Yeah, I have read about the things they want to do which are mostly cryptography related and which have absolutely zero reasoning as to _why_ they can't be done with ActivityPub. Signing all your messages? Yeah, you can append Ed25519 signature chains to any ActivityPub message. You could publish your base public key when creating an account. You could make software that hides all this beneath an abstraction layer so the end-user never has to deal with it. Exporting all your data and uploading it to another server is something you can already do with Mastodon. E2EE message? Coming soon to Mastodon. It's all things you can do _very easily_ on top of a fault tolerant federated public messaging system.

ActivityPub of course uses the same cryptographic certification virtually all the web does, just TLS/HTTPS and relies on the host cert to be valid. But if you wanted to add further levels of verification like pubkeys and sigs, there's absolutely nothing stopping you.