While building kazarma, a bridge between Matrix and ActivityPub, we saw how hard it was for communities to organise privately using ActivityPub apps. Privacy is, however, a real and growing need; either to prevent harassment, to opt out of the "narcissistic arena" or the "attention economy" which can be cultivated in public online spaces, or for other organisational needs.

This private space mechanism is at the core of Matrix, since it was made for group messaging. Differences in the design of protocols like this one affect how easy or hard it is to implement certain functionalities, but they also have indirect social implications on the power distribution among the network.

Following a comparison of ActivityPub and Matrix for use in private social spaces, we will showcase the user interface of Posca, for bringing together multiple social tools (chat, forums, microblogging and media sharing) in a single Matrix client.

The protocol comparison will primarily consider Matrix and ActivityPub, due to our familiarity and experience with these protocols, but it may consider other protocols. The goal is not to fuel pointless protocol wars, but to provoke fruitful discussion and reflect on important questions.