The new apt-get (>=0.6.42) introduced package authentication in order to improve security (supports verifying signed apt repositories). You can still install non-authenticated packages, but if you want to take advantage of this feature follow the procedure below, using gpg, an encryption and signing tool. Please note that not all the deb repositories implemented this feature, if you upgrade from such a repository you'll get a "packages could not be authenticated" warning.

If you encounter following kind of warning (when running apt-get update) :

W: GPG error: ftp://ftp.belnet.be testing Release: The following signatures couldn't be veri
ailable: NO_PUBKEY 010908312D230C5F


Follow the procedure below :

# open console terminal, type su and root password
# gpg --keyserver wwwkeys.eu.pgp.net --recv-keys 2D230C5F
# gpg --armor --export 2D230C5F | apt-key add -
# apt-get update


If you encounter following kind of warning (when running apt-get update) :

W: GPG error: http://secure-testing.debian.net etch/security-updates Release: The following signatures couldn't
be verified because the public key is not available: NO_PUBKEY 946AA6E18722E71E
W: You may want to run apt-get update to correct these problems


Follow the procedure below :

# Open console terminal, type su and root password
# wget http://secure-testing.debian.net/ziyi-2005-7.asc -O - | apt-key add -
# apt-get update


If you have other sources in sources.list file, you can get the key for them in a similar manner, just look for the key number in the error message.