Today, when I tried to <code>aptitude install</code> a package on Ubuntu the response was “WARNING: The following packages cannot be authenticated!”
I received a similar warning when I tried to use Synaptic Package Manager.
I checked and made sure that the software was trying to install from the official repository.
I’m not really sure of the cause, or when it started happening, but I have seemed to have fixed this by cleaning out most of the keys.