Add 'MSC3760: State sub-keys'

Author: andybalaam

proposals/3760-state-sub-keys.md

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+# MSC3760: State sub-keys
+
+## Problem
+
+Currently there are three main ways to limit who can overwrite a state event:
+
+ * If a user's PL is greater than the `m.room.power_levels` `state_default` field
+ * If a user's PL is greater than the `m.room.power_levels` `events` field for that event type
+ * If a state event has a state key which begins with an `@`, then the sender's mxid must match that state key.
+
+This is problematic if an unprivileged user needs to publish multiple state events of the same type in a room, but would like to set access control so that only they can subsequently update the event. An example of this is if a user wishes to publish multiple live location share beacons as per [MSC3489](https://github.com/matrix-org/matrix-spec-proposals/pull/3489) and [MSC3672](https://github.com/matrix-org/matrix-spec-proposals/pull/3672).  They will typically not want other users in the room to be able to overwrite the state event, so we need a mechanism to prevent other peers from doing so.
+
+## Proposal
+
+We propose adding an optional `state_subkey` top-level field that allows multiple state events to exist which all have the same `state_key` but represent different pieces of state.
+
+This means that the existing rules around access control to state events can be unchanged, but now one user can own multiple pieces of state that have the same `type`.
+
+For example, to post a live location sharing beacon from [MSC3672](https://github.com/matrix-org/matrix-spec-proposals/pull/3672):
+
+```json=
+{
+    "type": "m.beacon_info",
+    "state_key": "@stefan:matrix.org", // Ensures the sender has control
+    "state_subkey": "asset1",          // Does not clash with other events
+    "content": {
+        "m.beacon_info": {
+            "description": "Stefan's live location",
+            "timeout": 600000,
+            "live": true
+        },
+        "m.ts": 1436829458432,
+        "m.asset": {
+            "type": "m.self.live"
+        }
+    }
+}
+```
+
+State events that are missing `state_subkey` should behave exactly as if `state_subkey` had a value of `""`.
+
+Two state events with identical `type` and `state_key` should be treated as independent if they have different values of `state_subkey`.
+
+## Potential issues
+
+This change involves modifying the way the keys in a room's state map are constructed, so may be complex to implement.
+
+This will require a new room version.
+
+## Alternatives
+
+We could add a suffix to the `event_type`, as originally proposed in [MSC3489](https://github.com/matrix-org/matrix-spec-proposals/pull/3489). This can make life difficult for clients who expect a fixed type, and it makes it hard to control power levels for event types.
+
+We could stuff the mxid and subtype into the `state_key` field as proposed in [MSC3757](https://github.com/matrix-org/matrix-spec-proposals/pull/3757). This would involve changing the access control logic for state events, and objections were raised about the need to parse the state key's contents.
+
+We could allow additional events that set up access control on a state event.
+
+We could add a flag on the contents of the event (as originally proposed in [MSC3757](https://github.com/matrix-org/matrix-spec-proposals/pull/3757)), called e.g. `m.peer_unwritable: true` to say other users were prohibited from overwriting the event.  However, this is impractical because there isn't a good value for the state_key, which needs to be unique and not subject to races from other malicious users.
+
+## Security considerations
+
+The rules on who can modify a state event are unchanged by this proposal, which should simplify security concerns relative to the alternatives.
+
+Server implementors will need to ensure that the state_subkey has no effect on the access control of state events.
+
+This will require a new room version, meaning it will not affect old events.
+
+## Unstable prefix
+
+ * `state_subkey` should be referred to as `org.matrix.mscxxxx.state_subkey` until this MSC lands.
+
+## Dependencies
+
+None