Ubuntu - Fix "W: GPG..." key error

Sometimes, You run "sudo apt-get update" and get this kind of error:

W: GPG error: http://extras.ubuntu.com precise Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 16126D3A3E5C119

It is because your apt misses that public key. So, to fix this issue, just run the following command to add the need key:

$ sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 16126D3A3E5C1192

then run update again:

$ sudo apt-get update

If the above method does not work, try this:

$ 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