Cryptographic History of Work on the German Naval Enigma

was aggravated by the necessity for decoding two copies of most of the traffic, one for the Z watch and one for our crib room. In earlier days, when there were fewer keys, the Z watch had informed us of probable re-encodements [see Chap. ] and after the traffic had been translated the crib room girls entered up probable and possible cribs in the crib room "folders". When with the increase in keys the cribbing position became more complicated this system was both too chancy and too slow; it then became necessary to have two copies of the message decoded. When it became impossible for the decoding room to get through all they had to do, the solution adopted was to decode one copy only of Plaice (which was the biggest key at this time) and send this to the cribroom to be scanned before it went to the Z watch; this resulted in delaying the traffic approximately 12 hours.

21. A minor technical change was that the ringstellung on the 4th wheel which up till now had always been kept at Z (for no very good reason that we could see) suddenly began to vary in the same way as it did on the other 3 wheels. This was another example of a change made too late; in the days of short signals it would have been most serious since the reason that short signal cribs were 3 wheel and not 4 wheel jobs was that both the ringstellung and the message setting had fixed 4th wheel values - now however the change was merely a minor nuisance. Another rather tiresome event was a general breakdown of W.O. rules - whether by accident or design I cannot say.


22. These changes paled into complete insignificance before that introduced in April - no one can say that the German security service failed to keep going to the bitter end, for this last effort was one of the most formidable changes they had made. It was the introduction on Plaice of a set of 288 grundstellung, valid for a month, instead of having one grund for each day; the decision of which grund to use was automatically made for the operator by his choice of message trigrams. The discovery of what was happening and the reconstruction of the table of


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