public key is not available

если после выполнения комманды apt-get update вы видите следующее

W: GPG error: http://ftp.debian.org squeeze Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY AED4B06F473041FA NO_PUBKEY 64481591B98321F9
W: GPG error: http://security.debian.org squeeze/updates Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY AED4B06F473041FA

то вас спасёт комманда

apt-get install debian-archive-keyring && apt-get update