Cryptographic History of Work on the German Naval Enigma

patterns - known as "box-shapes" - independent of the stecker and determined only by the wheel order and Grundstellung at which the indicators are encyphered. With only 6 possible wheel orders catalogues could be made of all possible box shapes with the machine positions at which they occurred and thus the daily key could be worked out].

5. The Germans however realized eventually the weakness of the throw-on system and on May 1st 1937 a new indicating system was introduced. It is an astonishing thing that in spite of this realization (which they must have had, otherwise they would never have made the change since the throw-on system is extremely satisfactory from the operator's point of view) they compromised through use of this method no less than six keys* - two of major importance in the course of the war.

6. Returning to the Poles, this was a very sad blow for them, but nevertheless they succeeded in breaking May 8th by means of a crib. On the basis of external evidence one message was assumed to be a continuation (or FORT) of another - there was to be an echo of this years later when Offizier keys were broken by FORTS - and was assumed to start FORT followed by the time of origin of the other message, repeated and enclosed between Y's. Numerals at this time were represented by the top line of the German keyboard QWERTZUIO, Q=1, W=2, E=3 etc. An example quoted by the Poles was "continuation of message T.O.O. 23.30" which becomes FORTYWEEPYYWEEPY and the method of attack was hereafter known as "Forty Weepy". The method involved a series of assumptions that pairs of letters were unsteckered and, as 14 of the 26 letters were in fact unsteckered, these assumptions were right sufficiently often for the method to work. A similar attack was used during the war on BROWN, a German Air Force key with only six stecker pairs.

7. The Grund for May 8th was found through a typical flaw in the changeover from one system to the other. A torpedo boat, call sign AFÄ, had not received its instructions how to use the new indicating system and was therefore told, in a message sent in another cypher which the Poles

*Note: "Key" is used for an independent line of traffic with its own machine settings.


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