**TL;DR** Defective PRNG means every block was encrypted with the same known key, all we have to do is reverse the columnar encryption, which is the same as it usually is, except the matrix/grid gets transposed in the middle.

Original writeup (https://an00brektn.github.io/htb-cyber-apocalypse-crypto-short-and-sweet/#how-the-columns-have-turned).