MSC3673: Encrypted ephemeral data units

Author: stefanceriu

proposals/3673-encrypted-ephemeral-data-units.md

@@ -0,0 +1,45 @@
+# MSC3673 - Encrypting ephemeral data units
+
+## Problem
+
+With the introduction of [MSC2476](https://github.com/ananace/matrix-doc/blob/user-defined-edus/proposals/2477-user-defined-ephemeral-events.md)
+we now have the ability to send and receive custom, user-defined ephemeral data 
+units. This is a great mechanism for transferring short-lived data, applicable in
+a variety of situations where persistence is not desired.
+
+Unfortunately E2E encryption for EDUs isn't currently defined and some 
+situations, like live user location sharing, come with privacy concerns, moment 
+in which that becomes a problem.
+
+## Proposal
+
+This MSC proposes a generic mechanism for end to end encrypted ephemeral data 
+units, building on top of MSC2476](https://github.com/ananace/matrix-doc/blob/user-defined-edus/proposals/2477-user-defined-ephemeral-events.md)
+
+We would like to wrap them inside the standard encryption envelope:
+
+```json5
+{
+    "algorithm": "m.megolm.v1.aes-sha2",
+    "sender_key": "<sender_curve25519_key>",
+    "device_id": "<sender_device_id>",
+    "session_id": "<outbound_group_session_id>",
+    "ciphertext": "<encrypted_payload_base_64>"
+}
+```
+
+in which the `ciphertext` will contain the custom EDUs payload and which will be 
+sent to `rooms/{roomId}/ephemeral/m.room.encrypted/{txnId}`, similar to 
+encrypted timeline events .
+
+The Megolm keys required to decrypt this EDU should be shared over Olm just like
+regular encrypted timeline events.
+
+Clients will receive the encrypted payloads in the `/sync`s `ephemeral` 
+dictionary where `type` will be equal to `m.room.encrypted` and which can be 
+decrypted using the pre-shared Megolm keys.
+
+## Alternatives
+
+We are not aware of any other straightforward solution for sharing sensisitive 
+ephemeral data between users.