MSC2762: Allowing widgets to send/receive events

proposals/2762-widget-event-receiving.md

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+# MSC2762: Allowing widgets to send/receive events
+
+[MSC1236](https://github.com/matrix-org/matrix-doc/issues/1236) originally specified a Widget API
+which supports widgets being able to receive specified events from the client, and for widgets to
+be able to send more than stickers.
+
+Sticker support is already specified for widgets, though support for text and image events has been
+excluded from the initial specification, as has MSC1236's event receiving support. These components
+have been excluded from the specification due to lack of documentation and lack of reference
+implementation to influence the spec writing process.
+
+This proposal aims to bring the functionality originally proposed by MSC1236 into the widget
+specification with the accuracy and implementation validation required by modern MSCs.
+
+## Prerequisite background
+
+Widgets are relatively new to Matrix and so the terminology and behaviour might not be known to all
+readers. This section should clarify the components of widgets that are applicable to this MSC without
+going on a deep dive into widgets in general.
+
+Widgets are embedded HTML/JS/CSS applications in a client which use the `postMessage` API to talk
+to the client. This communication allows widgets to provide enhanced functionality such as sticker
+pickers (when applied to a user) or performance dashboards (in rooms).
+
+One of the first things that happens over this communication channel is a "capabilities negotiation"
+where the client asks the widget what permissions it wants, and the widget replies with its ideal
+set. The client then either decides or asks the user if the permissions requested are okay.
+
+All communication over the channel is done in a simple request/response flow, using actions to
+describe the request. For the capabilities negotiation, this would be the client sending the widget
+a request with an `action` of `capabilities`, and the widget would respond to that request with a
+response object.
+
+The channel in which communication occurs is called a "session", where the session is "established"
+after the capabilities negotiation. Sessions can only be terminated by the client.
+
+The Widget API is split into two parts: `toWidget` (client->widget) and `fromWidget` (widget->client).
+They are differentiated by where the request originates.
+
+For a bit of background, stickers are gated by an `m.sticker` capability and have a `m.sticker`
+action on the `fromWidget` API. If the widget was granted the capability and sent a valid request
+to the client, the client would send an `m.sticker` event to the currently viewed room as the
+user. This is all a bit confusing due to the naming of all the identifiers, but the principle
+is that there's prior art for sending events from widgets.
+
+## Proposal (sending events from widgets)
+
+As mentioned above in the prerequisite background, sticker messages can currently be sent over the
+Widget API but other events are not possible. To facilitate sending other event types to the room,
+some new capabilities are introduced to allow clients to easily differentiate between custom
+capabilities and custom event types (using the `m.sticker` convention could be confusing between a
+capability of `com.example.event` and an event type of the same name).
+
+The new capabilities are:
+
+* `m.send.event:<event type>` (eg: `m.send.event:m.room.message`) - Used for sending room messages of
+  a given type.
+* `m.send.state_event:<event type>` (eg: `m.send.state_event:m.room.topic`) - Used for sending state
+  events of a given type.
+
+Being able to send other kinds of events (EDUs, account data, etc) is not currently proposed.
+
+Clients SHOULD automatically deny `m.send.event` and `m.send.state_event` capability requests for
+known event types which do not match the descriptor. For example, `m.send.event:m.room.topic` should
+be denied, as should `m.send.state_event:m.room.message`.
+
+As with capabilities negotiation already, the user SHOULD be prompted to approve these capabilities
+if the widget requests them.
+
+For `m.room.message` specifically, the `msgtype` must be appended to the capability following a `#`.
+This ensures that widgets cannot interfere with encryption verification. It is expected that most
+widgets looking to use this functionality will request the following:
+
+* `m.send.event:m.room.message#m.notice`
+* `m.send.event:m.room.message#m.text`
+* `m.send.event:m.room.message#m.emote`
+
+To actually send the event, widgets would use a new `fromWidget` request with action `send_event`
+which takes the following shape:
+
+```json
+{
+  "api": "fromWidget",
+  "widgetId": "20200827_WidgetExample",
+  "requestid": "generated-id-1234",
+  "action": "send_event",
+  "data": {
+    "state_key": "",
+    "type": "m.room.topic",
+    "content": {
+      "topic": "Hello world!"
