The HITCON 2017 CTF “Data & Mining” challenge:

The file attached was a 230MB big pcapng file.

I think I solved this by accident. I was sifting through the data for a bit and started to exclude the flows with a huge amount of data as it was mostly compressed / unreadable to me.

In the remaining data I stumbled over a plaintext TCP stream on port 3333:

This contained the flag in plaintext: hitcon{BTC_is_so_expensive_$$$$$$$}

In retrospective, searching for the string hitcon in the packet data would have worked as well.