matrix-doc/proposals/2241-e2e-verification-in-dms.md

Key verification in DMs

Currently, key verification is done using to_device messages. However, since to_device messages are not part of a timeline, there is no user-visible record of the key verification.

As well, the current key verification framework does not provide any feedback when interacting with clients that do not support it; if a client does not support the key verification framework, there is no way for users to discover this other than waiting for a while and noticing that nothing is happening.

This proposal will solve both problems.

Proposal

The current key verification framework will be replaced by a new framework that uses room messages rather than to_device messages. Key verification messages will be sent in a Direct Messaging room. If there is no Direct Messaging room between the two users involved, the client that initiates the key verification will create one.

In this proposal, we use "Alice" to denote the user who initiates the key verification, and "Bob" to denote the other user involved in the key verification.

General framework

Requesting a key verification

To request a key verification, Alice will send an m.room.message event with the following properties in its contents:

• body: a fallback message to alert users that their client does not support the key verification framework, and that they should use a different method to verify keys. For example, "Alice is requesting to verify keys with you. However, your client does not support this method, so you will need to use the legacy method of key verification."

  Clients that do support the key verification framework should hide the body and instead present the user with an interface to accept or reject the key verification.

  The event may also contain format and formatted_body properties as described in the m.room.message msgtypes section of the spec. Clients that support the key verification should similarly hide these from the user.

• msgtype: m.key.verification.request

• methods: the verification methods supported by Alice's client

• to: Bob's Matrix ID. Users should only respond to verification requests if they are named in this field. Users who are not named in this field and who did not send this event should ignore all other events that have a m.reference relationship with this event.

• from_device: Alice's device ID. This is required since some verification methods may use the device IDs as part of the verification process.

Key verifications will be identified by the event ID of the key verification request event.

Clients should ignore verification requests that have been accepted or cancelled, or if they do not belong to the sending or target users.

The way that clients display this event can depend on which user and device the client belongs to, and what state the verification is in. For example:

• If the verification has been completed (there is an m.key.verification.done or m.key.verification.cancel event), the client can indicate that the verification was successful or had an error.
• If the verification has been accepted (there is an m.key.verification.start event) but has not been completed, the two devices involved can indicate that the verification is in progress and can use this event as a place in the room's timeline to display progress of the key verification and to interact with the user as necessary. Other devices can indicate that the verification is in progress on other devices.
• If the verification has not been accepted, clients for the target user can indicate that a verification has been requested and allow the user to accept the verification on that device. The sending client can indicate that it is waiting for the request to be accepted, and the sending user's other clients can indicate the that a request was initiated on a different device.

Clients may choose to display or not to display events of any other type that reference the original request event; but it must not have any effect on the verification itself.

Accepting a key verification

To accept a key verification, Bob will send an m.key.verification.ready event with the following properties in its contents:

• m.relates_to: an object with the properties:
  • rel_type: m.reference
  • event_id: the event ID of the key verification request that is being accepted
• methods: an array of verification methods that the device supports
• from_device: Bob's device ID. This is required since some verification methods may use the device IDs as part of the verification process.

(Note: the form of the m.relates_to property is based on the current state of MSC2674, but is independent from it since this MSC does not rely on any aggregations features.)

Clients should ignore m.key.verification.ready events that correspond to verification requests that they did not send.

After this, either Alice or Bob may start the verification by sending an m.key.verification.start event with the following properties in its contents:

• m.relates_to: an object with the properties:
  • rel_type: m.reference
  • event_id: the event ID of the key verification request that is being started
• method: the key verification method that is being used. This should be a method that both Alice's and Bob's devices support.
• from_device: The user's device ID.

If both Alice and Bob send an m.key.verification.start message, and they both specify the same verification method, then the event sent by the user whose user ID is the smallest is used, and the other event is ignored. If they both send an m.key.verification.start message and the method is different, then the verification should be cancelled with a code of m.unexpected_message.

After the m.key.verification.start event is sent, the devices may exchange messages (if any) according to the verification method in use.

Rejecting a key verification

To reject a key verification, Alice or Bob will send an m.key.verification.cancel event with the following properties in its contents:

• m.relates_to: an object with the properties:
  • rel_type: m.reference
  • event_id: the event ID of the key verification that is being cancelled
• reason: A human readable description of the code. The client should only rely on this string if it does not understand the code.
• code: The error code for why the process/request was cancelled by the user. The contents are the same as the code property of the currently defined m.key.verification.cancel to-device event, or as defined for specific key verification methods.

This message may be sent at any point in the key verification process. Any subsequent key verification messages relating to the same request are ignored. However, this does not undo any verifications that have already been done.

