Upgrading Fedora Linux using Fedup: Key is not Trusted by RPM

When I attempted to upgrade Fedora Linux using FedUp, I encountered an error that basically complains that fedup could not authenticate the downloaded Fedora Linux image because it did not have the public key to verity the image's GPG signature. In the error log, by default in /var/log/fedup.log, the error message resembles the following format,

             ......
[   152.927] (II) fedup.yum:check_keyfile() 
             checking keyfile /etc/pki/rpm-gpg/RPM-GPG-KEY-fedora-19-x86_64
[   152.991] (DD) fedup.yum:check_keyfile() 
             keyfile owned by package fedora-release-0:18-6
[   152.992] (DD) fedup.yum:check_keyfile() 
             package was signed with key de7f38bd
[   152.992] (II) fedup.yum:check_keyfile() 
             REJECTED: key de7f38bd is not trusted by rpm
[   152.997] (II) fedup:message() Downloading failed: 
             could not verify GPG signature: No public key
             ......

The above example corresponds to the result of upgrading to Fedora Linux 19. In the error message, the key will be different if we upgrade to a different version of Fedora Linux. However, if the system is missing the public key for the GPG signature of the Linux image that we are upgrading to, the same error will occur and the error message will be the same except the value of the public key. To resolve the problem, we need to manually import the public key to the GPG signature of the Linux image. The public keys to GPG signatures of the Linux images of various version of Fedora Linux are available at the following URL

           https://fedoraproject.org/keys

Since it is Fedora Linux 19 whose public key is not present in the system, we shall first locate the public key from the above page as illustrated below,

From the page, we can see that the primary public key is available in two URLs, one at the Fedora Project's website and the other at the gngpg.net website. We can import the key from either URL. The following shows that we import the public key from the Fedora Project's website.

rpm --import https://fedoraproject.org/static/BA094068.txt

Once we finish importing the public key, we can rerun fedup and the error should go away. The above method should work for other versions of Fedora Linux.