I’m not sure if this is in the correct section and I tried looking for my topic but can’t quite find the same subject. I am new to Let’s Encrypt / Certbot and it looks really good but I want to try something that I’m not quite seeing any post/documentation on. Basically I want to use my spare android phone as a web server ( Probably Palapa Web Server), and so I want to get https working with that. Is that possible? I see mostly info on using it for your own linux server and doing it remotely but the only topics I see involving Android are that Android browsers accept sites that use LE to display them, which is a completely different topic altogether.
As a starter, that would require that your Android based Web server running off your phone has a registered domain name and a computer name that both are found on a publicly reachable DNS server.
I don’t say it’s impossible, but I would be (positively) surprised if that’s the case.
Apart from that, any Web server software that knows how to serve https should be possible to setup to use a certificate from LE. You’d have to check the documentation for Palapa to know if it’s capable to do so, but before spending time on that I would ask myself the question if your Web server will be publicly reachable by name, not only by an IP address.
The short answer is yes, the long answer is yes but this really isn’t the place for that kind of support. LE can provide you certificates, but it’s not really their job to support every web server under the sun. But hopefully I can give you pointers!
You’ll need to take a look at various web server packages, such as kWS, jetty, or KSWEB and decide what suits your situation and needs best.
Then you’ll need to find a client that works on Android (or I’d actually recommend replacing Android with Cyanogenmod - it gives you much more control). I’d take a look at some of the shell clients, like acme.sh or letsencrypt.sh and see if they’ll work on the Android shell.
Then you’ll need to configure the server manually, that won’t be Let’s Encrypt specific, just get the thing working in HTTP first, and then adjust the server to use the LE certs.
Good luck! Report back if you get it going, I’m curious as to how you go! 
EDIT: There are many guides online, a quick search turned up stuff like this!
Thanks for the reply. I’m mainly doing this as a learning experience and I just successfully got a cert for my website using a virtual machine and a copy of Ubuntu Mate (Which I have set up currently to mimic a Raspberry Pi 3 I’m thinking about buying for this as well).
You posted a pretty nice link to a general purpose guide setting up a web server and honestly one thing did kind of turn me off. It mentioned Android going into sleep which I forgot about. I could use the app listed to keep it awake but I’d rather not keep the screen on so I may just attempt it just to see if I can (My old phone has more RAM than the pi 3 so I kind of hope it does work). It may be a couple of days before I get it working because I’m kind of busy currently but I’ll reply if I get it working. Might even make a guide just to help others out.
I’ll have to look more into this. I’m actually not seeing that this web server supports https so I may be out of luck with this one. I’ll keep looking around though at some other options and see which route I want to go. I’d rather it be a free server because this is mainly a learning experiment, though I could make use if it, if it did turn out. Thanks for the reply.
Ice Cold Apps makes an HTTPS and FTPS (and SFTP) capable server for Android. I’ve never used that mode but it looks superficially suitable. Well, to the extent “Android web server” makes any sense at least.
Hmm. (I haven't actually installed a web server on my phone so what I'm about to say is mostly extrapolated from what I have done.)
Don't confuse sleeping with the screen turning off. Apps like music payers are able to "keep awake" without the screen being on. And if the app was written to be a server, it should have the ability to run in the background (even on Android).
I know that guide said that the server those chose would sleep, but perhaps one of the other servers will stay awake? If not, a quick search returned apps like CPU Awake which appears to be able to keep the phone awake without keeping the screen on.
I hope you have success with this, I look forward to reading your guide 
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