Google announced in August 2014 that they want to encourage all websites to use secure browsing. At present this means acquiring an SSL certificate, installing it on your server and then referencing all pages by https (secure) rather than http (not secured). However this may not be the case in the future. We think that if HTTPS becomes universal, the security will be built into all servers and all browsers by default, obviating the necessity to buy SSL (secure server certificates) and eventually just using the web address without any http:// or https://
ref : https as ranking signal