What Happened on July 15th
The ENIGMA machine was introduced into the German Army and used for the transmission of coded messages. A simple German machine the size of a portable typewriter, ENIGMA allowed for security in communications by a process in which typed letters were replaced by a cipher text displayed on illuminated lamps. The cipher was symmetrical so entering the cipher text into another ENIGMA reproduced the original message. Security was provided by a set of rotor wheels and a series of patch cables whose arrangement was agreed upon previously.
ENIGMA was used extensively by the German military during World War II to transmit battle plans and other secret information. By December of 1941, however, British codebreakers managed to decipher the code, allowing them to routinely read most ENIGMA traffic.
An ENIGMA machine is on display at The Computer History Museum.