Concluding a key verification

When the other user's key is verified and no more messages are expected, each party will send an m.key.verification.done event with the following properties in its contents:

• m.relates_to: an object with the properties:
  • rel_type: m.reference
  • event_id: the event ID of the key verification that is being concluded

This provides a record within the room of the result of the verification.

Any subsequent key verification messages relating to the same request are ignored.

Although a client may have successfully completed its side of the verification, it may wait until receiving an m.key.verification.done (or m.key.verification.cancel) event from the other device before informing the user that the verification was successful or unsuccessful.

Other events

Key verification methods may define their own event types, or extensions to the above event types. All events sent as part of a key verification process should have an m.relates_to property as defined for m.key.verification.accept or m.key.verification.cancel events.

Clients should ignore events with an m.relates_to that have a rel_type of m.reference that refer to a verification where it is neither the requester nor the accepter.

Clients should not redact or edit verification messages. A client may ignore redactions or edits of key verification messages, or may cancel the verification with a code of m.unexpected_message when it receives a redaction or edit.

SAS verification

The messages used in SAS verification are the same as those currently defined, except that instead of the transaction_id property, an m.relates_to property, as defined above, is used instead.

If the key verification messages are encrypted, the hash commitment sent in the m.key.verification.accept message MUST be based on the decrypted m.key.verification.start message contents, and include the m.relates_to field, even if the decrypted message contents do not include that field. For example, if Alice sends a message to start the SAS verification:

{
  "content": {
    "algorithm": "m.megolm.v1.aes-sha2",
    "ciphertext": "ABCDEFG...",
    "device_id": "Dynabook",
    "sender_key": "alice+sender+key",
    "session_id": "session+id",
    "m.relates_to": {
      "rel_type": "m.reference",
      "event_id": "$verification_request_event"
    }
  },
  "event_id": "$event_id",
  "origin_server_ts": 1234567890,
  "sender": "@alice:example.org",
  "type": "m.room.encrypted",
  "room_id": "!room_id:example.org"
}

which, when decrypted, yields:

{
  "room_id": "!room_id:example.org",
  "type": "m.key.verification.start",
  "content": {
    "from_device": "Dynabook",
    "hashes": [
      "sha256"
    ],
    "key_agreement_protocols": [
        "curve25519"
    ],
    "message_authentication_codes": [
        "hkdf-hmac-sha256"
    ],
    "method": "m.sas.v1",
    "short_authentication_string": [
        "decimal",
        "emoji"
    ]
  }
}

then the hash commitment will be based on the message contents:

{
  "from_device": "Dynabook",
  "hashes": [
    "sha256"
  ],
  "key_agreement_protocols": [
      "curve25519"
  ],
  "message_authentication_codes": [
      "hkdf-hmac-sha256"
  ],
  "method": "m.sas.v1",
  "short_authentication_string": [
      "decimal",
      "emoji"
  ],
  "m.relates_to": {
    "rel_type": "m.reference",
    "event_id": "$verification_request_event"
  }
}

Alternatives

Messages sent by the verification methods, after the initial key verification request message, could be sent as to-device messages. The m.key.verification.ready, m.key.verification.cancel, and m.key.verification.done messages must be still be sent in the room, as the m.key.verification.ready notifies the sender's other devices that the request has been acknowledged, and the m.key.verification.cancel and m.key.verification.done provide a record of the status of the key verification.

However, it seems more natural to have all messages sent via the same mechanism.

Potential issues

If a user wants to verify their own device, this will require the creation of a Direct Messaging room with themselves. Instead, clients may use the current to_device messages for verifying the user's other devices.

Direct Messaging rooms could have end-to-end encryption enabled, and some clients can be configured to only send decryption keys to verified devices. Key verification messages should be granted an exception to this (so that decryption keys are sent to all of the target user's devices), or should be sent unencrypted, so that unverified devices will be able to be verified.

Users might have multiple Direct Messaging rooms with other users. In this case, clients could need to prompt the user to select the room in which they want to perform the verification, or could select a room.

Security considerations

Key verification is subject to the room's visibility settings, and may be visible to other users in the room. However, key verification does not rely on secrecy, so this will no affect the security of the key verification. This may reveal to others in the room that Alice and Bob know each other, but this is already revealed by the fact that they share a Direct Messaging room.

This framework allows users to see what key verifications they have performed in the past. However, since key verification messages are not secured, this should not be considered as authoritative.

Conclusion

By using room messages to perform key verification rather than to_device messages, the user experience of key verification can be improved.