Nginx

Update [2017 Aug 5]: Certbot has been developed by EFF and others as an easy-to-use automatic client that fetches and deploys SSL/TLS certificates. I would recommend using it.

Why

Since you are here, you probably know what Let’s Encrypt is and why it exists. If not, below is an executive summary (copied from here):

Anyone who has gone through the trouble of setting up a secure website knows what a hassle getting and maintaining a certificate can be. Let’s Encrypt automates away the pain and lets site operators turn on and manage HTTPS with simple commands.

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