Preparing to replace apt 0.8.13.2ubuntu4.6 (using .../apt_0.8.16~exp5ubuntu13.7_amd64.deb) ...
Unpacking replacement apt ...
Processing triggers for man-db ...
Setting up apt (0.8.16~exp5ubuntu13.7) ...
Installing new version of config file /etc/cron.daily/apt ...
Installing new version of config file /etc/apt/apt.conf.d/20changelog ...
ERROR: Can't find the archive-keyring
Is the ubuntu-keyring package installed?
dpkg: error processing apt (--configure):
 subprocess installed post-installation script returned error exit status 1