Thsi article is taken form :
http://blog.nachtarbeiter.net/2009/06/08/add-missing-gpg-public-key-to-debians-apt/
Add missing GPG public key to Debian’s apt.
If you get a message like this while installing a new package through apt…
GPG error: http://ppa.launchpad.net intrepid Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY C514AF8E4BA401C3
First copy the number that comes at the end of the error message.
In this case C514AF8E4BA401C3 and us it in the following 2 commands.
… you can do the following to get rid of it:
gpg --keyserver pgpkeys.mit.edu --recv-key C514AF8E4BA401C3
gpg -a --export C514AF8E4BA401C3 | apt-key add -
Works on Ubuntu and other derivatives of Debian all the same, of course. Hope this post will safe me some time the next time I run into this.
Alternative
You can get the keys via APT, in Debian at least. At the cost of not being able to actually verify that the packages you get are not modified, but that is a highly unlikely event.
apt-get install debian-archive-keyring
apt-get update