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Occasionally I encounter a specific cryptanalysis problem which I'd like to solve. (My most recent example can be found here, where I have cleartext/ciphertext/password/salt, and I'm trying to figure out the algorithm which was used.)

I understand that those kinds of questions/puzzles are off-topic for this site. ("Requests for analyzing ciphertext or reviewing full cryptographic designs are off-topic, as the results are rarely useful to anyone else and/or would be too long for this site.")

Now, I expect that there are people who actually like solving that kind of puzzle, who could help me.

My question: Where can I find such people? What forums or sites exist where such puzzles are on-topic, and have a chance of being answered?

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    $\begingroup$ puzzling.stackexchange.com is a good place for puzzles question. $\endgroup$ – Meysam Ghahramani Oct 4 '16 at 15:31
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As Meysam already noted in his/her comment, you have good chances of finding "puzzling" fans at the according StackExchange Q&A site:

Puzzling.SE

Puzzling.SE screenshot

(puzzling.stackexchange.com)

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    $\begingroup$ But that requires the question to be designed as puzzle, otherwise it almost certainly won't contain the information needed to figure it out. $\endgroup$ – CodesInChaos Oct 5 '16 at 9:34
  • $\begingroup$ @CodesInChaos True. OTOH it isn't obvious to me that that was asked. After all, the question (which was originally asked on our main site before I migrated it) asks "How to get eyes on a specific cryptanalysis puzz!e?" and explains that he/she wants to know where he/she can find "people who actually like solving at kind of puzzle". The logic answer to me (and some others) is that you’ll find puzzling fans at Puzzling.SE. I did not, and do not, see that the question asks "how to ask things in a specific way". Instead, it asks where to find puzzling folks: "What forums or sites exist where…" $\endgroup$ – e-sushi Oct 5 '16 at 11:33
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As e-sushi has noted, the proper place for puzzles on Stack Exchange is Puzzling.SE.

For questions about practical reverse engineering of encrypted or obfuscated data, however, a more appropriate choice would be Reverse Engineering Stack Exchange. They're still in beta, and their scope may not yet be quite as firmly established as here, but they do seem to be generally OK with well asked decryption questions.

Before asking such question (on any site), however, you really should read the meta post my previous link points to, and consider whether your question would be answerable at all.

In particular, note that any properly applied modern encryption method (using an actual secure cipher, as opposed to mere ad hoc obfuscation) will be effectively unbreakable without the key. In fact, properly encrypted data will generally be effectively indistinguishable from random bits, so, in the absence of metadata or documentation (or code!) identifying the encryption method used, you cannot even tell from the data itself how it has been encrypted.

Thus, in general, for a ciphertext encrypted using modern crypto to be decryptable at all, you must already know how it has been encrypted, or at least have a reasonable lead that others could follow up on (e.g. you've identified the encryption code in a disassembly, but can't tell exactly what it's doing). At that point, the problem then reduces to either:

  1. extracting the decryption key from the program or device it has been embedded into, or
  2. finding and exploiting a weakness in the way the encryption has been applied.

(The latter would be on-topic here at crypto.SE; however, note that something like 90% to 99% of the time, the only possible answer we could give would be "no, there's no obvious weakness there".)

The exception to that is if the encryption / obfuscation method used is homebrew, and exceedingly weak. Typically, such poor obfuscation schemes are easily identifiable by the fact that the obfuscated data does not look completely random, but has some obvious repetitive patterns.

Questions about analyzing such data (not "decrypt this for me!" but "how could I decrypt this kind of obfuscated data?") may IMO be on topic both here on crypto.SE as well as at reverseengineering.SE. As a general rule of thumb, I'd suggest that if your question involves analyzing any program code, it's likely to be a better fit for Reverse Engineering, whereas if it's purely about applying abstract cryptanalysis techniques, it would probably fit better here.

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