Info: https://www.nginx.com/resources/admin-guide/nginx-https-upstreams/
Source: http://nategood.com/client-side-certificate-authentication-in-ngi
This is SSL, so you'll need an cert-key pair for you/the server, the api users/the client and a CA pair. You will be the CA in this case (usually a role played by VeriSign, thawte, GoDaddy, etc.), signing your client's certs. There are plenty of tutorials out there on creating and signing certificates, so I'll leave the details on this to someone else and just quickly show a sample here to give a complete tutorial. NOTE: This is just a quick sample of creating certs and not intended for production.