SIGABA

Part of the World War II Cryptographic Simulation

We use two separate ciphers, the SIGABA and the TYPEX, to protect our messages. The SIGABA is used throughout our armed forces as the standard cipher. The TYPEX, on the other hand, is used for the highest level traffic between us and Britain. Since our British friends have guaranteed us of the security of TYPEX, our primary concern here is the SIGABA

SIGABA uses 15 rotors in 3 separate banks. The first 5 are called cipher rotors. These five actually encipher the text. The second, middle bank of rotors is the control bank. The final bank is called the index rotors. The Cipher and Control rotors have 26 contacts on each side and are interchangeable. The index rotors only have 10 contacts. The middle (3rd) control rotor rotates one position forward on each key press. After that rotor has rotated once, the one to its right (4th) is incremented one position. If the 4th rotor makes a full revolution, the 2nd rotor is then incremented. None of the other rotors rotate.

Let's consider an example of how it works. First, four electrical currents are first passed into the first control rotor, corresponding to the letters F, G, H and I. These currents are then transformed through the bank of rotors and come out on the right hand side. Here they are separated into one of 9 uneven groups and are passed to one of the contacts on the first index rotor. Here again they are transformed through the bank of rotors and emerge on the right hand side. They are finally grouped into one of 5 wires. Each wire is connected to a magnet which, if the wire is 'live', will increment its rotor. Because of the groupings, some of the currents can get grouped together and then act as a single current; however, at least one current will always get through because none are thrown away. Thus a minimum of 1 and a maximum of 4 cipher rotors are incremented on each key press. Because each key press increments the middle control rotor, the cipher rotors that rotate changes with each key press. This setup is known as the 'Stepping Maze' and is believed to be all but unbreakable.

Unfortunately, the complexity of the SIGABA makes it quite a heavy machine. This means we can only use it in the less mobile fronts of the war and on ships. However, this is also an advantage: it makes it much less likely that the enemy will steal a machine.