As the title says, Google doesn't give anything useful concerning this.
How do I set up and configure HTTPS/SSL for Sinatra apps?
How do I create a HTTPS route?
I have never used HTTPS for my apps before and have no experience tweaking Rack/whatever, so I appreciate detailed answers.
I guess you need to setup your Web-server, not Sinatra, to work with SSL. In Sinatra you can use the request.secure? method to check for the SSL usage.
SSL + Nginx: the first article, the second one.