I found this explanation of Enigma rotor movement on this website, but can't quite understand the methodology: Enigma rotation example

  1. A ⤑ B → D ⤑ C (rotor III in position B)
  2. C ⤑ C → D ⤑ D (rotor II in position A)
  3. D ⤑ D → F ⤑ F (rotor I in position A)
  4. F ↔ S (reflector B)
  5. S ⤑ S → S ⤑ S (rotor I in position A, reverse)
  6. S ⤑ S → E ⤑ E (rotor II in position A, reverse)
  7. E ⤑ F → C ⤑ B (rotor III in position B, reverse)

I follow steps 2 to 6 inclusive OK, but unsure about steps 1 & 7: I understand that the rotor moves before encryption, so I get the A>B>D, but why the final D>C? and similarly the end of step 7. I can't understand where that final step comes from. I guess there is a similar bit of complexity when the the rotors cause the next one to step ahead when the notches engage?

I'm only doing this for interest, as I used to use AES a lot in my job (now retired), encrypting electronic messages, so thought I'd get my head round Enigma to keep the brain ticking over.

Think I understand now (correct me if I'm wrong)!: Because the rotor has advanced, pressing A goes into rotor III at letter B, which encodes to D, but "comes out" in "position" C because the rotor has advanced.


I created a spreadsheet "picture" which explained to me how the encoding & wiring works to encrypt the letter 'A' twice, starting with all 3 rotors in the 'A' position.

[1]: https://i.stack.imgur.com/E2ChF.png

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  • $\begingroup$ If this is fully answering your question, you should accept it to clarify that. If something remains unclear, what? $\endgroup$ – fgrieu Jun 28 at 12:23
  • $\begingroup$ Nice. I think for a clearer understanding 4 status pictures are needed for encryption of two letters. One for the status before a keypress and next to it another one for the advancement of the rotor 3 ( 2 and 1 if needed) and the movement of the electric through wires. In short, it is Advance and Encrypt $\endgroup$ – kelalaka Jun 28 at 14:51

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