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 [a-zA-Z0123456+] without whitespaces, while some patterns are recurring (same characters are repeating in the same order a few times in the cipher).
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.
What I need would be a good advice on decrypting such ciphers 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.
Thanks in advance!