Cryptographic History of Work on the German Naval Enigma

Bruno Gamma (B C), would remain in force for the whole month. (2) Ringstellung (Clips or Rings). Tyre settings on all three wheels, reading from left to right. The tyre setting for the fourth wheel on the 4 wheel machine was always kept at Z until almost the very end of the war. There are 263 possible ringstellungs.

(3) Stecker. The cross plugging on the stecker board, usually ten pairs involving twenty letters; unplugged letters remain unchanged (i.e. as if plugged to themselves or "self-steckered"). There are about 140 million million ways of choosing ten stecker pairs.

(4) Grundstellung (Grund). This is a group of 3 or 4 letters, according to whether it is a 3 or 4 wheel machine, and is the position at which the machine is to set up to encode the message setting (see Section C).

15. It is worth noticing that the stecker has far more possible variations than any other part of the key (being in fact practically equivalent to an extra wheel with unknown wiring). Forms of the Enigma machine with no stecker are, therefore, a very much easier problem than those with stecker; the more self stecker a key has the nearer it approaches to the unsteckered Enigma and the easier to solve it becomes accordingly.

16. The point of the Ringstellung is to disguise the position at which the machine is set up for any particular message. Suppose we know that the window position for the start of a message is ABC; unless we know the Clip position it is useless to set the machine up at ABC because the body of the wheel will not be correctly placed and therefore of course the current will not go though the correct path.

"Paired Days".

17. One important weakness of German Naval Procedure was that the complete key did not change every day. On the "second day" of a pair the W.O. and Clips would be the same as for the first day only the Stecker and Grund changing. Normal procedure was for a completely new key to be chosen every other day though on some keys (e.g. Seahorse) a complete change was only made every 10 days. The effect of this was greatly to reduce the total time taken in breaking a series of days and also to make it easier to


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