I use up2date via KDE tray icon of Red Hat Network Alert. It shows me 163 updates and 0 ignored. When I Launch up2date it shows me three kernel in ignored and in the forward step, it lists many packages. Out of these when I try bash or foomatic or even Xfree, it starts downloading in a while but then it shows me a question dialogue saying "The package <packagenamewith version> does not have a valid GPG signature.
It has been tampered with or corrupted. Continue?" that the signature problem and gives asks a question if I want to continue with corrupted file foomatic or not. Naturally, my answer is no. Even when I click yes, it does nothing. Could anybody help with this?

I am doing this as a user and not as the root. However, it does ask me the password of root when I click "Launch Up2date"

The best thing to do would be to download the failed rpms separately, put them in the /var/spool/up2date folder. The next time you run up2date, the packages will be skipped as "already downloaded" and up2date will move on to the next file.