We run several (read: hundreds) of servers that are still running Debian 6 (Squeeze). A few months ago, we started seeing the following errors coming from the daily apt cronjob: "WARNING: The following packages cannot be authenticated!" When running apt-get update the following errors dump out:

W: GPG error: http://mirror.internode.on.net squeeze-backports Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 8B48AD6246925553
W: GPG error: http://mirror.internode.on.net squeeze-lts Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 8B48AD6246925553
W: GPG error: http://mirror.internode.on.net squeeze-updates Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 8B48AD6246925553

There are two ways to solve the problem:

apt-get install debian-archive-keyring will install all the keys you need.

If you want to install a specific key, then apt-key adv --keyserver pgpkeys.mit.edu --recv-keys 8B48AD6246925553 will do what you need. Obviously, adjust the key accordingly.