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If you've run sudo apt-get update and got the following error:

W: GPG error: https://storage.googleapis.com/cros-packages/79 stretch Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 78BD65473CB3BD13
W: The repository 'https://storage.googleapis.com/cros-packages/79 stretch Release' 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.

just run this command to manually download the key.
sudo apt-key adv --refresh-keys --keyserver keyserver.ubuntu.com

now you can safely

sudo apt-get update
sudo apt-get upgrade