As some people didn’t know how to solve this without knowing the keyword, I’ll tell you how I solved it.
I solved it as if it used a Quagmire II cipher, as the cryptanalysis suggested it had a peak in periodic IOC at period 15, and 6A hinted that didn’t use a caesar shift but was similar to Vigenere and Beaufort – so it was probably one of the Quagmire ciphers. If you didn’t know about the Quagmire ciphers, I’d say you should do some more research on ciphers and cryptanalysis if you want to do well in the later stages of this competition.
Another way that it could have been solved is by creating a hill climbing algorithm that stars with a 15 letter key (usually 15 random letters) and improves it, depending on what the IOC of the ciphertext would be after decrypting it with that 15-length key Vigenere. If a modification to the key (changing a letter in the key to a different letter) increases the IOC of the ciphertext, then keep it and carry on trying to improve the key in this way. At some point this algorithm won’t be able to find a modification to this key that increases the resulting text’s IOC, and at this point the key found by the algorithm should be any of the 26 possible caesar shifts of “AGRICOLAEMORTEM” (though it could get stuck in local maxima, so you may need to try this algorithm a few times using different starting keys if it doesn’t work the first time). After seeing that key, it is clear that it is probably the correct key as it is similar to some of the latin text we have seen in the part Bs of this competition. After decrypting it with “AGRICOLAEMORTEM” using Vigenere, it becomes an easily solvable affine shift (or a difficult keyword cipher) ciphertext.
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