tl;dr:
1. Notice that the serifin prime has only few high and low bits set and rest are 0
2. Brute force low bits, and recover high bits via Coppersmith
3. Calculate cubic roots for each of factors of modulus
4. Combine solutions with CRT

Full writeup: https://github.com/p4-team/ctf/tree/master/2019-11-16-asis-finals/serifin

Original writeup (https://github.com/p4-team/ctf/tree/master/2019-11-16-asis-finals/serifin).