NotesGerman high command had faith until the end of the war that Enigma was untouchable. However, the basic code had been broken by the Polish Biuro Szyfrow in 1932, with the help of a commercial Enigma machine and code books and manuals provided by the French. The French bought the documents from Hans Thilo Schmidt, who worked in the German Defence Ministry. Three Polish mathematics students, Jerzy Rozycki, Henryk Zygalski and Marian Rejewski, invented a machine they called a Bomba (different from Bletchley Park's Bombe) to identify repetitive letters.
In the 1930s, each encrypted message began with three letters that relayed the message setting; to reduce mistakes, the Germans repeated the three letters so there was a six letter cipher at the beginning of each message. The Poles analysed the message-settings with the Bomba and compared the first set of letters against the second, which let them break the code. As a result, the Poles could read about 75% of Enigma-encrypted messages by the early 1930s.
The Germans continually modified Enigma, however, and the addition of two more rotors in 1938 ended the ability of the Poles to decipher messages. In July 1939, just before the invasion of Poland by Germany, the Polish code breakers met with the British and French and passed on all the information they had amassed on Enigma, including clones of an Enigma machine with plug boards.
With this information, the British began working on the code at the Government Code and Cipher School at Bletchley Park. In April 1940, a team including Dilly Knox, John Jeffreys, Peter Twinn and Alan Turing broke the Luftwaffe and Wehrmacht operational ciphers for the first time. It would be an ongoing effort throughout the war, as the code had to be broken anew each day.
Aided by a captured Enigma machine and code books, Bletchley Park was decoding and reading over 4,000 Enigma-encrypted messages per day by 1942. The Y Service, a chain of radio intercept stations around the UK and Europe, intercepted German messages, which would then be sent to Bletchley Park. There, the cryptographers used the Bombes to identify pieces of text called cribs to break the ciphers. Cribs were words the cryptanalysts guessed to be in the message because German military communications often included specific phrases. For example, many messages included the words "Heil, Hitler" or a weather report, which was the crib used to break the code on D-Day. The crib would help analysts and the Bombes preclude certain rotor settings.
The Bombes would cycle through the potential rotor settings to reduce the number of settings that could be possibly used. What remained would then be deciphered by hand. Others at Bletchley Park would decipher the actual messages once the settings were discovered. The decrypted messages were then passed on to special liaison units to decide what intelligence (code named ULTRA) was useful and could be used. The Allies could not use everything gathered from the broken codes, as they risked alerting the Germans to the fact that the Enigma code had been broken. Nevertheless, decoded Enigma messages provided crucial tactical information in Europe and the North African campaigns.
After the end of the war in 1945, most Enigma machines were believed to have been destroyed. Still others were taken by the Allies, though several countries continued to use it as their principal cipher machine, including Norway, Germany and Austria. Versions of Enigma were refined and modified by the Americans and Russians, which provided the basis for the more sophisticated Cold War-era rotor-based cipher machines M-125 (Fialka) and KL-7 (ADONIS and POLLUX).
Enigma machines were quite rare until the end of the Cold War, when the fall of the Berlin Wall revealed that many had survived in Eastern Europe.
Colossus: The Secrets of Bletchley Park's Code-breaking Computers, ed. B. Jack Copeland
Code-breakers: the inside story of Bletchley Park, ed. F.H Hinsley and Alan Stripp
Alan Turing, Andrew Hodges
Enigma: The Battle for the Code, Hugh Sebag-Montefiore