Writeup

It was clear that it’s just a monoalphabetic cipher or fixed substitution cipher - so every letter is replaced by another one based on a fix alphabet.

I’ve never broken one of those before, but they are not hard if the ciphertext is long enough - and this one is.

I’ve worked in a way that every decoded letter is uppercase, it was easier for me to distinguish them from unknown ones.

One mistake I’ve nearly made was search-replacing - but that might have replaced already-replaced letters, so I’ve wrote a python script that goes letter by letter.

I’ve ran a few statistics on it and calculated letter frequencies. Based on the english letter frequencies I’ve found that G codes E. Based on that and the 3-letter groups I’ve also identified X as T and W as H.

I’ve also found T is S from 's.

Next I’ve guessed C as A from frequency, and found sAST and guessed it as LAST, so S is L - also O is I from is.

I’ve found SHApES HIS HEAa - clearly meant to be shakes his head.

I’ve continued to guess letters based on words, until I’ve had a full decoded sentence and could google that - and found that I’m decoding Mad Max 2: The Road Warrior’s script (link).

Based on that, I was able to decode ALL letters and get the flag inserted in the document.