I know many of you use Launchpad’s “Personal Package Archive” for updated and beta packages. I have about a half-dozen configured on my machine to give me the latest and greatest of my favorite applications. The one problem with a PPA though is that the packages can’t be verified when downloaded because you don’t have the GPG imported into your Apt Keyring. The following command is a template that you can use to import whatever PPA based keys you have warnings on:
sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys <key id>
You can gather the <key id> in the output of:
sudo apt-get update
You should see the warnings, along with the key id at the bottom of the output.
For each GPG key that you get a warning on, run the command above, and you’ll stop seeing the warnings. You will have imported that team/project’s GPG key into your keyring, considering them trusted.