groupoffice VS Cryptomator

In terms of running your own server with Activesync; obviously there is Microsoft Exchange, possibly Open-Xchange too (it isn't free), but you could deploy your own Zimbra server (a few options, they have open source but you need a bolt on for ActiveSync) or there is Group Office, which is open source and supports ActiveSync: https://www.group-office.com/

Take a look at Group Office. It has file management and sharing, email (with S/MIME support), contacts, tasks, and notes. Additionally, there are seamless integrations with either collabora and libre office online. It is open source and you can either self host or host with the developer. It is pretty well documented too.

I need a module built for the open-source project here: https://github.com/Intermesh/groupoffice

the best way to do this is with https://cryptomator.org

Before putting anything on a cloud service I would recommend 3rd party tools, like Cryptomator, to encrypt folders and such, then upload to a cloud service.

I use Cryptomator - https://cryptomator.org

I've used countless encryption "schemes" over the years, from True/Vera-Crypt to encrypted sparse bundles/images, and none have ever really felt right.

These days i tend to use Cryptomator[0] instead. It accomplishes what none of the others could do, which is transparent encryption across devices.

With Cryptomator, i simply create a vault somewhere in the cloud, stuff data in it, and i can access it from my laptop, phone or tablet, and not think much about it. It integrates into the normal file browsing APIs, and doesn't get in the way.

Because it does "per file" encryption, it also doesn't need to download a 20-100MB chunk from the cloud before decrypting, so it's rather fast (depending on file size of course).

[0]: https://cryptomator.org/

Cryptomator's arguably the most popular encryption software for cloud storage (you can give yourself zero-knowledge encryption by using them) - it's actually what they specialize & focus on (cloud encryption). It's 100% open source and Free to use on computers. On phones I believe it's just a 1-time fee of a few bucks ($13-14, then you have it forever) - note: their iOS offering is still new, so may be a bit unpolished at the moment.

This is the solution: https://cryptomator.org/

Cryptomator. It is made for uploading files securely to cloud storage, but works locally, is easy to use, and completely free for your use case.