I have installed NAS sever which is accessible from Internet by SFTP with certificate authentication. but I have a challenge:
I want to control who is using my server but If I send a certificate+username+server to a specific user, the receiver or user can share this information with everyone without us knowing, so we can not control who is using our server. is there any solution for it? if I can not exactly figure out who is using, at least I want to block sharing the username+password+certificate and use better solution than simple username and password. (should I use VPN? or hardware authenticators? software authenticators? using mac address?)