Is it even possible to implement E2E in the context of ActivtyPub? I mean, as far as I know, the federation doesn’t specify what content you send, only activities, groups and object definitions. There’s nothing stopping you from making the actual data E2E encrypted, altough making it so would be a hard problem.

On the other hand… As I’ve mused about in the other comments, it should be possible to create a fediverse app that serves as a self-hosted front-end for interacting with different fediverse apps. All of your personal data would live on it, and you are in full control. Which would also allow for a safe implementation of E2E, because you just publish your public key, and know that since the app is under your control, noone can get to it. However, this would mean that the other users whould have to use the same standart.

I actually really like that idea. If we can separate users from servers with content, so Lemmy instances would only host posts and comments, but DMs would be handled by the private user instances, it would make Fediverse a lot more private.

The only question standing in the way is - who hosts the content of the posts I make? If my home is programming.dev, and I post to lemmy.ml, do I send the post data through ActivityPub to Lemmy to host, or do I host in on programming.dev, and Lemmy.ml just gets the ID of the post? If it’s first one, making the self-hosted user frontend will be easy, since all you need is a few API calls to make posts, and the only storage you need is for DMs and your account details (which may actually static, so a faked webpage returning your data may suffice). If it’s the latter, then it will be a lot more difficult to easily self-host.