GPG invalid signature problem in ubuntu

Run bellow commands if there is problem with gpg key invalid

error while updating apt chache 

by using 

$ sudo apt-get update

this is very common issue sometimes happens after update ubuntu 

$ sudo apt-get clean
$ cd /var/lib/apt
$ sudo mv lists lists.old
$ sudo mkdir -p lists/partial
$ sudo apt-get clean
$ sudo apt-get update