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!

closed as off-topic by schroeder Oct 15 '15 at 16:58

  • This question does not appear to be about Information security within the scope defined in the help center.
If this question can be reworded to fit the rules in the help center, please edit the question.

  • And it's not base64 encoding? – RoraΖ Oct 14 '15 at 18:52
  • Try base64 decoding. Command: base64 -d [cipher/file] – xopis Oct 14 '15 at 19:00
  • @RoraΖ Didn't think of that before, but no - there's nothing readable after decoding from Base64. Any other ideas I might have missed to check? – UsuallyNot Oct 14 '15 at 19:01
  • The unreadable code is most likely the actual cipher text/binary data. Using base64 is just a means of making it easy to copy and paste for users of the challenge. – RoraΖ Oct 14 '15 at 19:04
  • 2
    I'm voting to close this question as off-topic because cross-posted to Crypto – schroeder Oct 15 '15 at 16:58