The Mailingsystem in Nextcloud is as far I noticed disconnected from the Mail app.
So on the first step I would like to get the gpg class and the mailing system to use it.
The second step would be from my point of view, to enable GPG key upload on contacts to encrypt share by Email messages. The third step would be an app for the Key management.
After that I have some Ideas to get GPG email working in the mail app, or an own app, but it is just an Idea.
The main Problem is, that you could do the encryption serverside-> then a admin would be able to get your key.
Or on the client as javascript-> there is an app which does it, but the key are on the client, which is maybe a bad idea, when sitting in an internetcoffee. And probably you don’t want to install an app every time, you want to check your emails.
Today I had the Idea, that it would be possible to splitt the key into two parts, since RSA is mostly a modluar power operation. This way the javascript cloud do the fist step of the encryption and the server the second part, but no system would get the full private key. But I have to think about, if it is a good Idea to splitt the RSA algorithm and how many side channel attacks this would allow.