acme-nano is a tool for generating HTTPS certificates that are signed by the Let's Encrypt Certificate Authority. acme-nano is less than 600 lines of code and has no external dependencies, making it easy to audit.
Install acme-nano (requires Go version 1.5+):
$ go get github.com/davidlazar/acme-nano
Register an account:
$ acme-nano register -account acme.key -email email@example.com
Authorize account to manage your domain:
$ sudo acme-nano authorize -account acme.key -domain example.com
acme-nano authorize -h for instructions on how to run the authorize
command without root.
$ acme-nano cert -account acme.key -domain example.com -chain
You'll probably want to run the cert command in a monthly cronjob.