Here’s a quick tip,
If you ever encounter this error(check the screenshot), the easiest thing that you could do to fix it AFAIK is to install the Y-PPA manager and let it try to import all the missing GPG keys.
Here’s how you can install it :
$ sudo add-apt-repository ppa:webupd8team/y-ppa-manager $ sudo apt-get update $ sudo apt-get install y-ppa-manager
Once you’ve installed the software, fire it up, either from the terminal or from your dash. Once you do that, select the “advanced options” from the menu and in the following menu(as shown in the screenshots), select “Try to import all missing GPG keys”.
After a few minutes(possibly more, this takes some time 😀 ), assuming all goes well, you’d have gotten rid of the warning that you’d been getting all this while.