matrix-doc/proposals/3956-extensible-events-encryption.md

MSC3956: Extensible Events - Encrypted Events

MSC1767 describes Extensible Events in detail, though deliberately does not include schemas for encrypted events. This MSC covers a replacement for m.room.encrypted in rooms.

Rationale: Splitting the MSCs down into individual parts makes it easier to implement and review in stages without blocking other pieces of the overall idea. For example, an issue with the way encryption is represented should not block the overall schema from going through.

Proposal

Author's note: There is fairly strong, and reasonable, opposition to having encryption be a content block. This could theoretically allow an event to be partially encrypted, which is undesirable.

Like m.room.message, m.room.encrypted is also deprecated in favour of a new m.encrypted event type. m.encrypted expects an m.encrypted content block, which is the current content schema for an m.room.encrypted event:

{
    // irrelevant fields not shown
    "type": "m.encrypted",
    "content": {
        "m.encrypted": {
            "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>"
        }
    }
}

This allows the m.encrypted content block to be reused by other event types, if required.

For clarity, this is not intended to allow unencrypted fallback on an encrypted event - doing so would be extraordinarily dangerous and is explicitly discouraged.

Potential issues

TODO: Address author's note in proposal body

TODO

Alternatives

TODO

Security considerations

TODO

Unstable prefix

While this MSC is not considered stable, implementations should use org.matrix.msc1767.* as a prefix in place of m.* throughout this proposal. Note that this uses the namespace of the parent MSC rather than the namespace of this MSC - this is deliberate.

Note that extensible events should only be used in an appropriate room version as well.