Cryptographic History of Work on the German Naval Enigma

message (2) If we could break one message could we then get into other messages by hand? The system of indication on Offizier (26 set-ups each represented by one of the letter A - Z and valid for a month) connected up with the whole month's traffic in a network and the problem of making headway from an initial break boiled down to this "Given the W.O., ringstellung and message set-up i.e. everything except the stecker, can the steacker be worked out without any knowledge of the content of the message?" It was known that on very long messages the answer was "Yes", but Offizier messages were not as long as all that. In the late Summer of of 1941, Yoxall started work on this problem and re-discovered the method of the "E rack" - a known technique but one whose importance in this connection had not been realized by us as it should have been - and caused a considerable sensation in breaking an "example" that had been set him on a length of 80 letters. This was particularly striking as it happened about a day after Turing - rightly recognized by all of us as the authority on any theoretical matter connected with the machine - had stated his opinion that 200 letters constituted a theoretical minimum. Yoxall must indeed have been inspired on this occasion as none of us ever succeeded in after days on a length of under 100 letters without some sort of crib.

13. This was a very important piece of work as it showed, what had previously not been believed, that if [we] could only get into one message we could probably read the whole month's traffic. We had read the Offizier for June (the June but not July Offizier having been pinched) and Noskwith, who was in charge of Offizier research from the crib room side, finally succeeded in producing a right crib - another feat which we had thought might well prove impossible on such non-routine traffic - and September Offizier was broken.

14. Once in, Yoxall's method proved itself in practice and the whole month's keys were read. As with almost all cryptography the first break was the hardest and except when traffic was very small we nearly always succeeded in breaking the Offizier for the month - though


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