Cryptographic History of Work on the German Naval Enigma
24. Fortunately this was available. At the same time that the W W book was captured the Kurzsignalheft and Kenngruppenheft i.e. the B bar code and indicator book was captured. B bars were short signals sent off by U-boats when on operations the code book providing a shorthand method of sending the information. A typical message might be
Cypher Decode Meaning
B'B'
[Written out vertically for convenience; text of course appears as QLTI SCYK AZEE TNFX OJ].
25. Cryptographic work by Ashcroft helped by N.S. IV had resulted in a considerable amount of the code book being recovered before its capture and 5 B bars using the same indicator had in fact been very plausibly cribbed and unsuccessfully tried on the bombe but material was not sufficient to make breaking possible until the pinch was made.
26. Even after the pinch no major use was made of B bars until the W W book changed. The W W cribs were extremely reliable and B bars were less good than they become later, so although we occasionally made use of B bar information when the W W information was not quite sufficient on its own their chief use was in the minor role of helping to break the paired day.
27. Now however we had to rely mainly on B bars and the results obtained certainly surpassed our expectations; from March 10th to the end of June 90 out of 112 Shark days were broken, the great majority on B bars (some were broken later on R.E's). After June, the improvement
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