Military Use of the Enigma

Tony Sale's
Codes and Ciphers

This is Page 3 of Tony Sale's sequence of pages on the Enigma, explaining how the Enigma was used in German military practice, and outlining the problem facing anyone trying to break the system.

The Message Key and Setting Sheets

As mentioned on page 1 of this sequence, the message key, (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 sugestions, 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:

1. Set the Enigma machine into the base configuration for the day as given in the setting sheet for the month.

2. Select a three letter start position, (the indicator), from which to encipher the selected three letter message key.

3. Turn the rotors to the indicator position, key in the message key, twice, and note down the lamps that light.

4. Turn the rotors to the message key letters and key in the message to be sent, noting down the lamps as they light.

5. Give the enciphered message plus its preamble to the radio operator for it to be transmitted by Morse code.

On receiving a message:

1. Set the Enigma machine into the same base configuration for the day from the setting sheet.

2. Turn the rotors to the indicator letters received in the preamble to the message.

3. Key in the next six letters to reveal the repeated message key as the lamps light.

4. Turn the rotors to the message key letters. Key in and decrypt the cipher text.

Now you can try decrypting an actual German message using the German Enigma machine that was used by 'Hester', played by Kate Winslet, in the film Enigma.

The Enigma configuration for that day was:
and here is the intercepted message:
U8K DE C 1806 = 49 = DHO GXS =

Click here and you will be taken to Tony Sale's on-line Enigma simulator where you can decrypt this message.

For full information about Tony Sale's work for the film Enigma go to these pages.

The complexity of the Enigma machine

As mentioned at the start, it is assumed that the interceptor has an Enigma machine. The protection against decryption then depends on the number of setting combinations that have to be tried in order to decipher a message.

The basic 3-rotor Enigma has 26x26x26 = 17,576 possible rotor states for each of 6 wheel orders giving 6x17,576 = 105,456 machine states.

For each of these the plugboard (with ten pairs of letters connected) can be in 150,738,274,937,250 possible states.

The total number of combinations is thus (even for the simplest military Enigma) of the order of 15,000,000,000,000,000,000, and then there is the ring-setting complication on top of this.

The task facing anyone trying to decipher a particular message is to find which one of these 15 billion billion settings has been used.

The Germans considered the task to be impossible, and certainly even a modern computer might take a year to work through this number of machine settings, if it simply tried them out in turn.

However we have already noticed how there are a colossal number of alphabetic substitutions but nevertheless you can solve simple cryptograms by using such facts as that E is the most common letter, which eliminate huge numbers of possibilities at once.

So the size of the possible message-key space is not the only consideration.

We shall continue in further pages to show how first the Polish mathematicians and then the codebreakers in Bletchley Park achieved the apparently impossible.

Back to the index page for Tony Sale's Codes and Ciphers in the Second World War.

This page is created by Tony Sale
( the original curator of the Bletchley Park Museum