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I read a ton of articles about the Enigma machine. And the one thing that I can't get is when the first rotor stars rotating.

Some articles says the rotor rotates after the first letter of message has been encrypted. So if the first rotor is set to 'A', the first input would go through 'A', the second - through 'B', etc.

But some articles say that the rotor starts rotating when you press a key. Only after that the machine encrypts the letter. I mean, I set the first rotor to 'A'. The I press a key to input the first letter of my message. When I press a key, the rotor goes to 'B' and the first letter goes through 'B'.

Which articles are correct?

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  • $\begingroup$ I think it resembles a revolver - it turns while you press it, because this way you can exert more force on the mechanism. A reverse turn would require a spring mechanism of some sort. There is a film on youtube by numerphile with an actual device. $\endgroup$
    – Konrad Gajewski
    Commented Aug 9, 2018 at 22:09

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Having had the privilege of using one of the original machines, I can tell you the sequence is the following: set the plugs, and set the rotors. Once you push the key, you can feel the force of the rotors turn, and the light turns on for the encrypted letter. The light turn off when you take pressure off the key.

This was on a 3-wheel Enigma, and I do not have any personal experience with the later models, but I will assume they work the same way.

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  • $\begingroup$ I can't pass up the opportunity to brag about my own amazing experience operating an original Enigma (also a 3-rotor M3 model). I happened to take a video which illustrates the action nicely and confirms your description: youtube.com/watch?v=ds8HoowfewA .. In the last keypress you can see the 2nd rotor move too. $\endgroup$
    – Mikero
    Commented Aug 9, 2018 at 2:45
  • $\begingroup$ if I understood you correctly, first the rotor turns, then the letter gets encrypted. Right? $\endgroup$
    – Igor234
    Commented Aug 9, 2018 at 5:15
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    $\begingroup$ @Igor234 yes. The rotor turns to encrypt the data. This is why the initial rotor positions are important. $\endgroup$
    – b degnan
    Commented Aug 9, 2018 at 11:42
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When you press down a key the rightmost rotor turns first, when the key hits the bottom the electric circuit is made and one of the lamps lights up. At some positions turning the rightmost will turn the middle one step also and then at some points the leftmost (on a 3 rotor enigma) turns.

On a 4 rotor enigma (used by the navy after 1942-2-2) the leftmost rotor never moves no matter how many letters are en/decrypted.

Now you may think it move one step each time but due to the mechanics most of the time it actually steps twice right after each other, this is called doublestepping and can be seen in https://www.youtube.com/watch?v=hcVhQeZ5gI4

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  • $\begingroup$ Your description of "double stepping" seems either wrong or at least highly unclear. What double stepping means, in the context of the Enigma, is simply the mechanical feature that causes the middle rotor to turn an extra step whenever the left (i.e. slowest) rotor turns. Here's one fairly clear and detailed description of it. $\endgroup$
    – Ilmari Karonen
    Commented Jun 17, 2019 at 8:17
  • $\begingroup$ The double stepping part is unclear but that's mostly because the question was about when the first rotor moves, not how double stepping happens, other questions deals with that part. $\endgroup$
    – lpaseen
    Commented Jun 17, 2019 at 16:35

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