tl;dr:1. Notice that the serifin prime has only few high and low bits set and rest are 02. Brute force low bits, and recover high bits via Coppersmith3. Calculate cubic roots for each of factors of modulus4. Combine solutions with CRT
Full writeup: https://github.com/p4-team/ctf/tree/master/2019-11-16-asis-finals/serifin