I checked some background information about the crypto war during World War II and posted it here:
Turing’s cipher story starts with a "mystery". ENIGMA (enigma) is derived from the Greek, and it is the cipher used during the war (in all military Among the codes of diplomacy, the most famous is probably the ENIGMA used by Germany in World War II), and it was Alan Mathieson Turing who cracked this code.
During World War II, Germany invented a seemingly undecipherable code "ENIGMA", which is a machine used for ENIGMA encryption and decryption. This code was widely used by the German army, including locating submarines on the Atlantic transportation line. These submarines sank the British ships at a frightening speed and were called "Atlantic Battle" by Churchill. Churchill was worried that the British army would be defeated due to a shortage of supplies, and the only solution was to stop the German submarine tactics. Cracking ENIGMA was one of the ways to stop the German army. If the British can decipher this information, they can determine the location and destroy the submarine. But for 13 years, the British and French believed that ENIGMA was undecipherable.
In response to this situation, the government established a new institution—Government Code and Cipher School (GCCS), headquartered in Bletchley Estate in Buckinghamshire. This problem was also handed over to Turing. Since September 1938, Turing has been responsible for ENIGMA's cryptanalysis. He led about 200 capable personnel in cryptanalysis, including even the world chess champion Alexander. The work of analysis and calculation is very complicated. 26 letters can replace 8 trillion puzzle letters in the "ENIGMA" machine. If you change the wiring, the change will exceed 2.5 quadrillion. Turing relied on his genius to design a deciphering machine. This machine is mainly composed of relays. It also uses 80 electronic tubes. The password is directly read by a photoelectric reader. 2000 characters can be read per second. It is called a "Turing bomb (Bombes)".
Turing’s method of deciphering ENIGMA is purely mathematical and theoretical. According to his colleagues’ memories, he created several new statistical theories in the work of deciphering codes, but none of them were published. Re-created by others, by A. The "sequential analysis" rediscovered and proposed by Wald is one of them. Sequential analysis is one of the important branches of mathematical statistics. Its characteristic is that when studying decision-making problems, instead of pre-fixing the sample size, sampling is done one by one until the sample provides enough information to make a decision appropriately. Among them, the application of "Sequential Probability Ratio Test (SPRT)" in cryptanalysis can significantly reduce the amount of keys required for related attacks.
Thanks to the efforts of this group, especially Turing’s excellent work, they mastered a complete set of methods to decipher the code, thereby understanding the movements of the German army, mastering the initiative of the war, and making outstanding contributions to the British-American coalition defeating Germany. On May 21, 1941, the deciphering team made a great contribution for the first time because it intercepted a secret telegram from Hitler to Admiral Raidel and sank the German battleship "Bismarck", which was then known as the world's most powerful battleship. In April 1943, Ichigo Yamamoto, commander-in-chief of the Japanese Joint Fleet, was about to fly to Kassili Bay on April 18. Kahili was intercepted and shot down only a few miles away.
Many of Turing's thoughts and foresights have been continuously verified after his death, and have always guided and promoted the development of computer science. The International Computer Association established the "Turing Award" in 1966 to specifically reward outstanding scientists who have made outstanding contributions to computer science research and the promotion of computer technology development.
Quoted from
"Turing-Code Breaker"
http://www.cms.org.cn/cms/zl/tuling/turing2012-4.pdf
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