consists (1) in encyphering EINS (the commonest four letter word in German Naval traffic) at all positions of the machine, using the W.O. and stecker for the day already worked out from the crib (2) In searching the traffic for combinations of letters which could be encypherments of EINS. Thus if AJLU was the result of encyphering EINS at machine position XYZ, say, and if AJLU occurred in one of the messages on the day's traffic a machine was set up at position XYZ and an attempt made to decypher the message; if the decypherment went EINS SIEBEN, say, the position was clearly correct while if it went EINS JSUVRN, say, it was equally clearly not correct and the EINS was just a fluke. In later days this encypherment of EINS and the sorting of the results against the encyphered message texts was done by the Hollerith section but this first "EINS catalogue" was made by hand using a modified Enigma machine - it must have been a perfectly prodigious labour!
22. It was now hoped that provided continuity could be maintained fairly rapid progress could be made using the Stecker rule. This proved rather over optimistic (I do not quite know why as, looking back, it sounds an eminently practical scheme) and little headway was being made when the long awaited "pinch" was made - not as a result of a planned attack but more or less by accident.
23. The "Narvik Pinch" was of an enemy trawler which was carrying a large assortment of miscellaneous objects included
(1) The Stecker and Grundstellung for April 23rd and 24th - the Stecker was scribbled on a small bit of paper and some days cryptographic work was done before it was discovered at all; an interesting example of the importance of retaining the most insignificant scraps for expert examination.
(2) Operator's logs giving letter for letter cribs for April 25th and 26th.
(3) Exact details of the method of working of the indicating system which confirmed Turing's discoveries and explained the use of the Schluessel kenngruppe (the top trigram used as a system discriminant - see Chap. I).