I am trying to decrypt a text(~900 characters) and I started with the assumption that its a Vigenère cipher as the one of the identifier tool identified it as Vigenère cipher and also the Index of Coincidence (IOC) for the ciphertext is 0.04492. So I used another tool to check the IOC values for different key lengths (up to 20) but I see the IOC for all the key lengths is not anywhere close to 0.06 which is typically the case for every piece of Vigenère ciphered text I have seen. And in my case, the IOC for all the key length (up to 20) is around 0.045. I think this means either of the two things

  • The key length is greater than 20 and I need to manually somehow compute the IOC for values > 20 as most of the tools available only offer key lengths up to 20

  • this is not a Vigenère cipher at all, in that case, I don't even know how to proceed in terms of identifying the correct cipher. Any advice will be appreciated Thanks

  • $\begingroup$ Welcome to Cryptography. Is this homework or a CTF? $\endgroup$
    – kelalaka
    Commented Aug 20, 2020 at 13:06
  • $\begingroup$ thanks, its not a homework, its a puzzle posted on a quant trading firm website $\endgroup$ Commented Aug 20, 2020 at 14:09


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