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Figure 3: SIGABA Cipher/Control Rotors - 

Normal Orientation [1]

Figure 4: SIGABA Cipher/Control Rotors - 

Reverse Orientation [1]

The index rotors are inserted in only the normal orientation (though as previously 

mentioned, they could be inserted in a reversed orientation), with the current position 

appearing as a right side up digit. However, unlike the cipher and control rotors, which 

appear in a decreasing order in the normal orientation, the index rotors appear in an 

increasing order in the normal orientation. 

Figure 5: SIGABA Index Rotor [1]

The machine is initialized by inserting the rotors into their respective banks within the rotor 

cage. However, before the rotors can be inserted into the rotor cage, certain decisions must 

be made regarding the key. First, the five rotors that will be used as cipher rotors and the 

five rotors that will be used as control rotors must be selected from the ten available rotors. 

Next, a permutation of the five cipher rotors and a permutation of the five control rotors 

need to be decided. After the permutation is selected, the orientations of the ten rotors must 

be selected. For each of the ten rotors, an initial starting position must be picked. Once that 

is done, the ten rotors are inserted into the cipher rotor bank and the control rotor bank 

within the rotor cage. After the cipher and control rotors are inserted, the five index rotors 

must be set and inserted. A permutation of the five index rotors must be selected, as well as 

their starting position. Index rotors can only be inserted in one orientation. Once the 

permutation and starting positions are chosen, the index rotors can be inserted into the 

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index rotor bank in the rotor cage. Once that is done, the machine is initialized with the key 

and is now ready to encrypt or decrypt a message.