matrix-spec-proposals/proposals/2399-reporting-no-key-sent.md

Reporting that decryption keys are withheld

When an encrypted message is sent in a room, the megolm key might not be sent to all devices present in the room. Sometimes this may be inadvertent (for example, if the sending device is not aware of some devices that have joined), but some times, this may be purposeful. For example, the sender may have blacklisted certain devices or users, or may be choosing to not send the megolm key to devices that they have not verified yet.

Currently, when this happens, there is no feedback given to the affected devices; devices that have not received keys do not know why they did not receive the key, and so cannot inform the user as to whether it is expected that the message cannot be decrypted. To address this, senders can send a message to devices indicating that they purposely did not send a megolm key.

A similar issue happens with keyshare requests; devices are not informed when other devices decide not to send back keys, and so do not know whether to expect to receive a key in response to the request.

Proposal

Devices that purposely do not send megolm keys to a device may instead send an m.room_key.withheld event as a to-device message to the device to indicate that it should not expect to receive keys for the message. This message may also be sent in reply to a m.room_key_request. The m.room.no_key event has the properties:

• room_id: Required if code is not m.no_olm. The ID of the room that the session belongs to.
• algorithm: Required. The encryption algorithm that the key is for.
• session_id: Required if code is not m.no_olm. The ID of the session.
• sender_key: Required. The key of the session creator.
• code: A machine-readable code for why the key was not sent. Possible values are:
  • m.blacklisted: the user/device was blacklisted
  • m.unverified: the user/devices is unverified
  • m.unauthorised: the user/device is not allowed have the key. For example, this would usually be sent in response to a key request if the user was not in the room when the message was sent
  • m.unavailable: sent in reply to a key request if the device that the key is requested from does not have the requested key
  • m.no_olm: an olm session could not be established. This may happen, for example, if the sender was unable to obtain a one-time key from the recipient.
• reason: A human-readable reason for why the key was not sent. If there is a code, this should be a human-readable representation of code. The receiving client should only use this string if it does not understand the code or if code is not provided.

An m.room_key.withheld event should only be sent once per session; the recipient of the event should assume that the event applies to all messages in that session. If a sender unblocks a recipient, it may re-use the existing session for which the recipient was previously informed that it was blocked, in which case, the recipient of the m.room_key.withheld message should assume that the event applies to all messages in the session prior to the index of the first key that it has received for that session.

A code of m.no_olm is used to indicate that the sender is unable to establish an olm session with the recipient. When this happens, multiple sessions will be affected. In order to avoid filling the recipient's device mailbox, the sender should only send one m.room_key.withheld message with no room_id nor session_id set. FIXME: how does the recipient determine which sessions the notification applies to?

Potential issues

This does not handle all possible reasons why a device may not have received megolm keys.

Security considerations

A user might not want to notify another user of the reason why it was not sent the keys. Sending m.room_key.withheld, or specifying the reason/code are optional.

Unstable prefix

While in development, clients will send events of type org.matrix.room_key.withheld.