Let’s Encrypt 免费，90天有效
The key principles behind Let’s Encrypt are:
Free: Anyone who owns a domain name can use Let’s Encrypt to obtain a trusted certificate at zero cost.
Automatic: Software running on a web server can interact with Let’s Encrypt to painlessly obtain a certificate, securely configure it for use, and automatically take care of renewal.
Secure: Let’s Encrypt will serve as a platform for advancing TLS security best practices, both on the CA side and by helping site operators properly secure their servers.
Transparent: All certificates issued or revoked will be publicly recorded and available for anyone to inspect.
Open: The automatic issuance and renewal protocol will be published as an open standard that others can adopt.
Cooperative: Much like the underlying Internet protocols themselves, Let’s Encrypt is a joint effort to benefit the community, beyond the control of any one organization.
网上有一些教程，大多linux环境下，少部分windows环境。这边主要讲windows环境下的使用。
首先去找一款软件 letsencrypt-win-simple
打开后，大概三个选项，网上教程大多使用M 手动创建，然后使用计划任务定期更新。
此外还有两个选项 webdav和 FTP，我这边使用FTP。这样就可以通过FTP上传和修改他要验证的文件。