Cryptographic History of Work on the German Naval Enigma

(4) The E' tables and description of how they worked. The "Long E bars" (Alfa - Funksignale) was a system devised for rapid communication by ships in action. The content of the message was first abbreviated by means of a standard phrase book (the Signalbuch) and then encyphered on the Enigma the message setting being denoted by a three figure number whose equivalent was given in the E' table in use. The signalbuch being in our hands these E bars were a fairly good source of cribs since their language was inevitably stereotype. [See Chap.]

24. Given the stecker and G/S April 23rd and 24th were easily broken and the "paired days" (days with the same W.O. and ringstellung - see Chap. I) April 22nd and 25th soon followed. April 26th presented a much nastier problem. By now it was known that at least two more wheels 6 and 7 had been introduced, these having been captured in early 1940, so that there were at least 7 x 6 x 5 = 210 W.O's to try. An attempt was made to break by hand methods but this attack - which would certainly have been successful in the old days of 6 stecker pairs - failed on the more difficult 10 stecker problem; the first hand break under these severe conditions was not made until June 1943 when a Shark (U-boat) key was broken by hand on a long re-encodement. [See Chap. ].

25. It was now May 1940 and the first bombe had just arrived so the crib for April 26th obtained from the operator's log was given to it to test its mettle and after a series of misadventures and a fortnight's work the machine triumphantly produced the answer.

26. April 26th being out, the paired day April 27th was soon broken and it was found that April 24th and 27th were on the same bigram table. This gave a clear programme for the future. (1) Every effort must be made to get out all the individual messages on April 24th and 27th and hence to find the value of the maximum possible number of bigrams on this table. (2) It was possible, owing to the fact that some trigrams (Verfahren kenngruppe) were more popular with operators than others, to identify the table in use on any particular day. Banburismus


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