Chrono Rage

Description

Your friend has developed a small local daemon on his server which opens a listening socket on localhost and performs privileged actions when clients connect and authenticate with a PIN. Unfortunately, he recently had his server attacked, despite all the efforts he put into securing his daemon.

He provides you with the source code of this Python daemon: as you’ll see, to secure the communication of the PIN on the socket, he uses rotating AES session keys. It also provides you with a pcap capture corresponding to suspicious activity on the part of a local client detected by its constant monitoring. Unfortunately, he has lost the AES key used at the time of this capture, so he can’t provide it to you. Finally, he has obviously redacted the PIN in the source code: you don’t need to know this sensitive secret!

Can you help him out by explaining how the attacker did it, and that he needs to change his PIN and correct his code as quickly as possible?

Note: the format of the flag is FCSC{PIN} where PIN is the server’s PIN.