tl;dr:

1. Notice the binary does only sort+xor
2. Recover keystream by encrypting black image
3. Recover sorting order by sending diagonal line

Full writeup: https://github.com/p4-team/ctf/tree/master/2019-09-02-tokyowesterns/meow

Original writeup (https://github.com/p4-team/ctf/tree/master/2019-09-02-tokyowesterns/meow).