A search for the word rotor on SE Crypto brings up fifty questions. Most of them are about the Enigma, of course, but some are about the Bomba and Bombe, and one is about the Fialka (a Russian electro-mechanical encryption machine). Contributions about the SIGABA (4) and the M-209 (4) do not come up because they are poorly tagged, some as classical-cipher. All of the devices mentioned thus far have rotors, the Bomba and Bombe being different from the encryption devices.

Given the importance of rotor-based encryption machines in the history of cryptography (both mechanical and electro-mechanical devices), why don't we create a tag that covers them? The tag of rotor-machines or rotor-based-encryption will include general questions about the math, theory, and practice of rotor-based encryption and help unify it as a topic on this website. See this question that is in need of proper tagging.

For details about rotor machines, please see this website.

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Now, if we were to be precise, we would make a distinction between rotor-based machines and pinwheel-based machines, but do we need to do that? Why not put them together under rotor-machines or rotor-based-encryption? Perhaps we could create a tag synonym named pinwheel-machines and explain our rationale in the usage guidance.

Pinwheel-based machines:

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So, should we create a rotor-machines tag?

  • $\begingroup$ It's a good question, but how many of these would not be about Enigma? If we can tag those correctly with enigma and / or classical-cipher, do we still need rotor-machine? That's in the end the question we should ask ourselves. On a different tangent: it's also classical-cipher, not the plural ciphers, so rotor-machine would make more sense; usually the question would be about one machine at a time. $\endgroup$
    – Maarten Bodewes Mod
    Commented Aug 13, 2021 at 18:28
  • $\begingroup$ I completely agree. I can do some research and tell you how many questions about individual rotor machines are not about Enigma. I can also find the number of questions about the math behind rotor machines, their history, general theory, etc. If someone searches the tag "rotor-machines", they will find a one-stop shop for the whole topic, and this will surely widen their understanding and perhaps pique their curiosity. First, I need to do a bit of research and answer your questions above. $\endgroup$
    – Patriot
    Commented Aug 14, 2021 at 2:30
  • 1
    $\begingroup$ cryptographic-hardware can cover modern cryptographic hardware, such as public-key crypto-accelerators. $\endgroup$
    – fgrieu Mod
    Commented Aug 16, 2021 at 7:08

2 Answers 2



Having a tag entitled rotor-machines will be beneficial. It will help users find information that relates to this important subject.

Supporting Information

1. The SE Crypto tag of classical-cipher does not fit sophisticated electromechanical rotor machines. Some of these provide security above $2^{76}$, which does not sound like a classical cipher.

Questions tagged [classical-cipher]

Ciphers invented before the era of computers and modern cryptographic theory. Classical ciphers typically operate on letters instead of bits and are usually designed to be implemented by hand or using simple mechanical devices.


Note the sophistication of the following electro-mechanical rotor machines that clearly go beyond classical cryptography:

  • TC-52, from 1952. 5-bit, used binary mixing with a PRNG included as a fall-back.

  • Hagelin HX-63, from 1964. It features re-inejection. Used by the French Army at that time.

  • KL-7, used heavily from 1952 to the 1970's, featured electrical rotors and reinjection. Declassified in March 2021. Last used (according to open-source information) by a Western military in 2012.

Not classical cryptography.


These questions would benefit from having a rotor-machines tag:

A Modernized Enigma?

Rotor that divides binary number by mod 3

About LFSR based M209 Cipher Machine

Rotor machines: secrecy of the wiring

How does the Fialka manage to have a letter encipher unto itself?

How to build an electro-mechanical public key cipher machine?

Are there any good examples of Contemporary Mechanical Cryptography?



We already have a tag for cryptographic-hardware.

Rotor machines are adequately labeled under the current tags of classical-cipher, Enigma, and cryptographic-hardware.


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