When I type *Ctrl-X X* (sic) in *screen*, the session is locked. What is the default password? Is it the same password which I use to log in to linux?

I ask because: I connect to several different remote linux servers via ssh, under various flavours of linux/unix, and have *screen* sessions running on all of them. When I lock the screen session and type my password, sometimes it works and on other installations it does not work and I therefore don't know what the password is (so I have to "quit" that screen session externally).

So, what is the default password of screen?

P.S.: I am never root. Version of screen used: Screen version 4.00.03 (FAU) 23-Oct-06