I am getting
W: GPG error: http:// archive.canonical.com quantal Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 40976EAF437D05B5 NO_PUBKEY 3B4FE6ACC0B21F32
The fix:
sudo apt-key adv --recv-keys --keyserver keyserver.ubuntu.com 40976EAF437D05B5