Cryptographic History of Work on the German Naval Enigma

6. Meanwhile Porpoise had followed Dolphin's example, splitting into Porpoise (Italy), Catfish (Aegean) and Bloater (Asiatic). Catfish re-used September Porpoise keys until December 15th when it became an independent key - by this time there was little traffic and no attempt was made to break it. Bloater used a rehash of September keys until November 15th after which it became an independent key and was unbroken until January 12th when it suddenly rejoined Porpoise - a mysterious but welcome event, the only other known case of a key merging with its "parent" being the seasonal reunion of Laube with Dolphin when the Baltic froze. Porpoise itself gave a great deal of trouble and only one day in October was broken; this was partly due to bad cribs and partly to a very unlikely reflector/4th wheel combination. Just as there were W.O. rules for the daily key so there were "rules" governing the choice of the reflector and fourth wheel over a series of months. Porpoise was ultimately handed over to the research section and after a struggle it was set on its feet again.


7. In November (actually on October 29th) Sucker became an independent key. Fortunately it almost at once started re-encoding the Hoofden weather message from Dolphin and this continued for most of the rest of the war. The re-encodement being simple under certain circumstances the two keys might actually be easier to break than if they had been one. For once in each month there was a "triplet" (3 consecutive days on the same key); suppose one month that this happened in Dolphin on the 7th, 8th and 9th and in Sucker on the 29th, 30th and 31st. Then from the 10th to the 29th the key would be out of step i.e. the Dolphin pairs would be 10/11, 12/13, 14/15 .... and the Sucker pairs 9/10, 11/12, 13/14 .... . Only one day in a pair is hard to break - the second day is easy because there is only one W.O. and a known ringstellung. Suppose now that we have somehow broken Sucker for the 9th, Sucker for the 10th follows (paired day) giving Dolphin for the 10th (Sucker/Dolphin R.E.), 11th Dolphin (paired), 11th Sucker (R.E.) and so ad infinitum. A similar phenomenon occurs when keys are in step and the R.E. occurs on consecutive days in the two keys


< previous

next >