proposal to move prev_content (#3442)

proposals/3442-move-prev-content.md

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+# MSC3442: move the `prev_content` key to `unsigned`
+
+Background: the Client-Server API specification
+[documents](https://matrix.org/docs/spec/client_server/r0.6.1#state-event-fields)
+a `prev_content` property to be included on state events. It gives the
+`content` of the previous state event with the same `type` and `state_key`.
+
+This property does not form part of the "raw" event as sent to federating
+homeservers via the Server-Server API; it is added to events served over the
+Client-Server API as a service to those clients.
+
+Adding properties at the top level of the event in this way is confusing: it
+makes it difficult to understand which properties are part of the raw event,
+and which added retrospectively.
+
+It is also inconsistent with other properties which are added by the local
+homeserver such as `age`, `redacted_because` and `transaction_id`, which are
+returned under `unsigned`: see https://matrix.org/docs/spec/client_server/r0.6.1#room-event-fields.
+
+## Further background information
+
+1. The specification for the [`GET
+   /_matrix/client/r0/notifications`](https://matrix.org/docs/spec/client_server/r0.6.1#get-matrix-client-r0-notifications)
+   API is unusual in that it requires `prev_content` to be returned under
+   `unsigned`.
+
+2. In general, in addition to returning `prev_content` at the top level as
+   specified, Synapse *also* returns it under `unsigned`, as proposed here.
+
+   The exceptions to this are `GET /notifications` and `GET /sync`, where
+   Synapse returns `prev_content` *only* under `unsigned`. As noted above, this
+   is compliant with the spec for `/notifications`, but appears to be a spec
+   violation for `/sync`.
+
+3. Dendrite returns `prev_content` *only* under `unsigned` (and is therefore
+   theoretically in violation of the spec).
+
+## Proposal
+
+It is proposed that the specification be updated to move the `prev_content`
+property to the `unsigned` object, bringing it in line with `age`,
+`redacted_because` and `transaction_id`.
+
+This will affect:
+ * Events returned by the Client-Server API.
+ * Events sent to Application Services by the Application Services API.
+
+## Potential issues
+
+The proposed change may break compatibility with existing clients and
+Application Services.
+
+In practice, any client supporting Dendrite or `/sync` requests via Synapse
+must already support the new location, so this is not a breaking change for
+such clients. There may be Application Services which rely on `prev_content` at
+the top level; however these can be safely updated to use the new location,
+since Synapse populates both locations.
+
+The problem can be further mitigated by homeservers populating the key at
+*both* locations for as long as they support older versions of the
+specification, as Synapse does today.
+
+## Alternatives
+
+1. We could double-down on the existing situation and require that
+   `prev_content` always be populated at the top level (possibly including
+   `/notifications`).  That is unattractive for the reasons set out in the
+   introduction to this proposal. Furthermore, in practice it will introduce
+   *more* compatibility problems, due to the implementations which currently do
+   not comply with the spec.
+
+2. We could require that homeservers populate the property at *both*
+   locations. However, setting aside the added complexity for homeserver
+   implementations, it also increases the chances of a situation as follows:
+
+    * Some clients use one property, some use another.
+    * A homeserver developer tests against a subset of clients, all of which use
+      one property.
+    * In consequence, some clients are incompatible with some homeservers.
+
+## Security considerations
+
+When an event is sent over federation, the sending homeserver can freely add
+properties to the `unsigned` object, without them being covered by the
+hashes or signatures. Some homeserver implementations (including Synapse)
+will then pass any such properties on to clients and application services.
+
+In principle then, a malicious or buggy homeserver implementation could add an
+incorrect `prev_content` property under `unsigned`, and send it over
+federation. A receiving homeserver, implementing the *current* specification,
+might pass that property straight through to a client, and the receiving client
+would be unable to tell that it was actually incorrectly populated by the
+remote homeserver.
+
+This is a more general problem that exists today, and which requires a more
+general solution to `unsigned` objects - see
+https://github.com/matrix-org/synapse/issues/11080.  This proposal does not
+materially affect the problem.
+
+## Unstable prefix
+
+No unstable prefix is required, since the behaviour proposed is already
+implemented by both Synapse and Dendrite.