After adding a repository to /etc/apt/sources.list I got the following error on update:

W: GPG error: http://ppa.launchpad.net intrepid Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 30B02BFA99E5A6E7
W: You may want to run apt-get update to correct these problems


To fix this, the missing public key must be imported into apt, which can be done the following way:

Switch to root:

sudo su -


Get the gpg public key from a key server:

gpg --keyserver subkeys.pgp.net --recv-keys 99E5A6E7


output:

gpg: requesting key 99E5A6E7 from hkp server subkeys.pgp.net
gpg: /root/.gnupg/trustdb.gpg: trustdb created
gpg: key 99E5A6E7: public key "Launchpad PPA for eBox platform unstable" imported
gpg: Total number processed: 1
gpg: imported: 1 (RSA: 1)


Import it into apt:

gpg --export 99E5A6E7 | apt-key add --


output:

OK


At last; get back to "normal" and perform the update again: (should run smoothly this time)

exit
sudo apt-get update