Cryptographic History of Work on the German Naval Enigma

37. On June 17th the Northern U-boats split away from the rest of Dolphin and formed a separate key of their own, Narwhal. From June 17th - Sept. 1st it was merely a stichwort of Dolphin but after this it became entirely independent. It was not on the whole a difficult key being closely linked by R.E's with Plaice and Shark.

July, August.

38. Little of interest happened in July and August. Mediterranean keys, due to our regrettable military successes in this area, became steadily harder. Turtle (the Mediterranean U-boat key) and Grampus (Black Sea) were both abandoned in August owing to the increasing difficulty in breaking and their steadily decreasing importance. Porpoise continued to be broken but the capture of crib centres made it much more difficult than before; there had also been a certain amount of re-encoding between Dolphin and Porpoise arising from Dolphin keys held in Southern France - these keys were now destroyed and the R.E's vanished.

39. There were two events of some technical interest in connection with Bonito. (1) The breaking of a day on a depth crib - three messages had the same indicator and using our knowledge of "beginners" (the name used for standard beginnings of messages) we were able to crib enough of the three messages to make a "menu" for the bombe. Depth cribbing was very common when we Banburised, as the work done in Banburismus enabled us to align messages correctly in relation to each other, but had fallen into desuetude since; this revival on Bonito gave us our first break into this key on a crib, all previous breaks being on throw-on or alphabet menus. (2) Dolphin "Stab" was broken on an R.E. from Bonito.

40. "Stab" was a sort of sort of super-Offizier key. It was used in just the same way as the ordinary Offizier, i.e. a message was first encyphered in Stab and then in the main key, but whereas ordinary Offizier had the same W.O. and ringstellung as the main key Stab had an entirely different key. This combined with the smallness of the traffic, made it practically unbreakable and this was the first (and only!) time we had


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