Part of the World War II Cryptographic Simulation

Our primary cipher for traffic between Britain and the United States is the TYPEX. This system is similar to the Enigma machine used by the Germans. It uses three regular cipher rotors, one reflecting rotor, and two additional rotors to the left that do not rotate. However, the TYPEX does not have the Plugboard that the Germans believes gives their machine such protection. The additional rotors to the right are supposed to provide the extra security required.

The TYPEX is based on the same principles of all rotor machines, first developed by Edward Herbern. Each rotor has 26 contacts representing the 26 letters of the English alphabet. When a live current is put in contact with one of the contacts, it is transformed through a maze of wiring to another letter. While difficult to explain, the effect is quite simple. Consider a wiring like this:

A B C D E F G H I J K L M N P O Q R S T U V W X Y Z
Y T M F X A Z K R P N H O Q J L I E G D C W V S U B

When the contact 'A' receives an electrical current it transforms the current to 'Y'. Going backwards, if the 'Y' contact on the output side received a current the current would be transformed to 'A'. The TYPEX uses five rotors for monoalphabetic substitutions like this one. By itself, this would offer little security; however, this is where the rotor mechanism comes in. Each time a key is pressed on the machine, the third rotor from the left rotates one position. Whenever the first rotor reaches a 'notched' position, the fourth rotor rotates. If the fourth rotor reaches a 'notched' position, it and the sixth rotor rotate. Unlike the Enigma, which typically only has one notch per rotor, the TYPEX uses multi-notched rotors. This means that the rotors rotate both more often and, so it would seem to the cryptanalysis, more randomly.

The rotation of the rotors is the basis for the safety the TYPEX provides. If the letter 'A' was given to the third rotor when it had rotated to the 'B' position, compensation for the rotation must be provided. Thus, instead of arriving at the 'A' position as expected, the letter would go to the position one before that, Z. This applies to all the rotors that rotated. In addition, if the first two rotors were put in at offset positions the offset would have to be compensated for as well, it just wouldn't change.

This is a rather difficult concept to comprehend. Here is a worked example that should help you:

The letter 'A' is typed, the 'A' current is sent to the first rotor
If the first rotor has been put in at an offset position, this offset must be compensated for.
Rotor1 receives the offset input, transforms it through its wiring, and sends it to Rotor2.
The offset must again be compensated for, Rotor2 transforms and sends it to rotor3.
Offset is compensated for, rotor3 transforms.
Offset, rotor4 transforms
Offset, rotor5 transforms
The offset between the moving fifth rotor and the stationary reflector is compensated for, the letter is again transformed, offset is compensated for going back to the fifth rotor.
The letter is then sent back through the rotors, compensating for offset and transformation, and emerges fully enciphered on the other side.
Now the third rotor is, as always, rotated. If the third rotor was at a notched position, the fourth rotor will rotate. If the fourth rotor is at a notched position, the fifth rotor rotates as well. The first two rotors and the reflector never rotate.

Decryption is as simple as typing the encrypted message in with the same settings. There is no special 'decryption' setting because the reflector reciprocates the message, making it seem that the message is being 'forced back the other way'.

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