Sample Message
one (of potentially several) in a QSO

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Color Code

DEFINITION OF ‘KEY’

Key data for an Enigma machine is seen as columns of information found on the monthly key pages. 
Each row of data specifies the initial settings for the machine for one date only.  

The initial settings specify
1. Walzenlage: the installed positions of the rotors;
2. Ringstellung: the rotor ring settings; 
3. Steckerverbindungen: the plug board jumper positions;  and 
4.  Kenngruppen:  key line validation groups.

KENNGRUPPEN

The Kenngruppen is composed of a three-letter group plus two randomly selected letters, e.g. the writer's initials.   The first three letters are used as a simple check that the correct key is being referenced for the current message.   There are four Kenngruppen options per day which may be cycled in column order.   
For our example of 12 DEC 2014,  the first option is FFF, which  in the example has become FFFSV.   Note that the Kenngruppen is the first five-letter group in the message and is not encrypted.  It is therefore not decryped either but treated always as plain text.   It is however included in the letter count!

TRIGRAMS

When composing a message, the sender decides upon two random trigrams,   e.g.  HWJ  and  YVP.    
The two trigrams appear at the end of  the message preamble line.   They are used to generate a third trigram which becomes the start position for the rotors for both encryption of plain text and decryption of cypher text for messages carrying that date. 

After the rotors have been selected and installed with their rings set according the the key, the machine cover may be closed.  The rotor start positions must then be set by rotating the rippled outer wheel of each rotor while viewing the rotor position through the small window, until the required letter of the first trigram,   (in this example)  H, W or J appears.

The second trigram, YVP,  is now typed on the keyboard and the result written down as each of three lamps sequentially illuminate, in this example it will be J, D and C.  This specifies the rotor start positions for encrypting the message proper, and are set on each rotor before beginning work with the second and subsequent five-letter groups.          (The first five-letter group is the Kenngruppen and remains in plain text at all times).

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