The World War 2 German Enigma Encryption Machine

The German Enigma is probably the best known of the WW2 cipher machines used by either side in the conflict. It is an electro-mechanical device that utilizes a stepping wheel system to ‘scramble’ a plain text message to produce a code by what is known as poly-alphabetic substitution.

The number of cipher text alphabets is astronomically large – a fact that led The German military authorities to believe, wrongly as it turned out, in the absolute security of this cipher system. In the case of the three rotor machine with Stecker board 150 million million combinations.

Enigma’s principle.

The Enigma machine consists of a number of components.  A message to be enciphered is input from a keyboard that is similar to an old mechanical typewriter. The signal leaves the keyboard and passes through a plug-board (know as Stecker from the German word for plug) where, if the plug-board socket contains a connector, its identity is switched in a mono-alphabetic substitution. If the particular socket does not contain a plug, the identity of the input character is unchanged. From the plug-board the signal passes to the entry rotor which passes it to the first of a series of three wheels. Each of these wheels has twenty-six contacts on each of its faces which are in electrical contact each with its adjacent wheel or wheels.

Inside each wheel the contacts are cross-wired in a random fashion so that the identity of an in- coming character is changed three times as it passes through the three wheels.  With each keyboard input the extreme right-hand wheel moves one position. Once it has rotated completely, the wheel to its left steps once, and in turn, one full rotation of that wheel makes the next left hand wheel to step once.  After passing through the three wheels the signal reaches the reflector that performs two functions. First, it changes the signal’s identity once more, and second, it sends the signal back, in the reverse direction, through the three wheels to the entry stator.

From here it passes back to the plug-board and then to the lamp-board where a lamp corresponding to the now enciphered character is illuminated.  It was the bi-directional nature of the plug-board’s substitution process and also the fact that, because of the reflectors’ function in the enciphering process, no plaintext character can ever encipher to itself.  These two weaknesses m the system were to be exploited to great effect by the Allied crypt-analysts.


How the machines were set up and used is an important part of both the development and later on the breaking of the encrypted massages. Three people were used to encrypt a message. The first would key in the letters which would make the lamp-board display an encrypted letter, the second operator would then write this down and the third person would send the data by Morse code further split up into four letter groups. At the receiving end three people would repeat the process in reverse to decipher the message.

Because of the nature of encryption and deciphering it was paramount that two machines at either end of a communications link were set to identical settings. Those settings were either taken from a set of tables or changed according to another set of parameters. The set-up a machine was as follows.

1: Select three wheels from a set of five in the case of the Army and Air Force machines or from a set of eight in the case of the Naval machines. Another machine (called an M4) needed a further selection of a fourth wheel and a corresponding reflector.

2: Set the ring-setting on each wheel and arrange these wheels in the machine in the order prescribed for that set up. The ring-setting is the position of the rings before they are revolved by keystrokes during use.

3: Set up the plug-board (Stecker) according to the same instructions. Normally, ten sets of plugs were used leaving six letters  ‘self-steckered’.

Enigma was then ready for use.

The History Of Enigma.

The first model was built in 1916 and patented by Arthur Scherbius in Germany in 1918 but it initially aroused little interest until the advent of Radio Communication. In 1922 they were shown at an exhibition in Berlin and sold commercially. The German Navy bought their first machines and began using them in 1926 followed by the Army in 1931. It was the Army who added the Stecker-board. In 1932 Polish mathematicians were recruited to break the Enigma. Marian Rejewski invented his “characteristics” based on the German error of double enciphering the message key.

In 1936 the Poles managed to acquire and thoroughly examine a machine. One was sent via the polish postal service for delivery to the German Embassy in Poland, but the Poles managed to delay delivery over a weekend. In the two days available, they took the machine apart, and thoroughly examined every part and detail. They reassembled it and dutifully delivered it on the Monday morning without suspicion.

After that they set about making them and had a very good understanding of them. Unfortunately no tables were included with the machine they examined, the table being used to choose and set the rotors and Stecker settings. Up until 1937 the settings were sent at the beginning of a message, but then the German Navy added the Bi-gram tables to conceal the message key.

Our Bletchley Park UK (BP) decoding centre was opened in 1938 when war in Europe was looming and in response to increased radio traffic that had been observed carrying coded massages. In 1939 a famous meeting took place in the Pyry Forest when the Poles told everything they knew to the British and French authorities, and gave them copies of the Enigma machines.  The French did not seem to do much with their machine or the information, but the British knew exactly what was going on and how valuable the gift was. In that same year the number of wheels available was increased to five.

In August 1939 BP began work in earnest with the machines and knowledge they had received form the Poles. Alan Turin and his teams knew what the Enigma was trying to do in scrambling messages and looked for weakness in the system. He worked out how the Bi-gram tables worked, and first broke a few German Naval Enigma messages. He soon realized that they needed to “number crunch” large amount of data, and designed a machine that he called a ‘Bombe’ to do that job.  The Turin Bombes as they were known, used thousands of valves to help in deciphering. These machines could run through masses of combinations very quickly and “looked” for patterns and coincidences. These were then given back to the people who finished the deciphering.

In 1940 huts 3 & 6 were set up at BP to break German Army and Air Force Enigma. In January that year they made their first breaks into the Air Force codes at BP by Dilly Knox in The Stable Yard, and by Marian Rejewski with the French at Chateaux Vignilles in Paris. Also that year BP took delivery of the first Bombes code named Victory.

In 1941 Hut 8 was set up for Navel code breaking. The capture of Bi-gram tables gets them going using Alan Turin’s “Banbarismus” techniques. However, in February 1941, U-boats started using a forth wheel, this was called the shark code. This caused a nine month black out of any deciphering which nearly lost the Allies the Battle of the Atlantic.  With more Bombes coming on stream outstations were opened at Eastcote and Stamnore in 1942. D block opened in 1943 at BP where now over 3000 people were working on all aspects of code breaking. That same year four wheel Bombes came on stream, and finally later that year the new codes were cracked, allowing us to discover the deployment of the U-boats, resulting in them being largely withdrawn from the Atlantic.  Another help in the confirmation of codes broken was to was ask the RAF to bomb a certain area, knowing this would be mentioned in German reports, and then the listening and deciphering of information.

In 1944 the Germans introduced the plug-able reflector, UKWD, known as Uncle Walter D (also known as the UHR). This was a switch-able Stecker box, which was changed on the hour and hence its name. By the 6th June the breaking of the Enigma codes was crucial in the planning and implementation of D-Day. By the end of the war some 10,000 were employed at Bletchley Park, when some 10 rotor wheels were in use.

Compiled for Hastings and Electronic Club UK Magazine, the Vital Spark. 20 November 2002.

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