if you deleted or facing problem while updating Ubuntu 12.04 because of bad signing keys here is quick fix
you can fix this by two methods
First :
open terminal and type
sudo apt-key adv –keyserver keyserver.ubuntu.com –recv-keys 10126D3A3E5C1192
replace 10126D3A3ESC1192 with your missing or bad key
Second :
sudo rm /var/lib/apt/lists/* -vf
sudo apt-get update && sudo apt-get upgrade
sudo apt-get update && sudo apt-get upgrade
enjoy !
