# download repo key
wget https://pkg.c3voc.de/repo.key -O repo.key
 
# check repo key!
echo "5389e5e6ff79b9439ca4ff7e3acf5020eb3478de51c50da4ac15e278d05259c8 repo.key" | sha256sum -c
# should report 'repo.key: OK'
 
# add key
sudo apt-key add repo.key
 
# add source
echo "deb http://pkg.c3voc.de/ buster main" | sudo tee /etc/apt/sources.list.d/pkg_c3voc_de.list
 
# update sources
sudo apt update

cat << EOF | sudo tee /etc/apt/preferences.d/voc.pref
Package: *
Pin: origin "pkg.c3voc.de"
Pin-Priority: 9001
EOF