openvpn
CA = Certificate Authority PKI = Public Key Infrastructure
https://github.com/OpenVPN/easy-rsa/blob/master/doc/Intro-To-PKI.md
https://community.openvpn.net/openvpn/wiki/EasyRSA3-OpenVPN-Howto
https://wiki.debian-fr.xyz/Serveur_OpenVPN (french)
https://openvpn.net/community-resources/revoking-certificates/