I was suing squueze-lts repos from a while but suddenly I start getting following error when performing apt-get upgrade (after apt-get update)
WARNING: The following packages cannot be authenticated!
So what I did to sort it.
Removed squeeze-lts repos and pinning then
apt-get install debian-keyring debian-archive-keyring
Then I re-add squeeze-lts repos and pinning and re-run apt.
Voila, warning gone and everything back to to normal.