I am the author of F*EX, an http based filetransfer service:
The (web)server is written in Perl for UNIX, it does not use any
additional webserver like apache or nginx.
https is handled via stunnel.
My development server fex.rus.uni-stuttgart.de has a standard certificate
from the provider of our university.
This particular service is limited to members of the University of
Stuttgart, but the software is free and anyone can download it and run a
server of its own. See https://fex.rus.uni-stuttgart.de/FAQ/meta.html#Who_else_is_using_F_EX
Other FEX admins with own servers might want to use letsencrypt
certficates. Therefore I should write an own FEX letsencrypt client or
integrate the Linux certbot in some way.
There are lots of tutorials how to use certbot with common webservers like
apache or nginx, but this does not help me.
How can I integrate letsencrypt in my own server software?
In need some kind of HOWTO, tutorial for the bloody beginner, etc.
How do I start? Which documents can help me?