Reverse it! Put the flag inside ictf
run ltrace to get the secret key, enter the secret key in the print flag function.
After that you will get a string. As you maybe saw with strings its probably the encrypted flag with aes-ecb
Use ghidra or some other tool and find the key. You will find the key in 1 function. Its not static, so you need to reverse that.
Its just adding 0x3, and then finally you can decrypt it to get the flag.(You can use cyberchef, python or some other languages/tools)