Professionally converted for accurate flowing-text e-book format reproduction, this important NSA publication tells the exciting story of the German Enigma cryptographic machine and the heroic work to read Enigma messages. Some historians claim that World War II could have gone on for as much as two more years, with an untold loss of life, had it not been for the Allies' ability to read Enigma messages. Those messages could not have been read without the Bombes and the men and women who built and operated them.As the German military grew in the late 1920s, it began looking for a better way to secure its communications. It found the answer in a new cryptographic machine called "Enigma." The Germans believed the encryption generated by the machine to be unbreakable. With a theoretical number of ciphering possibilities of 3 X 10114, their belief was not unjustified.1 However, they never reached that theoretical level of security. Nor did they count on the cryptanalytic abilities of their adversaries. The Enigma machine based its cipher capabilities on a series of wired rotor wheels and a plugboard. Through a web of internal wiring, each of the twenty-six input contacts on the rotor were connected to a different output contact. The wiring connections of one rotor differed from the connections on any other rotor. Additionally, each rotor had a moveable placement notch found on an outer ring. The notch forced the rotor to its left to step one place forward. This notch could be moved to a different point on the rotor by rotating the outer ring. The Germans followed a daily list, known as a key list, to indicate where the notch should be placed each day. Another complication to the machine involved the plugboard, which the Germans called a "Stecker." The plugboard simply connected one letter to a different letter. This also meant that the second letter automatically connected back to the first. Again, the key list indicated which letters should be connected for that day. Each day, the Germans followed the key list to plug the plugboard connections, select the rotors to be placed in the machine, change the rotor notch placement, and place the rotors in the left, center, or right position within the machine. Finally, the code clerk chose which three letters were to appear through three small windows next to the rotors. These letters indicated the initial rotor settings for any given message, and the code clerk changed those settings with every message he sent.