+    }
+  }
+}
+```
+
+Under `data`, the `state_key` is omitted if the widget is not sending a state event. The other
+properties of `data` are required.
+
+The client is responsible for encrypting the event before sending, if required by the room. The widget
+should not need to be made aware of encryption or have to encrypt events.
+
+If the widget did not get approved for the capability required to send the event, the client MUST
+send an error response (as required currently by the capabilities system for widgets). If the widget
+was approved, the client MUST only send the event to the room the user is currently viewing.
+
+The client SHOULD NOT modify the `type`, `state_key`, or `content` of the request unless required for
+encryption. The widget is responsible for producing valid events - the client MUST pass through any
+errors to the widget using the standard error response in the Widget API.
+
+For added clarity, the client picks either the `/send` or `/state` endpoint to use on the homeserver
+depending on the presence of a `state_key` in the request data. The client then forms a request using
+the `type`, `state_key`, and `content` by matching those against the endpoint's parameters, after
+encryption if required.
+
+If the event is successfully sent by the client, the client sends the following response:
+
+```json
+{
+  "api": "fromWidget",
+  "widgetId": "20200827_WidgetExample",
+  "requestid": "generated-id-1234",
+  "action": "send_event",
+  "data": {
+    "state_key": "",
+    "type": "m.room.topic",
+    "content": {
+      "topic": "Hello world!"
+    }
+  },
+  "response": {
+    "room_id": "!room:example.org",
+    "event_id": "$example"
+  }
+}
+```
+
+*Note: Widget API responses are a clone of the request with an added `response` field.*
+
+Both fields of the `response` are required and represent the room ID in which the event was sent,
+and the event ID of that event.
+
+With this new approach, the `m.sticker` capability and associated action are deprecated in favour of
+this MSC. If this proposal is able to land in the specification before the widgets spec has a first
+release, the `m.sticker` approach described in the prerequisite background section is not to be
+included in the release (existing clients may still support it for legacy purposes).
+
+## Proposal (receiving events in a widget)
+
+In addition to being able to send events into the room, some widgets have an interest in reacting
+to particular events that appear in the room. Using a similar approach to the sending of events,
+a new capability matching `m.receive.event:<event type>` and `m.receive.state_event:<event type>`
+are introduced, with the same formatting requirements as the `m.send.event` and `m.send.state_event`
+capabilities above (ie: `m.receive.event:m.room.message#m.text`).
+
+For each event type requested and approved, the client sends a `toWidget` request with action `event`
+is sent to the widget with the `data` being the event itself. For example:
+
+```json
+{
+  "api": "fromWidget",
+  "widgetId": "20200827_WidgetExample",
+  "requestid": "generated-id-1234",
+  "action": "send_event",
+  "data": {
+    "type": "m.room.topic",
+    "sender": "@alice:example.org",
+    "event_id": "$example",
+    "room_id": "!room:example.org",
+    "state_key": "",
+    "origin_server_ts": 1574383781154,
+    "content": {
+      "topic": "Hello world!"
+    },
+    "unsigned": {
+      "age": 12345
+    }
+  }
+}
+```
+
+The widget acknowledges receipt of this request with an empty `response` object.
+
+The client SHOULD only send events which were received by the client *after* the session has been
+established with the widget (after the widget's capabilities are negotiated). Clients are expected
+to apply the same semantics as the send event capabilities: widgets don't receive `m.emote` msgtypes
+unless they asked for it (and were approved), and they receive *decrypted* events.
+
+## Security considerations
+
+Because the widget can implicitly decrypt room history, it is absolutely imperative that clients
+prompt for permission to use these capabilities even though the capabilities negotation does not
+require this to be done. Clients which approve the capabilities proposed by this MSC without
+asking the user first are strongly frowned upon. There are very few use cases where not asking for
+the user's permission is valid.
+
+## Unstable prefix
+
+While this MSC is not present in the spec, clients and widgets should:
+
+* Use `org.matrix.msc2762.` in place of `m.` in all new identifiers of this MSC.
+* Only call/support the `action`s if an API version of `org.matrix.msc2762` is advertised.