Dear Maintainer,
sometimes synaptic says that the packages are not authenticated. Here
are the steps to reproduce the problem in my system:
* Select one or some packages (today I did this with the apache2
  packages I have to upgrade)
* Right click, select "Mark for removal"
* Click "Cancel" in the window that asks confirmation to remove related
  packages
* Reselect the same (apache2 packages in my case) packages as before
* Right click, select "Mark for upgrade"
* Select "Apply" in the toolbar

Now synaptic tells that the packages I am about to install are not
authenticated.

To make the warning go away I have to unmark the packages, reload, and
then select "Mark for upgrade".

This happens everytime I made the mistake to click on remove instead of
upgrade.

Kind regards,
Mark