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99. THE ENIGMA MACHINE Children are often fascinated by the task of sending secret messages. These usually involve some simple rule of substituting letters, or of perhaps using the first letters in successive words of some given text, or of lines in a page. In times of war it is important for a nation to be able to communicate with its forces in a way which the enemy will not understand. To this end, the Germans in World War II used the enigma machine, which turned a plain text message into a secret code. This code was transmitted by wireless or other means, andthen decoded in another enigma machine. The parameters for the exact coding and decoding were changed every day, making the system very difficult for the Allies to break. Today there is a whole science of cryptography which deals with the problems of coding and decoding. Most of today's methods use great computer power, and often involve very large prime numbers.
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