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I was considering asking a question about designing an encryption process for a science fiction/alternative history piece that I'm working on, but I'm not too sure if it would belong here or on Crytography.SE. That fictional process can be summed up as "Use a Vigenere cipher and change the key every two weeks", so it strikes me as being pretty borderline.

Where would that sort of question be better placed?

EDIT: following requests for more details, this is set in the 1910s, so modern cryptography probably wouldn't apply. This fictional process is thought up by a very senior executive, who may not have a background in this sort of thing, but has the clout to force it through.

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  • But what will the question be about? If you're planning to ask if your invented cipher is secure, probably neither site will be appropriate.
    – Arminius
    Commented Mar 30, 2018 at 13:10
  • @Arminius it probably wouldn't be secure anyway, since the Vigenere was broken some time broken in the 1860s (long before this is set). The question would be more about an in-universe attempt to mitigate that.
    – user81469
    Commented Mar 30, 2018 at 13:16
  • So... What would the question be? This currently looks like it would be off topic as the usual mitigation recommendation would be to use strong, proven crypto.
    – Rory Alsop Mod
    Commented Mar 30, 2018 at 15:30

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In modern information security, Vignere is broken and therefore irrelevant. The only secure option is to use good ciphers, so this question precludes information security given the current state of the art.

Cryptography probably won't be interested, as these algorithms in general are completely broken. They are of passing historical interest at best. It would be like asking a modern doctors about the balance of humors.

I believe Worldbuilding.SE is the best forum, as there seem to be a lot of technically-minded folks there who are interested in niche and fantastic scenarios. That said, crypto is an uncommon area of interest.

I'm have only been on Worldbuilding.SE once or twice, so I could be wrong. Regardless of which forum is ultimately correct, my response would be:

If you use a very long key sequentially---and replace it before you run through it completely---then it is effectively a one-time pad and is very secure. (Provided the new key/pad is distributed securely.)

Each message must indicate where on the pad to begin, otherwise a missing or incomplete message would render subsequent messages indecipherable. Or perhaps you could neglect this issue and let it become a plot point.

For the sake of consistency, your characters should understand that their total biweekly communication cannot exceed the key length. This would add some terseness to the messages and potentially some plot tension if one character needs to communicate as his pad is running out.

If this "due diligence" mitigation of Vignere brokenness needs to be overcome, you have options. The adversary can simply start intercepting and making copies of their pads. Alternatively, the one-time pad may not be truly random, and the adversary could determine how it is created.

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  • I lurk quite a bit on Worldbuilding, and it never occurred to me to ask it there; I just assumed it would be most on-topic here or on Cryptography. Given that the piece I'm working on is set roughly a hundred years ago, that probably is the best place for it.
    – user81469
    Commented Mar 30, 2018 at 17:43
  • @PhilipRowlands For what it's worth, I asked a well-received question about cryptography on Worldbuilding, and it seems to have been on-topic.
    – forest
    Commented Mar 31, 2018 at 3:11
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    @DoubleD Why does an algorithm or technique being broken make it irrelevant? It may be off-topic for other reasons, but it is perfectly acceptable to ask questions about broken techniques.
    – forest
    Commented Mar 31, 2018 at 3:12
  • @forest From the bit I've seen on crypto.se, they tend to deal primarily with usable, modern cryptography. I don't have the background to post there, however, so I haven't crawled their meta looking for the rules. Maybe it would work, but I suspect not.
    – DoubleD
    Commented Apr 2, 2018 at 17:44
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Often open-ended questions don't fit particularly well on any Stack Exchange site, since the format is designed to elevate one correct answer. Depending on what exactly you're asking, it might be better suited for a discussion forum like reddit's r/crypto.

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