These questions tend to plague unmoderated discussion sites about crypto, and they're almost always crap. I think any question of the form “here's a bunch of bytes, how do I break them” should be summarily closed. This blanket ban could be mentioned in the FAQ.
I'll cite somewhat similar examples on other Stack Exchange sites as precedents:
- On Science Fiction and Fantasy, we have a specific ban on “genre classification” questions (is X science fiction?). The reason for this ban is that these questions are pretty uniformly bad (the answer is yes or no depending on who you ask, for uniformly uninteresting reasons).
- On Programmers, there are specific question types that are considered unwelcome, such as “what language should I learn?” (where in 99.99% of cases there's no better answer than “whatever you like”) or “what salary should I ask for?” (where in 99% of cases any answer would be completely specific to the asker's case).
This would be a ban on questions that are specifically about identifying a crypto system from a given bunch of bytes. Questions about cryptanalysis techniques are obviously welcome here, even including questions such as this (not a great question because the asker didn't come back with more information, but still a good fit for the site). A couple of signs this question is ok are that it doesn't have or need a byte dump, and that there's a specific story behind it.