a. Early work on Enigma:
Nearly all the early work on German Naval Enigma was done by Polish cryptographers who handed over the details of their very considerable achievements just before the outbreak of war. Most of the information I have collected about pre war days comes from them through Turing who joined G.C.C.S. in 1939 and began to interest himself in Naval Cyphers which so far had received scant attention.
The Heimsoeth + Rinke machine which was in use throughout the war and which I described in the first chapter was not the first machine to he used by the German Navy. In the 1920s, the so-called O Bar machine had been in use. This had 3 wheels and no Stecker and the curious characteristic of 29 keys - the modified vowels O, U and A being included. Of these 29 symbols X always encyphered as X without the current entering the machine and the remaining 28 letters were encyphered in the normal way. The tyres of the wheels necessarily had 28 letters printed on them and it was decided that the letter which had been omitted was the modified O: hence the name of the machine which was broken by the Poles and the traffic read.
The O Bar machine went out of force for fleet units in 1931 when the present machine was introduced and gradually disappeared altogether. The new machine had originally been sold commercially by the Swedes: as sold by them it had no Stecker and it was they who recommended the boxing indicator system which enabled so many Navy Cyphers to be read.
When the German Navy first started to use the machine there were only 3 wheels in existence instead of the later 8 and only 6 Stecker were used. The reflector in force was reflector A and boxing or throw-on indicators were used. Then we come to the account of how Porpoise was broken the weakness of this indicating system will be fully explained. For the time being let it be said that a cypher using this system is almost certainly breakable.
Having obtained photographs of the keys for 3 months, during which period the wheel order obligingly remained unchanged, the Poles broke the wiring of wheels 1, 2, and 3 by a ‘Saga', a long and complicated hand process which I shall not attempt to explain. Having obtained all the details of the machine, they were able to read the traffic more or less currently with the help of the indicating system and catalogues of 'box shapes'. These cata-
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