Every month the operating instructions specified daily or more frequent changes to several variables.
A typical daily "key" gave the clerk instructions for the first three steps of the enciphering procedure.
1. The wheel order (Walzenlage): the choice and position of the three wheels to be used (e.g., I-V-III).
2. The ring-setting (Ringstellung) of the left, middle, and right wheels (e.g., 06-20-24 denoting FTX).
3. The cross-plugging or "steckering" (Steckerverbindungen) (e.g., UA PF etc.).
Envision yourself as a German strategist and you have to send the bombing details to the German spies in Britain.
Match the settings given below to the Enigma simulator to encode message from the German Officials.
Rotors: V VIII II
Reflector: C
Plugboard: AE BF CM DQ HU JN LX
Message: HHBlitzIIBristolXam
The cipher clerk would set up his machine accordingly. Until the end of April 1940, he then continued as follows:
1. He turned his three wheels to a position chosen at random, the "indicator-setting" (e.g., JCM).
2. He twice keyed his own randomly selected choice of text-setting, or "message-setting" (e.g., BGZBGZ).
3. This came out as the "indicator" (e.g., TNUFDQ).
4. He set his wheels at BGZ and keyed the clear text of the message, thus obtaining the enciphered text,
letter by letter.
The message as transmitted included four elements, as follows:
1. The preamble, transmitted in clear before the message itself, showing call-sign,
time of origin, and number of letters in the text; this was followed by his chosen indicator setting
(e.g., JCM) (No. 4 above).
2. A five-letter group comprising two padding letters (Füllbuchstaben) followed by the three-letter "discriminant"
(Kenngruppe), e.g., JEU, which distinguished various types of Enigma traffic and showed which of many "keys"
(sets of operator instructions) were being used. The latter were known at Bletchley by cover-names such as Kestrel,
Light Blue, etc.
3. The six letters of the "indicator" TNUFDQ (No. 6 above).
4. The enciphered text of the signal, in five-letter groups.