Cryptographic History of Work on the German Naval Enigma

of shots. Now, however, it miraculously settled down and remained a first class crib until June 1944 - and that was a very long life in the cribbing world. The German security service spotted the crib (they spotted most of them all right) and sent a signal to the originator in Shark saying "Biscay Weather appears daily at an almost identical time, much of the text is identical, the length identical, the beginning identical and it is always repeated on the "Ireland" frequencies." Only the last sentence of this reproof (sent in January 1944) seems to have sunk in as the sole effect was that in future the originator sent out a separate version on the Ireland frequency, providing two cribs instead of one (not that the second crib was a very good one as it happened). From now until the end of the war every Shark day was broken - September 12th was the last day that failed to come out.


15. In November Dolphin began to look rather unpromising and a faint (but very faint) gleam of hope sprang up in some optimistic Banburists' hearts that maybe the good old days might yet return. However a thorough investigation of the whole Dolphin cribbing position by Mahon speedily disposed of this unworthy aspiration and produced several new cribs. This was done by an analysis of a number of apparently poor cribs which had too many different forms to be usable; on close investigation it was noticed that these different forms occurred at regular intervals and were due to different operators being on shift. The shift system was three shifts 0 - 9, 9 - 16, 16 - 24 and if A, B and C represent the three operators the rota was BAC/ACB/CBA/BAC/ACB etc. If A invariably used one form, B one of two others and C was security conscious and had no regular form at all then although at first sight the crib would look hopeless, having 3 common forms and a large number of other possibilities, on analysing into shifts it should produce a first class crib one day in three, two alternatives one day in three and only when C was on duty would it be hopeless. The first use made of shifts was by Wylie in Spring 1942 on a Channel Weather crib; this revival of it was of very great value to us.


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