Can someone help me understand this message "packages cannot be authenticated"

[Jonathan Carter (highvoltage)]

Hi John

john wrote:
> I have an LTSP server running Jaunty. I want to install apticron on
> the server.  Apt wants to install an MTA along with apticron which is
> fine with me, however
> it wants to install postfix AND exim4 and it also warns that the
> packages cannot be authenticated. Can someone help me understand why
> this might be happening.

For security purposes, mirrors are usually signed so that when you're on
a connection that can't be completely trusted, you'll know that the
server you're getting packages from is the one that you initially added.
On you're side you'll need to add the public key for
us.archive.ubuntu.com, I'm not sure why it's not on your system, it
might be that they have changed their key and now you'll have to import
a new public one.

According to this forum post:
http://ubuntuforums.org/archive/index.php/t-964342.html
this should work:
sudo gpg --keyserver wwwkeys.eu.pgp.net --recv-keys 106A92EF1E5F9737
sudo gpg --armor --export 106A92EF1E5F9737 | apt-key add -
sudo apt-get update

HTH

-Jonathan