While updating with command $sudo apt-get update I got warning,
W: GPG error: http://archive.canonical.com jaunty Release: The following signatures couldn’t be verified because the public key is not available: NO_PUBKEY *************
This means you have lost the signing key, or never got it.
To fix it, use command:
$sudo apt-key adv –keyserver keyserver.ubuntu.com –recv-keys
Where 3CB9FD148F374FEF is the my hexadecimal missing key to be verified. Replace it with your own. 🙂