How to fix this BADSIG issue?

Question

W: GPG error: http://repo.vivaldi.com/stable/deb stable Release: The following signatures were invalid: BADSIG 793FEB8BB69735B2 Vivaldi Package Composer KEY06 <packager@vivaldi.com>
E: The repository 'http://repo.vivaldi.com/stable/deb stable Release' is not signed.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.

Answer 1

You have to download their GPG key and add it to the system by using command below:

cd /tmp
wget -c http://repo.vivaldi.com/stable/linux_signing_key.pub
sudo apt-key add linux_signing_key.pub

then update package lists again by sudo apt-get update as usual.