When you upgrade to a new version of Ubuntu/Debian, then you may see the following message:
Code:
WARNING: The following packages cannot be authenticated!
  foo bar
Searching google doesn't turn up good hits for fixing this, so i provide here the solution to this problem, which is really simple:
Code:
sudo apt-get remove debian-keyring debian-archive-keyring
sudo apt-get clean
sudo apt-get update
sudo apt-get -y install debian-keyring debian-archive-keyring