If you get this error message:

W: GPG error: http://people.debian.org unstable Release: The following signatures couldn’t be verified because the public key is not available: NO_PUBKEY 212253A4F641D1A6
W: You may want to run apt-get update to correct these problems

This is because you don’t have the public key for Randall’s Nvidia Debian packages! The following instructions are from

http://wiki.chrismoonlight.de/doku.php?id=wissen:computer:linux:gpgupdateerror

gpg –keyserver pgp.mit.edu –recv-keys F641D1A6

gpg –armor –export F641D1A6 | sudo apt-key add -

sudo apt-get -u update

However I would have thought that installing either the debian-maintainers package or the debian-keyring would have the required key as the description of each packages says it’s has the keyos of the Debian maintainers:

debian-maintainers – GPG keys of Debian maintainers

debian-keyring – GnuPG (and obsolete PGP) keys of Debian Developers