[shmapf]

Does activity pub not use public key cryptography? (I don't know the answer to this but assumed it would have)

If each post is encrypted and signed by the author, and Sig chains are verified by servers when recieving updates that each link in the chain is an approved follower, then while posts could be more widely reshared by recipients, they wouldn't show up as from the original author, because the sharer and the Sig wouldn't match.

Seems to me this would be equivalent to using a centralised system like Facebook, where anyone with access to your posts can just copy paste to re share them.