Attach Wireshark to the program and capture the network traffic. This might already give you some insight if it's a very simple authentication mechanism - maybe

Attach Wireshark to the program and capture the network traffic. This might already give you some insight if it's a very simple authentication mechanism - maybe you're even able to edit your hosts.txt file and make a fake server yourself. If that fails, get comfortable with x86 assembly and dive into the code. If you're lucky, they left some symbols for the function names, which makes your life much easier.