When doing a
# dnf system-upgrade reboot on an ARM64 system you may encounter a stream of GPG errors, followed by reboot, without upgrade.
To fix this, simply import the F28 GPG key in advance, as done for Rawhide:
# rpm --import /etc/pki/rpm-gpg/RPM-GPG-KEY-fedora-28-primary
After this, the system-upgrade should proceed correctly..
Robert Gadsdon. July 6, 2018