Install this debian package - xtrlock_2.0-12_i386.deb - with the command "dpkg -i xtrlock_2.0-12_i386.deb" and execute as follows:
export DISPLAY=127.0.0.1:0.0
xtrlock
This will show you a blue lock on the screen. The only way unlock again is to type the password of the user who locked the screen. The characters you type will not be displayed on screen