I've been keen on IT Security for a long time now and I've learned a lot about networking & security. However trying a "decryption challenge" I'm lacking what I think is basic encryption/decryption knowledge.
So I've got a ciphertext without knowing about the algorithm or key used encrypting the plaintext. I assume that this cipher is not "too hard" (no AES e.g.) to crack, because it would be an overkill for a simple challenge.
I want to mention that I would like to crack it on my own, but I need advice in which direction I should go - what's most likely to be used, how I can verify if some encryption algorithm is used or not, etc.
About the cipher: The Cipher contains [cghijklmnopqrsuvwxyzABCDEFGIJKLMNOPQRSTUVWYZ0-6+] without whitespaces, while some patterns are recurring (same characters are repeating in the same order a few times in the cipher). I'm also quite sure the ciphertext isn't hashed as I'm supposed to read what the result is. Cracking a 1000+ characters cipher won't be the "challenge".
What I've already done:
I tried a frequency analysis to see if it's just substituted monoalphabetically sign for sign - without success (many results staying at ~8%, no one above 10%)
I had a look at classic transpositioning and substitution methods which lead me to encryptions like Vigenére, Autokey, Beaufort and so on. However, those are designed to work with non-numeric alphabets as far as my understanding goes.
I calculated the "Index of Coincidence" being about 0.06, which should tell me that problably a substitution is used (I haven't yet figured out completely how this works in detail)
I already tried to separate the cipher at points where suspicious, recurring "words" are, just to find something that seems like separator-signs. However it's not much clearer afterwards and I'm not sure how to go on.
I did already base64 decode as suggested, but the result was even less helpful. Looks more unrelated than before, just many special characters mixed up with periods and numbers here and there.
What I need would be a good advice on how to find out which cipher may be used as I'm currently running out of ideas what to try.
I hope somebody can help me out here as I'm really interested in the topic, but somehow stuck.
Thanks in advance!