their own with the messages in front of them and trying to get it out by genius would have got nothing but a headache. This sort of cooperation was vital all through but the W W's and B bars provide a very clearcut and complete example.
33. Secondly the extreme folly of linking higher grade with lower grade cyphers comes out very clearly. The W W's exhibit a sort of Rake's Progress in the cypher world. (1) We have Shark a four wheel key and as such almost unbreakable at that time (2) It is reduced to a three wheel key by use of the machine in a simplified form on W W's (3) The security of this is made to depend on that of a trigram substitution, breakable by hand methods (4) In the second month of its use the trigram substitution is repeated so that the text is virtually in clear!
34. Having described the main current of events in the September - June period, we may retrace our steps a little and deal with other miscellaneous happenings. On February 19th, amidst sentimental lamentations, we left the old Hut 8 and moved to Block D. The hut had by this time become too small for us - we had grown from about 30 people in early 1941 to 125 and were still increasing - and the move was clearly right although we all had a feeling for the old hut which made our new residence seem very bleak at first.
35. In March the bigram tables changed again. Now however there were 70 bombes we had no difficulty at all in building up the new bigram tables - to our relief these were not pinched so we had the satisfaction of completing them unaided. As there were 9 different tables and we were now 10 strong, one man went without (I can't remember now who was the victim) and we each had our own table over which we pored constantly, getting out new bigrams, correcting old ones, tracking down complicated contradictions, until we almost knew all the bigram equivalents by heart. It was a fascinating business - quite comparable in interest with Banburismus, depth cribbing, Offizier stecker, "dotteries" or any of our more regular occupations.