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I received this message when updating Operating System from the AppCenter. How could this update not be from a trusted source? Could it be that my AppCenter has been compromised?
There is a repository key missing. To fix this, you can add the key in a terminal window with the following command:
wget --quiet http://download.opensuse.org/repositories/home:Horst3180/xUbuntu_16.04/Release.key -O - | sudo apt-key add -
Required, but never shown