tl;dr:
1. Notice that CRC32 has to have a cycle after at most 2^32 steps
2. Find the cycle length and calculate the remainder to know how many steps we really need to do
3. 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

Original writeup (https://github.com/p4-team/ctf/tree/master/2019-10-19-seccon/crc).