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I understand that the ring setting of an Enigma rotor changes the alignment between the internal wiring and the letters on the ring. For example, in the diagram below, rotor I has ring setting 2 (B), which causes current entering at position A (the center) to flow through the wiring from Z to J (internal positions) which corresponds to A -> K based on the visible indices on the rotor.

However, we could achieve the same result with ring setting 1 by using Z as the top character, which would cause current to flow from Z->J (internal positions) corresponding to Z->J on the visible indices.

This leads me to believe that the only real impact the ring setting has on the encryption is that it changes the location of the notch (that steps the next rotor) relative to the internal wiring. For plain texts that are long enough to step at least one rotor this will result in a substitution that cannot be recovered by permuting the rotor starting positions alone.

Is there anything else I'm missing?

Rotor I with ring setting B and ring setting F

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  • $\begingroup$ I have come to te same conclusion. Be keen to hear from someone who knows. Cheers, Stan. $\endgroup$ Mar 17 at 7:59
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Mechanically the only effect of the Ringstellung is to determine the rollover points as you surmise. This is easily inferred from descriptions of Enigma models in Alan Turing "Prof's Book where "comic strip" and paper Engimas only use the Ringstellung information to determine rollovers.

Cryptanalytically the effect is that if the message indicator or "window position" of the machine is known but the Ringstellug is not, then nothing is revealed about the "wheel position" or "rod position" as it was referred to at Bletchley Park. There were instances when such information was known, but it was not always easy to bring to bear on the cryptanalysis. A notable exception was the Herivelismus exploitation of human error that noted that common message indicators were often correlated with Ringstellung settings. This was a very useful step in separating out one part of Enigma settings and solving for it separately early on in the war.

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