particularly useful for e.g. openvpn
apt install easy-rsa
e.g. for an openvpn server
    cd /etc/openvpn/
    make-cadir ca2
    cd ca2/
    ./easyrsa init-pki
    ./easyrsa build-ca
generate server key and cert
    ./easyrsa gen-req openvpn-server nopass
    ./easyrsa sign-req server openvpn-client
generate client key and cert
newuser=openvpn-client
    ./easyrsa gen-req $newuser nopass
    ./easyrsa sign-req client $newuser
ls -lF /etc/openvpn/server/easy-rsa/pki/issued/$newuser.crt
ls -lF /etc/openvpn/server/easy-rsa/pki/private/$newuser.key
FW https://community.openvpn.net/Pages/EasyRSA3-OpenVPN-Howto
https://www.marksei.com/vpn-openvpn-tutorial/