The History of Hut Eight

changes of tables, and even of system within the categories, so that a complete picture of Short Signals involves a study of several German documents, which are in our hands. In the following pages all documents marked with an asterisk are available and should be read in conjunction with this chapter. Documents containing indicators are marked (I), Containing code books (C).

ii. Uses of Short Signals.

Any Short Signal had a peculiar cryptographic importance provided that the book on which it was based was known sufficiently to make some form of cribbing possible, and provided that it was possible to deduce the message setting before the keys of the day on which it had been cyphered were broken. This implied that the indicator tables were at least partially available.

The chief uses of Short Signals were:

a). For assisting in the technical problem of finding the Ringstellung when the Stecker and Wheel Order were known.

b). For breaking the Stecker on paired days.

In this case wheels, clips, and message setting were known but not Stecker. With a crib of about 10 letters the Stecker was found by a hand process. The method was equivalent to testing by hand all Stecker assumptions at one position of the bombe, not only the relative positions but the actual positions of all links on the menu being known.

c). For breaking keys of a completely new pair of days.

Menus of bombe strength were often made from one or more Short Signals; more than one signal could be used because from the indicating system the correct relative positions of all signals were known.

The disadvantages of the Short Signal menus (which normally involved rather scrappy cribs) prevented their extensive use until the 4th wheel had been introduced on Shark in February 1942. From this time, until the summer of 1943, Short Signals provided the only practicable method of breaking Shark - for a very particular reason.

All printed indicator tables for Short Signals were designed for use on Dolphin, Shark, and other keys, and only gave three wheel settings for messages. These were converted into 4 wheel settings by an order saying that the Reflector Wheel was always to be placed at A. This order was in force until November 1944 and throughout this period the clip on the Reflector Wheel was kept at A. All Short Signals therefore were effectively encyphered on a 3 wheel machine and menus based on them were 3 wheel jobs.


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