Yesterday, I encountered a situation about which I really have no idea. When I attempted to open a remote directory of another Ubuntu PC, there came a dialog asking me for ``Unlock Keyring.''
Then I went to get some help from Google searching, and found the link. I followed the guidance, and didn't notice too many details in that article, and when I restart my Ubuntu today, I can't login into the desktop environment!
Fortunately, I have learned how to brows using elinks in virtual terminals, so I went back to the link just mentioned and found what I have done yesterday. Then I remove the setting did yesterday in the /etc/pam.d/gdm. Reboot, and everything was normal as usual.
I still don't know very well about the concept and related setting of the keyring. Listed below are some links I found might be useful for (my own) further reading.