There is a script to add all the missing keys for the PPA repositories of Ubuntu in your system.

In terminal, add a PPA and run the script\\
<code>
sudo apt-add-repository ppa:nilarimogard/webupd8

sudo apt-get update

sudo apt-get install launchpad-getkeys
</code>

To run the script, in terminal again\\
<code>
sudo launchpad-getkeys
</code>