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Figure 6: ECM Mark II Rotor Cage [9]

2.2 Encryption

During the encryption process, the plaintext is entered using the keyboard. When a key is 

pressed on the keyboard, a signal is generated that is sent to two of the three banks of rotors 

in the rotor cage. The first signal is sent to the left side of the cipher rotor bank. It is then 

permuted through the five cipher rotors to produce the ciphertext. The second signal is sent 

to the right side of the control rotor bank. However, the signal is handled differently from 

the signal sent to the cipher rotor bank. 

For the control rotors, the signal is used to energize the input into the control rotor bank. 

Four inputs to the control rotor bank are energized when a key is pressed on the keyboard. 

The four inputs are always ‘F’, ‘G’, ‘H’, and ‘I’ regardless of which letter was pressed on 

the keyboard. These four signals are then permuted through the control rotors in a right to 

left fashion. Once the four signals emerge from the left side of the control rotor bank, the 

signals go through an ORing to determine which inputs for the index rotor bank are 

energized. The following table shows the index rotor bank inputs that are energized for the 

different control rotor outputs.

10

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