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Quite possibly the most famous encryption device of all time, the Enigma Machine of World War II was a German marvel of cryptography. With more possible combinations of codes than there are atoms in the universe, it was easy for the Germans to become overconfident about their machine. That overconfidence was a weakness, as the Allies were able to decipher the Enigma, and thus gain access to critical German communications.

The actual machine is built into a wooden box small enough to be carried by a single man. When the box is opened, there is a typewriter style keyboard. Above the typewriter are lamps for each letter.

Whenever someone presses a key, one of the lights illuminates. This corresponds to the encrypted letter.

What does the actual encryption of the text is a complicated system of three rotors. Each rotor can be set to any letter of the alphabet, thus making the encryption key. A rotor is a device that substitutes one letter for another, according to the wiring of that rotor. What makes the enigma stronger than a simple substitution cipher is that it has three of these rotors, and they move after each letter of a message. Thus, the encryption system is altered with each letter encrypted.

The method with which the rotors progressed can be equated to a speedometer. For example, the readouts for the rotors would progress like this:

AAA, AAB, AAC...AAX, AAY, AAZ, ABA, ABB, ABC...AZY, AZZ, BAA, BAB

In addition to these rotors, though, the Enigma had several additional "security" features, some of which, to the chagrin of the Germans, actually reduced security:

• There was a "reflecting rotor." Traditionally, plaintext goes in one end of a machine, and ciphertext comes out the other; with the enigma, though, the electrical signal comes out on the same side it went in on, but on a different letter. This caused several unique features:
• Because a letter comes out on a different letter than it went in on, it is impossible for the enigma to have any ciphertext letter be the same as a plaintext letter. This is a major weakness in the enigma, and it allowed the allies to greatly reduce the time necessary to crack codes.
• The machine does not need different modes for encrypting and decrypting, as the allied SIGABA does. All that a person needs to do is set the machine up with the appropriate settings for the encryption key, and start typing. If the text being entered is ciphertext, plaintext will come out. If the text being entered is plaintext, ciphertext comes out. Very convenient.

• There were five standard issue rotors; however, only three could be used in the machine at any given time. As a result, there were 60 possible combinations of rotors (5 nPr 3). Germany would change the rotors used in the machine and their starting positions every two days.

• The Enigma used by the German Forces during World War II included a plugboard. Basically, what this did was to allow the user of the enigma to swap any letter for any other letter at a location other than the rotors. There was one plug for each letter of the alphabet, and 26 sockets. All that had to be done was insert the plug into a socket, and the letter was swapped. The use of a plugboard (also sometimes called Steckerboard) increased the number of combinations of enigma settings by a factor of ten to the fifteenth power.

• Different branches of the German forces used different types of Enigmas. The Enigma used by the Navy was different from the Army, which was different from the Railway. Each branch had no idea what the other branch was using for its encryption. This secrecy meant that if an enigma from one service was compromised, other branches would not have to get overly concerned. An example is the Navy, which used four rotors instead of three. Additionally, they incorporated extra rotors to make the reflection system stronger. Finally, they picked four rotors from a set of eight to use in the machine, as opposed to three from five.