In the age before digital electronics and computers, code breaking the ciphered messages produced by the Enigma machine was almost impossible even if the code breaker had a working copy of the Enigma machine, as long as he didn't know the right combination of initial electric and mechanical settings, which were also periodically changed. Additional security procedures such as double encryption and using codes in the original free text message made it even harder to code break an Enigma message, so no wonder the German experts were over confident about the security of the Enigma machine despite the suspicions raised during the war.
Tremendous and continuous efforts were required to code break the German Enigma machine, to be able to continue to do so despite German improvements, and to do so fast enough before the achieved information was too old to be useful. The improvements made by the Germans several times before and during the war, both to the Enigma machine and to its operating procedures, enhanced Enigma security and made it harder to code break. Some of these changes caused "darkness" periods which lasted for months, in which the German Enigma messages could not be deciphered. The longest "darkness" period was with the German Navy's Enigma machine, which could not be deciphered for four years between 1937-1941, and remained the most difficult to code break of all German Enigma networks (army, Luftwaffe, etc), because the German Navy used the most advanced Enigma machine type, and used it with the most secure operating procedures.
The great efforts to code break the Enigma required the combined efforts and talents of brilliant mathematicians, code breakers, intelligence officers, and communications experts, deeply familiar with German language and mentality and with radio operator mentality and procedures. It also required daring and skillful combat operations, mainly at sea, some carefully planned and some exploiting rare opportunities. The Enigma code breakers also used an increasing number of machines, nicknamed "Bombe", which provided electro-mechanical computing power which helped them significantly shorten the process of deciphering the Enigma messages.
The first British "Bombe" machine was designed by Alan Turing, a genius mathematician which developed the theoretical model of the computer, and served during World War 2 as a senior code breaker at the British code breaking center at Bletchley Park. The advanced British "Bombe", was derived from the pre-war Polish "Bombe" (bombs) machine, developed by Polish code breakers who slowly deciphered pre-war early type Enigma messages in 1933-1938.
An additional security effort was needed, not just to keep the secret from the German intelligence despite an increasing number of various witnesses, but to also carefully balance between the desire to use the information achieved from reading German Enigma messages, and the constant risk that the results of combat operations based on that information will lead the German intelligence to conclude that The Allies read their Enigma machine messages. Many thousands of lives were at stake at either side of this delicate balance.
Overall, it is estimated that the success of the efforts to code break the German Enigma machine helped The Allies defeat Nazi Germany two years earlier than they would without it, saving countless lives, and making it one of the most successful intelligence operations in history. It it also important to note that German code breakers could read a significant part of the coded messages transmitted by The Allies.
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