Cryptographic History of Work on the German Naval Enigma

grunds involved breaking a very large number of individual messages and it was only the existence of Fillibuster that made it possible to do this in a reasonable time. By the end of the month most of the table had been built up and the great bulk of the traffic could be decoded immediately.

23. In May, a new table appeared on Plaice and Dolphin also took up the system using an independent grund table. Combined with this a change of bigram tables was announced for May 5th. As the method of building up new bigram tables depended on a single grund being used we were faced with a major cryptographic problem. The proposed method of solution is too technical to examine here; the rough outline was

(1) Using all our "message breaking" apparatus - chiefly Hypo and Fillibuster to build up the Plaice and Dolphin grund tables as much as possible in the period May 1st - 5th before the bigram tables changed.

(2) To find as many bigram values as possible by using Sucker, Shark and Narwhal messages all of which had remained on the old grund system

(3) When some progress had been made with the tables a modified form of the old technique based on a single grund could be used to utilize messages broken out on Plaice and Dolphin. This was all going to need a great deal of skilled assistance and six cryptographers were borrowed from other sections to help with the work. Distribution difficulties - not surprisingly - were however too much for the Germans and the new tables failed to come into force; when captured there proved to be 15 of them rather than 9 so that problem would have been even worse than we anticipated.

24. It is interesting to conjecture how far we should have survived this change. It would certainly have taken three months to get straight during which period only a portion of the messages on each key broken could have been decyphered. Whether or not we could have kept in sufficient touch with the traffic to maintain cribs is difficult to estimate; cribs were getting very bad anyway at this time but this was because of the approaching end and the necessity of continued breaking would imply an extension of the war and consequently more stable conditions.


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