sudo apt-get clean all
sudo apt-get update

amazon-ebs: W: GPG error: http://archive.ubuntu.com/ubuntu artful InRelease: Splitting up /var/lib/apt/lists/archive.ubuntu.com_ubuntu_dists_artful_InRelease into data and signature failed
amazon-ebs: E: The repository 'http://archive.ubuntu.com/ubuntu artful InRelease' is not signed.