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MSC2241: Key verification in DMs
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# Key verification in DMs | ||
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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. | ||
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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. | ||
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This proposal will solve both problems. | ||
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## Proposal | ||
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The current [key verification | ||
framework](https://matrix.org/docs/spec/client_server/r0.5.0#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](https://matrix.org/docs/spec/client_server/r0.5.0#id185) room. If | ||
there is no Direct Messaging room between the two users involved, the client | ||
that initiates the key verification will create one. | ||
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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. | ||
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### General framework | ||
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#### Requesting a key verification | ||
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To request a key verification, Alice will send an `m.room.message` event with the | ||
following properties in its contents: | ||
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- `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." | ||
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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. | ||
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The event may also contain `format` and `formatted_body` properties as | ||
described in the [m.room.message | ||
msgtypes](https://matrix.org/docs/spec/client_server/r0.5.0#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. | ||
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Key verifications will be identified by the event ID of the key verification | ||
request event. | ||
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Clients should ignore verification requests that have been accepted or | ||
cancelled, or if they do not belong to the sending or target users. | ||
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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: | ||
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- 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. | ||
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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. | ||
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#### Accepting a key verification | ||
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To accept a key verification, Bob will send an `m.key.verification.ready` event | ||
with the following properties in its contents: | ||
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- `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. | ||
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(Note: the form of the `m.relates_to` property is based on the current state of | ||
[MSC2674](https://github.com/matrix-org/matrix-doc/pull/2674), but is | ||
independent from it since this MSC does not rely on any aggregations features.) | ||
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Clients should ignore `m.key.verification.ready` events that correspond to | ||
verification requests that they did not send. | ||
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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: | ||
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- `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. | ||
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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`. | ||
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After the `m.key.verification.start` event is sent, the devices may exchange | ||
messages (if any) according to the verification method in use. | ||
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#### Rejecting a key verification | ||
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To reject a key verification, Alice or Bob will send an | ||
`m.key.verification.cancel` event with the following properties in its | ||
contents: | ||
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- `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](https://matrix.org/docs/spec/client_server/r0.5.0#m-key-verification-cancel), | ||
or as defined for specific key verification methods. | ||
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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. | ||
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#### Concluding a key verification | ||
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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: | ||
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- `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 | ||
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This provides a record within the room of the result of the verification. | ||
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Any subsequent key verification messages relating to the same request are | ||
ignored. | ||
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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. | ||
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#### Other events | ||
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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. | ||
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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. | ||
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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. | ||
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### SAS verification | ||
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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. | ||
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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: | ||
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```json | ||
{ | ||
"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" | ||
} | ||
``` | ||
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which, when decrypted, yields: | ||
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```json | ||
{ | ||
"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" | ||
] | ||
} | ||
} | ||
``` | ||
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then the hash commitment will be based on the message contents: | ||
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```json | ||
{ | ||
"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" | ||
} | ||
} | ||
``` | ||
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## Alternatives | ||
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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. | ||
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However, it seems more natural to have all messages sent via the same | ||
mechanism. | ||
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## Potential issues | ||
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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. | ||
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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. | ||
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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. | ||
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## Security considerations | ||
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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. | ||
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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. | ||
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## Conclusion | ||
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By using room messages to perform key verification rather than `to_device` | ||
messages, the user experience of key verification can be improved. |