patch the program to jump the calls to set_timer and get_key, calculate the xor of the flag stored in the program with "picoCTF{", run in gdb and before the call to decrypt_flag set $rdi to the calculated value. see original write up for details

Original writeup (https://xnand.github.io/2018/10/15/picoCTF2018-be-quick-or-be-dead-3.html).