Enigma was the Allied code name for the German cipher machine, used throughout the Second World War. Developed at the end of the 1920s, it was an electro-mechanical encoding device, and was the mainstay of all German military communications. What follows is not a history of the Allied efforts to crack Enigma; rather it is a physical description of the machine itself.

Appearance

The Enigma machine was enclosed in a wooden case, approximately 18 inches wide, two inches deep and standing ten inches high. On opening the front cover one found a telephone exchange-type plugboard with 26 pairs of sockets, which could be coupled in any sequence using short leads.

Opening the top cover revealed a typewriter-style keyboard used by the operator to input his message. Behind the keyboard was a display panel, made up of windows, each one marked with a letter of the alphabet; this illuminated as the operator pressed the keys on the keyboard, thus showing the encoded text.

Behind the display panel were three (or four) wheels, each of which could be set in one of 26 positions and which rotated in turn with every press of a key. The wheel settings were changed daily and the three wheels installed in the machine could be chosen from a choice of five available wheels.

The Encoding Principle

When a key was pressed, for example the letter 'B', an electrical current passed to the plugboard, where it could either be diverted to another letter by a connection lead, or otherwise pass straight through if no lead was used. From here it was transmitted to the right hand side 'entry disk' of the wheels. This 'entry disk', hardwired into the machine, had 26 contacts on the right hand side and connected to 26 contacts on the left, though obviously not straight through.

From the output of the entry disk, the current passed through three wheels in series. These wheels had 26 pins on their right-hand side, wired to 26 contact plates on their left. Each wheel was wired in a different manner; for example, the RH terminal one in wheel A would be wired to LH terminal 18, whereas in wheel B, terminal one would be linked to terminal seven. Once the current had passed through the three wheels in series, it arrived at the left hand side 'reflector disk'. Here it was once again diverted and returned through the three wheels, with its trajectory again being altered at each stage, back through the entry disk, the plugboard and finally back to the display panel; this resulted in the lighting of a letter.

After each key depression, the right hand-wheel rotated one position. After a complete rotation of the right-hand wheel, the next wheel in line would rotate one position, and so on. The effect of this was that pressing the letter K on the keyboard repetitively would produce different outputs on each occasion.

Another element in the wheels was the ring around the rim of each wheel; this was marked with the 26 letters of the alphabet and could be freely rotated and then locked into position. The position of these rings was defined in the daily settings, which defined that the ring should be rotated until a certain letter was aligned with a red dot on the inner portion of the wheel. These rings served two purposes. The first was that the ring position defined the turnover point of the next wheel. The second was that in operating conditions, the wheels were covered with the exception of just three small windows, on the outer cover of the Enigma machine; the sender of a message would have to transmit these letters in clear (unencrypted) to let the receiving station know the starting position of the wheels for that particular message. Interestingly, one fault with the Enigma system, which would later be exploited by Allied cryptographers, was the fact that an A would never return an A, B would never be B, etc.

Equipment Settings

The settings for the day included the following information:

• The plugboard settings

• The wheel order (that is, which wheels were to be used in which positions)

• The ring settings for each wheel (which letter was locked into position against the red dot on the inner wheel)

• The initial setting - the initial wheel position for all messages

These settings were unique for each day and for each type of code.

The Four-wheeled Enigma Machine

German U-boats used a variation of the system used by the German army, airforce and surface fleets. This machine was similar to the version explained above, apart from the fact that four wheels were installed, chosen from among the eight wheels supplied to the U-boat commander. These machines were used exclusively by the U-boats and their shore contact stations.

Operating Procedures

These procedures were those used for naval Enigma:

The operator would first set up the machine in accordance with the daily settings, wheels, wheel positions, and ring positions, plugboard connections and initial wheel setting.

The operator would then select a three-letter group (trigram) at random, from the trigram book, for example LXZ. He would then encipher this on the Enigma machine with the wheels set to their initial settings, which might be ALC. The resulting three letter group, say QSD would be used as the wheel position setting or message setting, for that particular message. The operator then turned the wheels to the message setting QSD and then enciphered his message.

The method for indicating the message settings to the receiving station was as follows:

In addition to the initial trigram LXZ, the operator picked out a second trigram at random, say BFA. These trigrams were then written down one above the other after a random letter selected by the operator was added to the beginning of one trigram and the end of the other. At this point, the two lines of letters would look something like this:

C B F A
L X Z B

Each vertical pair of letters in the group was then converted into the equivalent two letter group (bigram) using a bigram table held by each operator; thus the bigram for C/L would become R/E.

After each of the four sets of letters were converted, the two lines would look something like this:

R V M K
E Y P W

The letters were then manipulated to appear in the following format:

R E V Y
M P K W

These groups were then transmitted, in clear, at the beginning and end of the enciphered message. The addressee would then, with the help of the same tables used by the transmitting operator, work out the message settings used in transmission, set his Enigma machine to the same wheel settings, type in the enciphered text, and, hey presto, out came plain language on the display panel.

Conclusion

This entry is not meant to be a history of the Enigma machine and its alterations and improvements over the 1930s and '40s. Neither does it touch on the years of work that went into cracking the code, by the likes of Alan Turing at Bletchley Park. However, it does show the complexity of an incredible machine, invented over 70 years ago, and gives an idea of the extreme difficulties overcome by the British Intelligence Services in their battle to counter it.

Interesting Links

To see an Enigma machine in operation is also a unique experience.

Further Reading

• Enigma: The Battle for the Code by Hugh Sebag-Montefiore is an excellent factual account of events at Bletchley Park and the battle to crack naval Enigma codes.

• Enigma, a 1996 novel by Robert Harris tells the story of a fictional mathematician posted to Bletchley Park. The historical research behind this book has been excellently done and it gives a good insight into Bletchley during the Second World War. It has recently been adapted to film, but is the film ever as good as the book?