Have you seen this during a apt-get update:

?> sudo apt-get update
Ign http://dk.archive.ubuntu.com trusty InRelease
Hit http://ppa.launchpad.net trusty InRelease
Hit http://security.ubuntu.com trusty-security InRelease
Hit http://dk.archive.ubuntu.com trusty-updates InRelease
Ign http://extras.ubuntu.com trusty InRelease
......
Reading package lists... Done
W: There is no public key available for the following key IDs:
1397BC53640DB551

Then you are missing a public key in the database.

To solve the problem do:

?> sudo -i bash
[sudo] password for <username>:

?> apt-get install debian-keyring debian-archive-keyring

?> gpg --keyserver keyserver.ubuntu.com --recv 1397BC53640DB551
gpg: requesting key 640DB551 from hkp server keyserver.ubuntu.com
gpg: /root/.gnupg/trustdb.gpg: trustdb created
gpg: key D38B4796: public key "Google Inc. (Linux Packages Signing Authority) <linux-packages-keymaster@google.com>" imported
gpg: no ultimately trusted keys found
gpg: Total number processed: 1
gpg: imported: 1 (RSA: 1)


?> gpg --export 1397BC53640DB551 | apt-key add -
OK

?> exit

Now will it be possible to update the package system.