Using what you learned in this section, determine the type of encoding used

[HoshmandEnt]

as you can see the challenge says "To get the flag, you can send a 'POST' request to 'serial.php', and set the data as says decode the encoded flag and send that to the server so it means that you have to send the request with serial as your param for example "curl -s "SERVERIP:PORT/serial.php" -X POST -d to get the Flag!!!