public key is not available???

if you have worked with apt, installing applications from external sources, i'm sure you should have seen this message at least once or twice

Hit http://security.debian.org etch/updates/main Sources
Fetched 1331B in 0s (2153B/s)
Reading package lists... Done
W: GPG error: http://www.ultramonkey.org sarge Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 03C0023E05410E97
W: You may want to run apt-get update to correct these problems

there are few workarounds, but here goes the most trusted one on the job!

first run..

apt-key update

and try

apt-get update

if the issues is still there.. run following command to fetch the missing public key. make sure to substitute the correct PUBKEY in place of <PUBKEY ID> from your warning message.

gpg --keyserver wwwkeys.eu.pgp.net --recv-keys <PUBKEY ID>

and add the key to the keyring

apt-key add /root/.gnupg/pubring.gpg

and issue..

apt-key update

then..

apt-get update

you are done.