MSC4296: Mentions for device IDs

Signed-off-by: Johannes Marbach <n0-0ne+github@mailbox.org>

Author: Johennes

proposals/4296-device-id-mentions.md

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+# MSC4296: Mentions for device IDs
+
+The [`m.mentions`] content property on events allows senders to include an indicator about whether
+the event should triggger a specialised notification for some or all room members. Among others,
+this can be used to direct events at a certain recipient group within the room. It is, however,
+not currently possible to direct events at a certain subset of devices within a room. This can
+be helpful, for instance, in cases where devices with different capabilities are participating
+in the room and the sender wants to provide a hint on which devices the recipient should pick up
+the message. This proposal makes it possible to mention specific devices via the existing
+[`m.mentions`] mechanism.
+
+## Proposal
+
+A new optional property `device_ids` is introduced in [`m.mentions`] to allow specifying an array
+of device IDs to be mentioned by the message.
+
+```json5
+"m.mentions": {
+  "device_ids": ["ABC1234"]
+}
+```
+
+It is legal for `room` and `user_ids` to be present within `m.mentions` simultaneously to `device_ids`.
+
+## Potential issues
+
+None.
+
+## Alternatives
+
+None.
+
+## Security considerations
+
+Device IDs are already disclosed when sending messages in encrypted rooms. Allowing their use within
+`m.mentions` does, therefore, not present an additional metadata leak.
+
+## Unstable prefix
+
+Until this proposal is accepted into the spec, implementations should refer to `device_ids` as
+`de.gematik.msc4296.device_ids`.
+
+## Dependencies
+
+None.
+
+[`m.mentions`]: https://spec.matrix.org/v1.14/client-server-api/#definition-mmentions