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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

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  • 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
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    I'm voting to close this question as off-topic because cross-posted to Crypto – schroeder Oct 15 '15 at 16:58