Raspberry pi Server - SSLPi:
SSLPi was designed to enable secure communication between a Raspberry Pi and various clients. Important is that it is easy to install and NOT using a external cloud service.
Direct communication between the Raspberry Pi and the client is established using OpenSSL.
Only by means of configuration you can one integrate your own applications.
It can be quite time-consuming to enter the correct combination of OpenSSL commands and arguments, the software is set up using an installation program (script).
In order for the Raspberry pi server to be accessible from the Internet, it requires a fixed IP address or a domain name.
If you do not own a fixed Ip Adress you can use one of the DynDNS Services. Either your router can integrate DynDNS Services or you install DDNS client.
Your Raspberry must have a fixed IP address in your internal, private ethernet, so that you can tell your router where to send the data packets via IP forwarding.
The detailed description and Software can be downloaded from Github.