There is no public key available for the following key IDs: 1397BC53640DB551

Seeing the above warning? Then you're probably using Google Chrome in Linux, and you need to update the public key:

# wget -q -O - https://dl.google.com/linux/linux_signing_key.pub | apt-key add -

(NB: Modified for Debian systems- the version in links below uses Sudo.)

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