in order to automatically download missing authentication keys for third party apt-get repositories I suggest a handy script published by Trevino (I haven’t tested it yet though).

sudo apt-get update 2> /tmp/keymissing; for key in $(grep "NO_PUBKEY" /tmp/keymissing |sed "s/.*NO_PUBKEY //"); do echo -e "\nProcessing key: $key"; sudo gpg --keyserver subkeys.pgp.net --recv $key && sudo gpg --export --armor $key | sudo apt-key add -; done

Related posts:

1. Adding the key of a launchpad PPA repository
2. update VirtualBox signing key 54422A4B98AB5139
3. cannot update initrd image due to missing kernel
4. une-gconf-default gconf file missing error while updating
5. WordPress 2.5 automatic plugin upgrade error