I've been trying to make an algorithm work like the real Enigma machine, fully customizable. I use two online emulators to check that my encoded message is correct :

Currently, without the plugboard, it seems that it works correctly... until some weird rotors configuration happens. So here is the exact setup : The leftmost rotor is V The middle rotor is I The rightmost rotor is III

The initial configuration is ZPK. The ring setting is AAA.

After some letters, according to my algorithm, the rotor configuration goes:

  • ZPV
  • ZQW
  • ZQX

I am aware of the double stepping mechanism, and 26 keys pressed later we should have

  • ZQV
  • ZRW
  • ASX

BUT, both online simulation have this sequence:

  • ZPV
  • ZQW
  • ARX

As is rotor I going from P to Q is the turnover, while all the other sources I've seen say it's from Q to R (Royal Flag Wave Kings Above...)

Am I crazy, or did I miss something?

Thanks for your help!

  • $\begingroup$ The biggest problems with such questions are how we can sure that your code is correct and that kind of problem is off-topic here. I'm confused with the rotor configuration prints since we need only 3 characters shown on the top of the rotors. $\endgroup$
    – kelalaka
    May 19, 2022 at 19:46
  • $\begingroup$ Well, let's not talk about my code, in fact. My question is the following : Considering the rotors are V, I, III, how is it possible to get the following sequence : ZPV, ZQW, ARX ? $\endgroup$
    – Damien
    May 19, 2022 at 20:09
  • $\begingroup$ Rotors move like a counter, I don't see that in your sequence. $\endgroup$
    – kelalaka
    May 19, 2022 at 20:27
  • $\begingroup$ ZPV, ZQW, ZQX isn't like a counter? How is ZPV, ZQW, ARX more coherent regarding Enigma? $\endgroup$
    – Damien
    May 19, 2022 at 20:48
  • $\begingroup$ Uh, shouldn't that end with ZQB? $\endgroup$
    – Maarten Bodewes
    May 20, 2022 at 0:33

1 Answer 1


The sequence ZPV, ZQW, ARX is quite correct -- the first time the middle rotor turns is not due its own notch, but to that of the fast rotor (which indeed turns from V to W for "Wave"). Recall that the notch of the fast rotor when set to V allows the middle rotor's gear to be engaged.

Now when the middle rotor turns again one letter later, it is because of its own notch, which is exposed when the middle rotor shows the letter Q (preceding R for "Royal"). When a key is pushed, the lever engages the leftmost rotor's gear and the middle rotor's notch, turning both -- this is of course the famous double stepping anomaly.


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