Get:1 http://archive.ubuntu.com/ubuntu xenial InRelease [247 kB]
Ign:1 http://archive.ubuntu.com/ubuntu xenial InRelease
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W: GPG error: http://archive.ubuntu.com/ubuntu xenial InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 40976EAF437D05B5  NO_PUBKEY 3B4FE6ACC0B21F32
W: The repository 'http://archive.ubuntu.com/ubuntu xenial InRelease' is not signed.
N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.