W: GPG error: http://security.debian.org etch/updates Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY XXXXXXXXXXXXXXXX
W: GPG error: http://ftp.debian.org etch Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY XXXXXXXXXXXXXXXX
W: You may want to run apt-get update to correct these problems
get’s fixed by
gpg --keyserver wwwkeys.eu.pgp.net --recv-keys XXXXXXXXXXXXXXXX
apt-key add /root/.gnupg/pubring.gpg
apt-get update
as stated here
Substitute the missing key for XXXXXXXXXXXXXXXX