The message key which was the complete and exact configuration of the machine in its starting position, had to be conveyed to the intended recipient of the message.

The Germans, following Scherbius's original suggestions, decided to specify exactly everything except the rotor start position for each 24-hour period. This was achieved by pre-printing setting sheets — a month's settings on one sheet, which was distributed by courier.

Geheim! Secret indeed! This is an example of the setting sheets used:

The sheet shows a line of entries for each day in the month.

On the top line, the day 31 is followed by the rotor order for assembling the rotors on their shaft, in this case: I V III.

The next three numbers are the ring settings. The setting sheets says that the ring on the left hand rotor should be turned until 6 (letter F) is by the catch, 20 (letter T) on the middle rotor and 24 (letter X) on the right hand rotor.

Next come the Steckerverbindungen or plug connections. There are ten pairs of letters and the two-pin plugs at each end of a piece of wire are plugged into the letter sockets given, i.e. U to A, and so on.

For the moment we can ignore the last section (the Kenngruppe) which was used in various different ways at different times by the German armed forces.

Both sender and intended receiver have identical copies of the setting sheet and so both can put their machines into exactly the same 'base position' for a given 24-hour period.
Now what remains is for the sender to choose, and then convey to the recipient, a rotor start position. The principle was that this should be a different start position for every message sent.
The method for this was to use the Enigma machine itself to encipher this crucial part of the message key.
There were different systems used by different forces at different times, but we will describe the simple system used before the war by the German Army and Air Force.

First, with the machine in the base configuration, the rotors were turned to a start position selected by the operator but sent in clear in the message preamble. Then the operator keyed in one after another the three letters actually to be used for the start of the message.
The lamps that lit (the encipherment) were then also sent in the message preamble.

But the Germans made a bad mistake in devising this system. They required the operator to key in the message key letters twice in succession and transmit the six letters that resulted. This was a primitive form of error-correcting code, ensuring that this vital message key arrived correctly, despite possibly bad radio connections.

But it meant transmitting redundant information, and this mistake gave the Polish analysts their great success in the period just before the outbreak of war.

Sending and receiving a message using the simple Enigma indicator system To send a message: Set the Enigma machine into the base configuration for the day as given in the setting sheet for the month. Select a three letter start position, (the indicator), from which to encipher the selected three letter message key. Turn the rotors to the indicator position, key in the message key, twice, and note down the lamps that light. Turn the rotors to the message key letters and key in the message to be sent, noting down the lamps as they light. Give the enciphered message plus its preamble to the radio operator for it to be transmitted by Morse code. On receiving a message: Set the Enigma machine into the same base configuration for the day from the setting sheet. Turn the rotors to the indicator letters received in the preamble to the message. Key in the next six letters to reveal the repeated message key as the lamps light. Turn the rotors to the message key letters. Key in and decrypt the cipher text.

The complexity of the Enigma machine