The original of this document is held in the United Kingdom
The National Archives, Kew, Richmond, Surrey, TW9 4DU. Reference HW 25/3.
In the period from about 1931 to April 30, 1937 the Naval cipher used much the same indicating system as the other German service enigma ciphers, viz. the 'boxing' method recommended by the firm that sold the 'commercial' enigma. With this system as well as the set up of the machine consisting of wheel order, Ringstellung and Stecker, there was a window position fixed for the day, and known as the 'Grundstellung'. When it was desired to encipher a message one first chose three letters at random from a list of about 1700 trigrammes e.g. ZLE. One then set the machine to the Grundstellung and enciphered ZLEZLE. The resulting six letters were put at the beginning of the message, and the remainder of the message consisted of the result of enciphering the plaintext with pre-start window position ZLE. (This differs from the other boxing indicating systems in that most of these allow the trigramme such as ZLE to be chosen at random instead of from a restricted list.
The weakness of this indicating system is that a great deal of information is given away about the 'Grundstellung'. If there were no Stecker, and a known diagonal and the traffic amounted to 100 messages per diem it would be possible to find the connections of the machine, and if there were Stecker but the connections of the machine were known it would be possible to find the keys every day from the same amount of traffic. To explain the possibility of finding the keys let us suppose that the following were a set of indicators for one day's traffic.