Before encrypting a letter the first rotor advances by one, right? So there could be a way, once the first rotor turns 26 times, make the second rotor advance two positions instead of one. Or three. Did the Enigma take advantage of it?

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    $\begingroup$ Advantage? It would not add to security if it was done always; actually, always advancing two steps in the second rotor would half the keyspace. $\endgroup$
    – marstato
    Feb 5, 2016 at 14:31
  • $\begingroup$ Yes, agreed. I meant the operators could set it. $\endgroup$ Feb 5, 2016 at 15:01
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    $\begingroup$ This could be a duplicate of Is the logic for how the enigma machine worked documented somewhere?. Which makes me ask: what research have you done? The information describing the “wiring and setup” procedure is described in (eg) Enigma operator manuals which are freely online for research purposes, as well as the material mentioned in the several answers to the question I just pointed to. For my own clarification purposes: I assume you don’t expect someone to simply copy-and-paste relevant sections from the operator manual as an answer, or do you? $\endgroup$
    – e-sushi
    Feb 5, 2016 at 20:19
  • $\begingroup$ It would be interesting if the second and subsequent rotors advanced at 25 steps of the prior rotor instead of 26. This would make the longest possible path before finding a repeat in the sequence. It might also confound efforts to break the code. Just a thought. The Germans might have thought this to be too inelloquent for their liking, but it seems a curious thought. $\endgroup$
    – Enigmatic
    Jan 15, 2017 at 4:11

1 Answer 1


No, there was no way on the Enigma machine to change the behavior of the rotor rotation.

Rotation on the Enigma was a fixed mechanical property of the rotors like the gears in a clock. However, other cryptographers noted this vulnerability in the Enigma and improved it in follow on rotor machines particularly the British Typex and the American SIGABA aka ECM Mark II aka M-134.

The Typex had five non-reflecting rotors (watch this video for why reflecting rotors were a flaw in the Enigma), but the first two were stationary. They could be set and acted like the Enigma's plugboard but did not have the flaw of being reciprocal, a key means of defeating the Enigma's plugboard (watch this video for why).

Typex type 22 or 23 with two plugboards

Typex type 22 or 23 with two plugboards

The other three rotated, but Typex rotors came in two pieces. An wiring insert which scrambled the letter, and a sleeve which determined when it rotated. Each sleeve had different numbers of notches in different locations making it difficult to work out when the rotors would rotate. They could even be reversed giving yet more variation.

SIGABA also had varying rotation but electric signals rather than mechanical notches. The original used a paper tape with holes in it to allow or block an electrical signal. Each keypress read and advanced the tape which could contain many holes. Each keypress could turn any and all (or none) of the rotors.

SIGABA rotor unit

SIGABA rotor unit

The next version did away with the paper tape and instead used yet more rotors (control and index rotors) to generate a 5 bit signal, each bit indicated which scrambling rotor (code wheels in the illustration below) to turn. This eliminated the paper tape and instead rotation was determined by setting the rotation rotors according to daily code.

This provided SIGABA with very good pseudo-random behavior for the time and it is not known to have been defeated during its service lifetime.


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