MSC3061: Sharing room keys for past messages (#3061)

* initial version

* use MSC number

* address comments from review

* fix list

* fix markdown

Co-authored-by: Richard van der Hoff <[email protected]>

---------

Co-authored-by: Richard van der Hoff <[email protected]>

proposals/3061-shareable-room-keys.md

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+# MSC3061: Sharing room keys for past messages
+
+In Matrix, rooms can be configured via the `m.room.history_visibility` state
+event such that historical messages can be visible to all Matrix users
+(`world_readable`), all room members (`shared`), room members from the time
+that they are invited to a room (`invited`), or room members from the time that
+they join a room (`joined`).  However, currently in encrypted rooms, rooms with
+the history visibility set to `world_readable` or `shared` are effectively
+set to `invited` since other members generally do not send new members the keys
+to decrypt messages sent before they were invited or joined a room.
+
+We define a "shared-history" flag that identifies keys for messages that were
+sent when the room's visibility setting was set to `world_readable` or
+`shared`.  This allows clients to know which keys are "safe" to share with new
+members so that they can decrypt historical messages.  We also give examples of
+ways in which this flag can be used.
+
+
+## Proposal
+
+A room key (such as a megolm session) is flagged as having been used for shared
+history when it was used to encrypt a message while the room's history
+visibility setting was set to `world_readable` or `shared`.
+
+If the client does not have an `m.room.history_visibility` state event for the
+room, or its value is not understood, the client should treat it as if its
+value is `joined` for the purposes of determining whether the key is used for
+shared history.  This is in contrast with the normal processing of
+`m.room.history_visibility` which defaults to `world_readable` when there is no
+`m.room.history_visibility` state event or its value is not understood.  This
+is done so that, in the event of a bug that causes the client to fail to obtain
+the state event, the client will fail in a secure manner.
+
+Internally, a client may use any mechanism it wants to keep track of this flag.
+When a room key is marked as having been used for shared history:
+
+- `m.room_key` and `m.forwarded_room_key` messages used to share this key have
+  a `shared_history` property set to `true` e.g.
+
+  ```json
+  {
+    "type": "m.room_key",
+    "content": {
+      "algorithm": "m.megolm.v1.aes-sha2",
+      "room_id": "!room_id",
+      "session_id": "session_id",
+      "session_key": "session_key",
+      "shared_history": true
+    }
+  }
+  ```
+
+- the [`SessionData` type](https://spec.matrix.org/unstable/client-server-api/#definition-sessiondata)
+  in key backups (that is, the plaintext object that gets encrypted into the
+  `session_data` field) of this key has a `shared_history` property set to
+  `true` in the decrypted JSON structure e.g.
+
+  ```json
+  {
+    "algorithm": "m.megolm.v1.aes-sha2",
+    "forwarding_curve25519_key_chain": [
+      "hPQNcabIABgGnx3/ACv/jmMmiQHoeFfuLB17tzWp6Hw"
+    ],
+    "sender_claimed_keys": {
+      "ed25519": "aj40p+aw64yPIdsxoog8jhPu9i7l7NcFRecuOQblE3Y"
+    },
+    "sender_key": "RF3s+E7RkTQTGF2d8Deol0FkQvgII2aJDf3/Jp5mxVU",
+    "session_key": "AgAAAADxKHa9uFxcXzwYoNueL5Xqi69IkD4sni8Llf...",
+    "shared_history": true
+  }
+  ```
+
+  and,
+- the [`SessionData` type](https://spec.matrix.org/unstable/client-server-api/#key-export-format)
+  used in key exports has a `shared_history` property that is set to `true` for
+  this key e.g.
+
+  ```json
+  {
+    "algorithm": "m.megolm.v1.aes-sha2",
+    "forwarding_curve25519_key_chain": [
+      "hPQNcabIABgGnx3/ACv/jmMmiQHoeFfuLB17tzWp6Hw"
+    ],
+    "sender_claimed_keys": {
+      "ed25519": "aj40p+aw64yPIdsxoog8jhPu9i7l7NcFRecuOQblE3Y"
+    },
+    "sender_key": "RF3s+E7RkTQTGF2d8Deol0FkQvgII2aJDf3/Jp5mxVU",
+    "session_key": "AgAAAADxKHa9uFxcXzwYoNueL5Xqi69IkD4sni8Llf...",
+    "shared_history": true
+  }
+  ```
+
+When a client obtains a key that has the `shared_history` property set to
+`true`, then it flags the key internally as having been used for shared
+history.  Otherwise, the key should not be flagged as such.
+
+When the room's history visibility setting changes to `world_readable` or
+`shared` from `invited` or `joined`, or changes to `invited` or `joined` from
+`world_readable` or `shared`, senders that support this flag must rotate their
+megolm sessions.
+
+Clients may use this flag to modify their behaviour with respect to sharing
+keys.  For example, when the user invites someone to the room, they may
+preemptively share keys that have this flag with the invited user.  Other
+behaviours may be possible, but must be careful not to guard against malicious
+homeservers.  See the "Security Considerations" section.
+
+## Potential issues
+
+Room keys from clients that do not support this proposal will not be eligible
+for the modified client behaviour.
+
+The suggested behaviour in this MSC is to only share additional keys when
+inviting another user.  This does not allow users who join the room but were
+not invited (for example, if membership is restricted to another space, or if
+the room is publicly joinable) to receive the keys.  Also, if the inviter does
+not have all the keys available for whatever reason, the invitee has no way of
+receiving the keys.  This may be solved in the future when we have a mechanism
+for verifying room membership.
+
+## Alternatives
+
+Rather than having the sender flagging keys, a client can paginate through the
+room's history to determine the room's history visibility settings when the
+room key was used.  This would not require any changes, but has performance
+problems.  In addition, the server could lie about the room history while the
+user is paginating through the history.  By having the sender flag keys, this
+ensures that the key is treated in a manner consistent with the sender's view
+of the room.
+
+Rather than using a boolean flag, we could include the history visibility
+setting as-is.  For example, a `history_visibility` field could be added, which
+is set to the history visibility setting (e.g. `world_readable`).  This
+produces an equivalent effect, but it pushes the processing of the history
+visibility setting to the receiver rather than the sender.  For consistency, it
+is better for as much of the decision-making done by the sender, rather than
+the receiver.
+
+## Security considerations
+
+Clients should still ensure that keys are only shared with authorized users and
+devices, as a malicious homeserver could inject fake room membership events.
+One way to ensure that keys are only shared with authorized users is to only
+share keys with users when the client invites them, as the client is then
+certain that the user is allowed to be in the room.  Another way is to have a
+mechanism of verifying membership, such as the method proposed in
+[MSC3917](https://github.com/matrix-org/matrix-spec-proposals/pull/3917).
+
+## Unstable prefix
+
+Until this feature lands in the spec, the property name to be used is
+`org.matrix.msc3061.shared_history` rather than `shared_history`.