I am a CS student who is in an introductory Cryptography course. I am not looking for the complete plaintext of this message, as I am not looking to cheat. I am simply stuck, and have no idea what to do for this part of an assignment. I have attempted frequency analysis and have a link to it below. I have attempted substituting in E, T , trying trigraph THE etc. at the most common points but have not been able to deduce anything. To my knowledge there is no easy way to brute force this as I would have to try every keyword (170k words in the english language) at every position in the alphabet (42). Its possible, but crazy inefficient.

We are instructed to launch a ciphertext-only attack on the given ciphertext. Our alphabet is 42 characters: 26 upper case English letters,10 digits, and 6 punctuation signs including "," (comma), "." (period), " (whitespace)", ";", ":", "!". This keyword cipher involves this.

So far in the class I have cracked an affine cipher and Vignere using the Kasiski method. I have also done a couple other coding projects. I have been searching for hours on ways to potentially tackle this variant of the keyword cipher but I am at an utter loss. I am not seeking someone to decode the ciphertext but rather provide a potential method for me to tackle this or lead me in the right direction. Partial decryption is fine, but I really want to solve this on my own. I am trying to find the keyword, but ultimately I just have to decrypt this. If I can somehow do that without finding out the keyword that is a viable method. Our textbook thoroughly covered decryption methods for other ciphers, but doesnt have a description on how to tackle this at all beyond frequency analysis which I have been unsuccessful at.

Here is the ciphertext

Here is what I have done so far (Includes frequency analysis)


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