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There have been a number of questions along the lines of "what cipher was used to encrypt this data", and they have all been (rightly) closed as off-topic (as discussed here).

I think it's correct to close these questions. However, we're also a site dedicated to spreading knowledge, so it may be nice to have a canonical answer to these questions that lays out why this is a bad question AND why these questions are generally impossible to answer for any modern cipher (i.e. in most cases, it is fundamentally impossible to distinguish between AES and DES based on a single ciphertext, and even distinguishing between symetrical and asymetrical ciphers can be hard). We could also include resources about how to distinguish historical ciphers, if there are any, but I'd be mainly interested in having a single, canonical, good explanation for why these questions are closed.

I personally like having such a reference question and answer (for example, I like to use this question and its excellent answers as a reference on "is this cipher secure" questions). Should we set up such a canonical answer to refer everyone asking such a question to? And if yes: On meta or on the main site?

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    $\begingroup$ meta if you want to explain people why this is a bad type of question (like we have with reference recommendation) and main-site if you want to actually give a canonical answer to all of these question (types) $\endgroup$ – SEJPM Apr 10 '16 at 12:14
  • $\begingroup$ Looking through my own past answers, here's one potential dupe target for at least some of these questions. $\endgroup$ – Ilmari Karonen Apr 15 '16 at 11:21

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