Cryptographic History of Work on the German Naval Enigma

of these limitations we might gradually have got to read a few of the Sonders carrying heavier traffic but with existing methods the large majority of messages would have remained unread. The "Sonders" are about the only type of key which make one feel the need for a statistical attack on Enigma instead of the normal method of cribbing. (1) With small traffic cribbing prospects are very poor (2) As there are really only 3 independent keys in all for each Sonder even an extremely laborious method of attack might have been worthwhile. Such a machine would, however, go far beyond any existing methods and I should think it extremely likely that it will never be possible to develop one.


18. Rorqual - a most un-Naval key, as it carried information about Allied air raids in the Ruhr - appeared on February 1st. It was broken during February and most of March, being of interest to Hut 3 though not to Naval Section. Towards the end of March cribs deteriorated and it became evident that the Ruhr was about to be overrun by the Allies. In early April the key was abandoned as of no further interest.


19. In March, Goby (Ursula) was broken. It first appeared at the beginning of the war and was a form of short signal using Shark keys and an indicating system of its own. A spirited but unsuccessful attempt was made to break it in January by assuming that two messages had identical content and having them both decoded by the Hollerith section at all 17,000 machine positions and the results compared to see if an identical pair could be found. A Narwhal message gave the show away in March and betrayed that Goby was using the 1939 E bar tables modified for the 4 wheel machine for its indicating system. Little traffic was ever sent and the interest of the result was mainly academic.

20. Staff shortage, caused a hold up in Plaice at this time. With very heavy traffic on a large number of different keys there had been most severe pressure on all the clerical staff particularly the decoders. This


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