Xor LOBYTE of output streams which uses Linear Congruential Generator with a = 0x1282, c = 0x1634, and m = 0xFEFEFEFE.
(Note that ax + c should be calculated in 32-bit)
https://en.wikipedia.org/wiki/Linear_congruential_generator

Original writeup (https://github.com/ironore15/ctf/tree/master/2020-AeroCTF/rewarmup).