1. We XOR the two ciphertexts from `output.txt` together
2. Since we know that our flag starts with `CHTB{`, we will XOR the result of `1.` with it, to see the first letters of our quote.
3. We find the appropriate quote that gave us one of the ciphertexts
4. XOR the quote with our result from `1.` to get the flag.

Original writeup (https://kblagoev.com/blog/phasestream-4-cyberapocalypse-2021-ctf/).