Se quando lanciate un apt-get update vi trovate con questo errore:

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

Ecco cosa dovete fare:

gpg –keyserver wwwkeys.eu.pgp.net –recv-keys XXXXXXXXXXXXXXX && apt-key add /root/.gnupg/pubring.gpg && apt-get update

Saluti!