program calls ptrace to interfere with debugger, no matter, use LD_PRELOAD to disable:
gcc -shared -fPIC -m32 ptrace.c
LD_PRELOAD=./a.out ./9674ea9858c5edaa760ff4a656802999-doors

Then, call doors with gdb in this order:

doorwithkey
door1
door0
door9001
door2006
door121
door3
door271
door111970
  doorBlue
    door777
	  door42
		dead end
	  door25
		door5i
		door42
		  dead end
  doorRed
    door34
    door111
	door87
	door66
	  => get the flag