroke of good fortune, one Bonito group getting stranded, in the confusion following the retreat from France, with only Dolphin keys. This gave rise to a series of R.E's between Bonito and Dolphin which though they only lasted a short time, gave us an opportunity to examine the traffic again for cribs; these now proved to much better and when the Bonito/Dolphin R.E's expired we were able to continue breaking on cribs for the rest of the war, only 4 pairs being missed. Bonito is another striking case of a key on which we should never have got started had it not been for the "throw-on" system of indication.


32. The chief event in May was Op. 20 G's break into April Seahorse


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