tl;dr:
1. Recover server certificate from pcap
2. Extract RSA public key
3. Factor modulus with Williams p+1 method
4. Forge private RSA key
5. Decrypt SSL traffic

Full writeup here: https://github.com/p4-team/ctf/tree/master/2017-12-09-seccon-quals/crypto_smooth

Original writeup (https://github.com/p4-team/ctf/tree/master/2017-12-09-seccon-quals/crypto_smooth).