I ran aptitude update and received the following error message.
W: GPG error: http://ftp.xxxxx.xxx testing Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 07DC563D1F41B907
Can't get in the pub? No. Not that. apt-get is telling me they have no public key to certify the download source is secure. The error message relates to Christian Merrilat's Debian Multimedia repository.
If you don't have it already, install the debian-archive-keyring package.
apt-get install debian-archive-keyring
But it doesn't fix the current problem. We need to update the key for Debian Multimedia.
Add their gpg key to the keyring. Copy the PUBKEY ID. In my case it was 07DC563D1F41B907. Request the key from the keyserver.
gpg --keyserver hkp://wwwkeys.eu.pgp.net --recv-keys 07DC563D1F41B907
gpg: requesting key 1F41B907 from hkp server wwwkeys.eu.pgp.net
gpg: key 1F41B907: "Christian Marillat
gpg: Total number processed: 1
gpg: unchanged: 1
Export the key from gpg and add it to your list of trusted keys, using apt-key
gpg --armor --export 07DC563D1F41B907 | apt-key add -
Thats about it. You should be good to go.