tl;dr:1. Notice that CRC32 has to have a cycle after at most 2^32 steps2. Find the cycle length and calculate the remainder to know how many steps we really need to do3. Notice that zlib.crc32 is much faster :)4. Run on all inputs and recover the flag
Full writeup: https://github.com/p4-team/ctf/tree/master/2019-10-19-seccon/crc