How to fix missing GPG keys Ubuntu 12.04

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

enjoy !