I came over the error:
Reading package lists... Done W: GPG error: http://ftp.no.debian.org lenny Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY AED4B06F473041FA W: You may want to run apt-get update to correct these problems
When running apt-get update .
The solution is actually to do a
apt-get install debian-archive-keyring