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

MSC3673 - Encrypting ephemeral data units

Problem

With the introduction of MSC2477 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 MSC2477

We would like to wrap them inside the standard encryption envelope:

{
    "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 /syncs 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.