RHEL packages are signed with the RH gpg key. RHEL 6 automatically verifies gpg sign of an rpm package before installing it. If the key is not installed, install it from installation cd / dvd. After mounting pass the commands: rpm --import /mnt/cdrom/RPM-GPG-KEY
or see the lists: rpm -qa gpg-pubkey*
It is very necessary to verify sign of rpm files before installing them by providing commands: rpm -K /tmp/updates/*.rpm (assuming rpms r under /tmp)