This is an issue that plagues me constantly. For each PPA you add to Ubuntu, you also need to import a GPG key for it. This is all fine and well – until don’t have the key and are unsure how to get it (often happens after a reformat). Try doing an update without a valid key and you’ll get the following:
Well we finally have a solution: launchpad-getkeys! This handy little tool will automatically determine which PPA’s require GPG keys and import them for you.
sudo apt-add-repository ppa:nilarimogard/webupd8 sudo apt-get update sudo apt-get install launchpad-getkeys
Images taken from this howtogeek article. For more information see here and here.