E is for… cracking the Enigma machine

The Enigma machine was invented by a German engineer, Arthur Scherbius, shortly after the First World War. The machine resembled a typewriter, and there were a number of varying types produced. It was adopted by the German armed forces as an unbreakable way of sending coded messages.

Polish codebreakers had broken Enigma in 1932, and with the prospect of war in 1939, informed the British of their success. Dilly Knox, one of the former British codebreakers of the First World War was convinced he too could break the system and set up the Enigma Research Section at Bletchley Park. Comprising of himself and Tony Kendrick, they were later joined by the great minds of Peter Twinn, Alan Turing and Gordon Welchman. 

Enigma I, A02192, the most common version in use by the Germans during WW2

Bletchley Park was covertly set up to decode intercepted Nazi messages during the Second World War. Working in the stable yard of Bletchley Park, the first wartime Enigma message was broken by the British team in January 1940. The Enigma devices typically changed settings every 2 hours, with up to 60 septillion possible combinations every day depending on the Enigma model.[1] The Bletchley staff worked around the clock every day to break the setting by hand, and it was quickly realised that a mechanical method for identifying the keys was needed to speed up the process. 

The Bombe machine was designed by Alan Turning, and created by engineers at the British Tabulating Machine Company in Letchworth. They were designed with multiple drums representing each of the Enigma machines rotors. This allowed for the potential setting that correlated to the enigma machine being used to transmit the message to quickly be checked by the Wrens drafted into Bletchley Park to operate them. The machines were housed in Huts 11, 11A and 11B in the Bletchley Park grounds, and the work was deemed oppressive by the women who undertook the long shifts in the dark, stuffy rooms.[2] However, the work was vital to the war effort, and without the effort both the codebreakers and the women put into intercepting the Enigma messages, the war could have taken a different course.

A working Bombe model in the Bletchley Park exhibition

By speeding up the process of breaking the daily-changing Enigma settings, Turning’s invention meant that staff were able to decode the messages far quicker than they had previously, allowing enough time for the information to be acted upon by the military.[3] The breaking of the Enigma code had huge impact across land, sea and air battles during the Second World War, including the sinking of the German battleship Bismarck in 1941, the battle of El Alamein in 1942, and the defensive preparations of the Germans prior to the D-Day offensive in 1944.[4]

The Enigma codebreaking team’s first messages, codenamed ‘Red’ at Bletchley Park, were intercepted from the German Air Force, using their General Operational Key.5 These messages were codenamed ‘Red’ because of the red pencil Welchman used to annotate them.[5] Regularly deciphering ‘Red’ messages from 22 May 1940, message in the Bletchley Park archive show they were regularly noted with the following: the encrypted message in groups of five letters as it was originally transmitted by Morse code, the decrypt of the same message printed on teleprinter tape glued to the page, and notes in red pencil indicating how a Bombe machine could be set up to try to find the settings used on the Enigma machine that encrypted the messages.[6]

Detail from the front of a ‘Red’ messages, sent by the German Air Force using their General Operational Key, annotated by Welchman

Messages sent using other Enigma keys were marked with different coloured pencils, as Welchman had done. Later, when Bletchley Park ran out of colours, they began naming the keys after other things, such as plants, insects and birds.[7] Various markings were used by the codebreakers to make sense of the Enigma messages, such as vertical lines to separate words, a curving line under words to indicate a name, and numbers written out in Arabic numerals below the text.[8]

The term ‘abstimspruch’ was adopted by the Enigma codebreakers, used as a ‘crib’ or a guessed piece of the message that gave the codebreakers a starting point to break the encryption.[9] This in turn could provide a whole day’s worth of messages on an Enigma network. Each Enigma message would begin with a ‘preamble’, the sender’s call sign, the time of transmission, the number of characters in the message and the indicator (a group of characters that helped indicate the Enigma message setting).[10] The hard part for Bletchley codebreakers was that the indicator only applied to that particular transmission and could not be used to work out other Enigma settings needed to read the message. 

The interception of these messages was extremely useful, noting things including the movement of individual, units or equipment. The decrypted Enigma messages would be passed on to the intelligence service Ultra, for analysis.[11] (Ultra will be explored later in the A-Z series). Throughout the remainder of the war Enigma messages continued to be broken and passed on to Ultra to help the Allied war effort.

You can learn more about the cracking of the Enigma code, and the Bombe machines at Bletchley Park’s permanent exhibition, ‘Hut 11A: The Bombe Breakthrough’ which, having personally visited, offers a great insight into the challenges posed by Enigma, as well as oral histories and objects from those working on the machines.

[1] Bletchley Park (BP), ‘Six Facts You Need To Know About The Bombe Machines,’ last accessed 5 April 2021, https://bletchleypark.org.uk/whats-on/hut-11a-the-bombe-breakthrough 

[2] BP, ‘Six Facts You Need To Know About The Bombe Machines,’ last accessed 5 April 2021, https://bletchleypark.org.uk/whats-on/hut-11a-the-bombe-breakthrough

[3] Ibid

[4] Ibid

[5] BP, ‘Collections uncovered 4,’ 2 May 2018, last accessed 5 April 2021, https://bletchleypark.org.uk/news/collections-uncovered-4

[6] Ibid

[7] Ibid

[8] Ibid

[9] Ibid

[10] Ibid

[11] Ibid

[Image 1] BP, ‘Collections uncovered 15,’ 27 March 2019, last accessed 9 May 2021, https://bletchleypark.org.uk/news/collections-uncovered-15

[Image 2] BP, ‘Hut 11A: The Bombe Breakthrough,’ last accessed 9 May 2021, https://bletchleypark.org.uk/whats-on/hut-11a-the-bombe-breakthrough

[Image 3] BP, ‘Collections uncovered 4,’ 2 May 2018, last accessed 5 April 2021, https://bletchleypark.org.uk/news/collections-uncovered-4

2 thoughts on “E is for… cracking the Enigma machine

  1. Very detailed. I like it a lot. I did know about Alan Turing and how the enigma machine helped in beating the submarine threat. As Dönitz had a lot of cummunication with his mostly inexperienced u boat crews.

    Great work they did indeed!


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