sudo apt-get update GPG Error

Problem:
Reading package lists... Done
W: GPG error: http://frozenbox.mirror.garr.it/mirrors/parrot stable InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY D7427F070F4FC7A6
W: The repository 'http://frozenbox.mirror.garr.it/mirrors/parrot stable 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.
W: There is no public key available for the following key IDs:
D7427F070F4FC7A6

Fixed:
sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys D7427F070F4FC7A6

Running:
Executing: /tmp/tmp.Tgnb9w75yQ/gpg.1.sh --keyserver
keyserver.ubuntu.com
--recv-keys
D7427F070F4FC7A6
gpg: requesting key 0F4FC7A6 from hkp server keyserver.ubuntu.com
gpg: key 0F4FC7A6: public key "Frozenbox Network (repository signature only) <info@frozenbox.org>" imported
gpg: Total number processed: 1
gpg: imported: 1 (RSA: 1)

Restart:
sudo apt-get update

Result:
Hit:1 http://ppa.launchpad.net/sunab/kdenlive-release/ubuntu xenial InRelease
Hit:2 http://security.ubuntu.com/ubuntu xenial-security InRelease 
Ign:3 http://packages.linuxmint.com sarah InRelease 
Hit:4 http://archive.ubuntu.com/ubuntu xenial InRelease 
Hit:5 http://archive.canonical.com/ubuntu xenial InRelease 
Hit:6 http://ppa.launchpad.net/webupd8team/tor-browser/ubuntu xenial InRelease 
Hit:7 http://archive.ubuntu.com/ubuntu xenial-updates InRelease 
Hit:8 http://packages.linuxmint.com sarah Release 
Get:10 http://frozenbox.mirror.garr.it/mirrors/parrot stable InRelease [11.7 kB]
Hit:11 http://archive.ubuntu.com/ubuntu xenial-backports InRelease
Fetched 11.7 kB in 2s (5,658 B/s) 
Reading package lists... Done

Tested in Linux Mint 18 - Sarah