Retrospect and Prospect.
The Nature of the Change.
1. June 1943 represents a turning point in the Hut's history; it is also roughly half way from the start of current exploitation [August 1941] to the end of the European war. This is therefore an appropriate point in the history at which to pause and take stock.
2. Up till then the following facts had conditioned our work (1) We had few keys. Until September 1942 only one main key (Dolphin) was being broken and even in June there were only three (Dolphin, Shark and Porpoise). Each of these had also its associated Offizier key (Oyster, Limpet and Winkle respectively). (2) Bombe time was a precious commodity and every effort had to be made to save it. [See the graph "Number of bombes available", remembering that bombes had to be tested and "run in" on arrival so that to get an idea of machines in action at any time the whole graph should be shifted forward about a month. (3) Straight cribbing (i.e. cribbing based on a thorough knowledge of past traffic) played a minor part in all our breaking. On Dolphin we had Banburismus, on Porpoise we used the "throw-ons" to give us Grundstellung alphabets and on Shark we relied on a highly specialized form of cribbing - W W's and B bars - in which knowledge of the traffic played little or no part. The breaking of daily Offizier steckers - once the original break for the month had been made on a crib - used a cryptographic technique in which about the only cribbing was to test for EINS or ERSTENS at the beginning of a message. The getting out of the 26 settings (i.e. the equivalents for the month of the letters A to Z) involved a fair amount of cribbing but not of a very difficult kind owing to the smallness of the traffic.
3. From June onwards (the change began earlier in some respects, but June is the best dividing date) the position rapidly changed. (1) The number of keys increased. Turtle was broken in July, Sunfish in August, Seahorse in September, Grampus in October and in 1944 Plaice,