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.