Stepping switches
The table demonstrates the switch positions as keys are pressed on a Purple Machine, assuming that Twenties Switches 1, 2 and three are the fast, medium and slow switches, respectively. The sixes switch continues to advance one position forward with each key press. The fast switch does the same until the sixes switch reaches position 25, at which point the fast switch stays still (position 4 in this case) and the medium switch advances forward one position (in this case to position 1).
If the sixes switch reaches position 24 and the medium switch is at position 25, the slow switch advances forward one (in this case from 5 to 6). The medium and fast switches remain still. The end result was that the twenties alphabets would not repeat until 15,625 letters had been written using the same machine settings.
The use of stepping switches also meant that unlike Enigma, letters could be enciphered as themselves, making encoded messages that much harder to break.
In order to encrypt a message, an operator would first wire each of the keys on the input keyboard to a plugboard, which was in turn connected to the stepping switches. This determined which letters were sixes and which were twenties. The settings were changed daily.
The operator would also need to know the initial positions for all four switches – for instance the sixes switch may be in position 9. Finally, they would designate which of the twenties switches would be the fast one, which was medium and which was slow .