Letsencrypt/readthedocsについて、ここに記述してください。

https://letsencrypt.readthedocs.org/en/latest/intro.html

Current Features 

    Supports multiple web servers:
        apache/2.x (tested and working on Ubuntu Linux)
        nginx/0.8.48+ (under development)
        standalone (runs its own simple webserver to prove you control a domain)
    The private key is generated locally on your system.
    Can talk to the Let’s Encrypt (demo) CA or optionally to other ACME compliant services.
    Can get domain-validated (DV) certificates.
    Can revoke certificates.
    Adjustable RSA key bit-length (2048 (default), 4096, ...).
    Can optionally install a http -> https redirect, so your site effectively runs https only (Apache only)
    Fully automated.
    Configuration changes are logged and can be reverted.
    Text and ncurses UI.
    Free and Open Source Software, made with Python.