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+# MSC3394: new auth rule that only allows someone to post a message in relation to another message
+
+Today, chat platforms are used broadly in order to broadcast information to large audiences.
+An example of this are [Telegram channels](https://telegram.org/tour/channels/).
+
+Matrix rooms can be used to a similar effect, if permissions are set so that regular
+users can't post any messages. Subscribers are fine with this most of the time, because
+they're usually more interested in reading new content by the room's owner than in
+replying to that content, or in reading what other subscribers have to say.
+
+But some times they may want to give feedback, or read other people's opinions; and that
+can't be done today because, by definition, this kind of room prevents any type of conversation.
+
+## Proposal
+
+In the context of this proposal, and based on [MSC3440](https://github.com/matrix-org/matrix-doc/pull/3440),
+a "message inside a thread" is defined as an `m.room.message` event containing an `m.relates_to` field
+with a relation type of `m.thread` and a "top-level message" is defined as an `m.room.message` event
+NOT containing such a field.
+
+I propose a new auth rule that only allows room members to post a message if it is
+in relation to another message.
+
+This will mean that regular users can't post top-level messages, but they can start threads
+and reply to them; this will allow for conversation inside the room while keeping the room
+clean for subscribers who just want the news; because the main timeline will only contain
+posts by the room's owner, and all the conversations will take place within threads.
+
+It may also be useful for additional purposes: for example a [client focused on social networking](https://matrix.org/blog/2020/12/18/introducing-cerulean) could use a room
+to store a user's posts, which could then be followed by other users; or a [matrix-based comment system](https://cactus.chat/) could show
+every post on a webpage in the same room, with all the discussions happening inside threads.
+
+Based on [existing auth rules](https://spec.matrix.org/latest/rooms/v7/#authorization-rules), the new auth rule I'm proposing
+could probably look like this:
+
+1. If type is m.room.message:
+  1. If the user's power level is higher than the power level for m.room.message events, allow.
+  2. Otherwise, check the new power level I want to introduce. If it is is higher than the user's power level, reject.
+  3. Otherwise, if the event doesn't have a relation, reject.
+  4. Otherwise, allow.
+
+As an example, let's say I want to set up my room so that only moderators can post top-level messages,
+but regular users can comment those messages inside a thread. I will set a power level of 50 for the
+"send messages" permission, and a power level of 0 for the new "send messages to threads" permission.
+
+When a user tries to send a message:
+
+1. If they are a moderator (PL of 50) they are allowed.
+2. Otherwise, if they are a regular user (PL of 0) and the message is inside a thread (it has a relation) they are allowed.
+3. Otherwise, they're not allowed.
+
+It is possible that the proposal above has errors of form, since I'm not familiar with auth rules. In that
+case I would welcome suggestions on how to improve it.
+
+## Alternatives
+
+It would also be possible to achieve the same result with threads-as-rooms; this would
+also allow for greater flexibility (e.g. allowing regular users to comment some posts,
+but not others) but it would also probably be more expensive.