Install gui-apt-key by entering the following commands in the terminal:
sudo apt update sudo apt install gui-apt-key
Description:
Graphical Key Manager for APT
The graphical frontend to the apt-key utility (gak) provides an easy to use interface to maintain digital keys for APT. They are required to authenticate Debian archives and prevent malicious packages to creep in.
Homepage: http://www.infodrom.org/projects/gui-apt-key/
Version: 0.4-2.2
Section: universe/admin