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I'm trying to decrypt a cipher that I suppose has been encrypted with Vigenére.

However I came about the following thought: The cipher consists of characters from [cghijklmnopqrsuvwxyzABCDEFGIJKLMNOPQRSTUVWYZ0-6+]. My thought: The characters seen in the cipher need to be in the alphabet used when encrypting via Vigenére. However this does not mean that this is the complete alphabet used - for example, the 7 could be present in the alphabet and just not a given result appearing in the cipher (maybe the match never appeared).

The presence of the 7 would affect the characters afterwards as it shifts the table-entries.

Real question: How do I know if I've gotten the right alphabet to start solving a Vigenére Cipher (without known key) if I can't imagine the alphabet at first? As I see the alphabet used in my cipher isn't a default a-z Alphabet.

Am I thinking too complicated? I think I have a misunderstanding, but can't get rid of it.

I would really appreciate every explanation to clear up my confusion.

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    $\begingroup$ I found out that most Vigenére-like cipher's "default alphabet" (e.g. Autokey, Beaufort, Running Key) consists only of A-Z or (in case of Gronsfeld) 0-9. Aren't they theoretically usable with any custom alphabet? That's the problem I'm facing. I don't know which alphabet to expect as it could be any. $\endgroup$
    – UsuallyNot
    Commented Oct 16, 2015 at 7:59
  • $\begingroup$ The text I supposed to be a Vigenére cipher actually wasn't one, it was a base64 encoded archive file. However, this does not change my question as I'm interested in it anyway. So, in case we'd know the alphabet is different from the default alphabet (a-z, case insensetive) - how would we react as we've not gotten any specific frequency distribution to rely on (like we do with languages). $\endgroup$
    – UsuallyNot
    Commented Oct 22, 2015 at 13:07
  • $\begingroup$ On the other hand we wouldn't be able to encrypt missing characters in our alphabet which are present in the default alphabet - so we would have to evolve our message around the given alphabet, which is clearly not the idea of encryption. In conclusion I'd say we would just extend the default alphabet instead of using a new one, so we'd be able to use additional characters. Of course, the sender and receiver should be aware of this. Did I made a mistake in my thinking or would it work like that? $\endgroup$
    – UsuallyNot
    Commented Oct 22, 2015 at 13:07

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