Post #4174504

@evan yay, kudos for addressing this! It's true that not all participants would want to identify themselves, e.g. to all friends of friends when replying to a friend's private post. I think it's OK for such repliers to still be identifiable to (only) their friend the OP. Otherwise, harassment would be more possible if the reply were completely anonymous, whereas if the OP knows their identity they can use social pressure to mitigate harassment. One challenge is if Alice posts and Bob replies, and Bob is blocking Charlie, then how does Alice (or Alice's server, e.g. an MLSGroup) know not to forward Bob's reply to Charlie, unless Alice knows that Bob is blocking Charlie? Blocklists are private information. One possible solution is to use Private Set Intersection (PSI) to determine the intersection of Alice's recipients and Bob's blocklist. PSI is slow and typically requires recalculating every time one of the sets changes, but Updateable Private Set Intersection (UPSI) can handle set changes quickly. 1/