Comment 26 for bug 1263540

 I deleted all the contents in /etc/apt/trusted.gpg.d/.

then run

sudo launchpad-getkeys - found here: http://www.webupd8.org/2010/05/automatically-import-all-missing.html
sudo apt-get update

.... this cleared the fault for me.

http://askubuntu.com/questions/608140/all-public-keys-are-missing-xubuntu-ubuntu-14-04