In Level05 we are pointed to the flag05 user directory. There we can find a .ssh directory so we can assume that flag05 uses ssh keys to login into his account and also a .backup folder: level05@nebula:~$ ls -la /home/flag05 total 36 drwxr-x--- 5 flag05 level05 4096 Nov 20 16:49 . drwxr-xr-x 43 root root 4096 Nov 20 2011 .. drwxr-xr-x 2 flag05 flag05 4096 Nov 20 2011 .backup -rw------- 1 flag05 flag05 8 Nov 20 16:49 .