MSC3083: Restricting room membership based on membership in other rooms

proposals/3083-restricted-rooms.md

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+# Restricting room membership based on space membership
+
+Draft join rule changes for [spaces](https://github.com/matrix-org/matrix-doc/pull/1772),
+this is meant to replaces the second half of [MSC2962](https://github.com/matrix-org/matrix-doc/pull/2962/).
+
+A desirable feature is to give room admins the power to restrict membership of
+their room based on the membership of one or more spaces, for example:
+
+> members of the #doglovers space can join this room without an invitation<sup id="a1">[1](#f1)</sup>
+
+We could represent the allowed spaces with a new `join_rule` - `restricted` - to
+reflect  the fact that what we have is a cross between `invite` and `public`. This
+would have additional content of the rooms to trust for membership. For example:
+
+```json
+{
+    "type": "m.room.join_rules",
+    "state_key": "",
+    "content": {
+        "join_rule": "restricted",
+        "allow": [
+            {
+                "space": "!mods:example.org",
+                "via": ["example.org"]
+            },
+            {
+                "space": "!users:example.org",
+                "via": ["example.org"]
+            }
+        ]
+    }
+}
+```
+
+This means that a user must be a member of the `!mods:example.org` space or
+`!users:example.org` space in order to join without an invite<sup id="a2">[2](#f2)</sup>. Membership in
+a single space is enough.
+
+If the `allow` key is an empty list (or not a list at all), then no users are allowed to join without an invite. Each entry is expected to be an object with the
+following keys, or a string representing the MXID of the user exempted:
+
+* `space`: The room ID of the space to check the membership of.
+* `via`: A list of servers which may be used to peek for membership of the space.
+
+Any entries in the list which do not match the expected format are ignored.
+
+When a server receives a `/join` request from a client or a `/make_join` / `/send_join`
+request from a server, the request should only be permitted if the user has a valid
+invite or is in one of the listed spaces (established by peeking if the server is not
+already in the space, see [MSC2444](https://github.com/matrix-org/matrix-doc/pull/2444)).
+
+Unlike the `invite` join rule, confirmation that the `allow` rules were  properly
+checked cannot be enforced over federation by event authorization, so servers in
+the room are trusted not to allow invalid users to join.<sup id="a3">[3](#f3)</sup>
+However, user IDs listed as strings can be properly checked over federation.
+
+## Summary of the behaviour of join rules
+
+See the [join rules](https://matrix.org/docs/spec/client_server/r0.6.1#m-room-join-rules)
+specification for full details, but the summary below should highlight the differences
+between `public`, `invite`, and `restricted`.
+
+* `public`: anyone can join, subject to `ban` and `server_acls`, as today.
+* `invite`: only people with membership `invite` can join, as today.
+* `knock`: the same as `invite`, except anyone can knock, subject to `ban` and
+  `server_acls`. See [MSC2403](https://github.com/matrix-org/matrix-doc/pull/2403).
+* `private`: This is reserved and not implemented. 
+* `restricted`: the same as `public` from the perspective of the auth rules, but
+  with the additional caveat that servers are expected to check the `allow` rules
+  before generating a `join` event (whether for a local or a remote user).
+
+## Security considerations
+
+The `allow` feature for `join_rules` places increased trust in the servers in the
+  room. We consider this acceptable: if you don't want evil servers randomly
+  joining spurious users into your rooms, then:
+
+1. Don't let evil servers in your room in the first place
+2. Don't use `allow` lists, given the expansion increases the attack surface anyway by letting members in other rooms dictate who's allowed into your room.
+
+The peek server also has significant power. For example, a poorly chosen peek
+server could lie about the space membership and add an `@evil_user:example.org`.
+
+## Unstable prefix
+
+The `restricted` join rule will be included in a future room version to ensure
+that servers and clients opt-into the new functionality.
+
+During development it is expected that an unstable room version of
+`org.matrix.mscXXXX` is used. Since the room version namespaces the behaviour,
+the `allow` key and the `restricted` value do not need unstable prefixes.
+
+## History / Rationale
+
+It may seem that just having the `allow` key with `public` join rules is enough,
+as suggested in [MSC2962](https://github.com/matrix-org/matrix-doc/pull/2962/),
+but there are concerns that having a `public` join rule that is restricted may
+cause issues if an implementation does not understand the semantics of the `allow`
+keyword. Using an `allow` key with `invite` join rules also does not make sense as
+from the perspective of the auth rules, this is akin to `public` (since the checking
+of whether a member is in the space is done during the call to `/join`
+or `/make_join` / `/send_join`).
+
+The above concerns about an implementation not understanding the semantics of `allow`
+could be solved by introducing a new room version, but if this is done it seems clearer
+to just introduce a a new join rule - `restricted` - as described above.
+
+## Future extensions
+
+Potential future extensions which should not be designed out
+include, but are not included in this MSC.
+
+### Checking space membership over federation
+
+
+
+### Kicking users out when they leave the allowed space
+
+In the above example, suppose `@bob:server.example` leaves `!users:example.org`:
+they should be removed from the room. One option is to leave the departure up
+to Bob's server `server.example`, but this places a relatively high level of trust
+in that server. Additionally, if `server.example` were offline, other users in
+the room would still see Bob in the room (and their servers would attempt to
+send message traffic to it).
+
+Another consideration is that users may have joined via a direct invite, not via access through a space.
+
+Fixing this is thorny. Some sort of annotation on the membership events might
+help. but it's unclear what the desired semantics are:
+
+* Assuming that users in a given space are *not* kicked when that space is
+  removed from `allow`, are those users then given a pass to remain
+  in the room indefinitely? What happens if the space is added back to
+  `allow` and *then* the user leaves it?
+* Suppose a user joins a room via a space (SpaceA). Later, SpaceB is added to
+  the `allow` list and SpaceA is removed. What should happen when the
+  user leaves SpaceB? Are they exempt from the kick?
+
+### Inheriting join rules
+
+If you make a parent space invite-only, should that (optionally?) cascade into
+child rooms? Seems to have some of the same problems as inheriting power levels, as discussed in [MSC2962](https://github.com/matrix-org/matrix-doc/pull/2962).
+
+## Footnotes
+
+<a id="f1"/>[1]: The converse restriction, "anybody can join, provided they are not members
+of the '#catlovers' space" is less useful since:
+
+1. Users in the banned space could simply leave it at any time
+2. This functionality is already somewhat provided by [Moderation policy lists](https://matrix.org/docs/spec/client_server/r0.6.1#moderation-policy-lists). [↩](#a1)
+
+<a id="f2"/>[2]: Note that there is nothing stopping users sending and
+receiving invites in `public` rooms today, and they work as you might expect.
+The only difference is that you are not *required* to hold an invite when
+joining the room. [↩](#a2)
+
+<a id="f3"/>[3]: This is a marginal decrease in security from the current
+situation. Currently, a misbehaving server can allow  unauthorized users to join
+any room by first issuing an invite to that user. In theory that can be
+prevented by raising the PL required to send an invite, but in practice that is
+rarely done. [↩](#a2)