matrix-spec-proposals/proposals/1717-key_verification.md

Key verification mechanisms

Key verification is an essential part of ensuring that end-to-end encrypted messages are secure. Matrix may support multiple verification methods that require sending events; in fact, two such methods (such as MSC 1267 and MSC 1543) have already been proposed.

This proposal tries to present a common framework for verification methods to use, and presents a way to request key verification.

Proposal

Each key verification method is identified by a name. Verification method names defined in the Matrix spec will begin with m., and verification method names that are not defined in the Matrix spec must be namespaced following the Java package naming convention.

If Alice wants to verify keys with Bob, Alice's device may send to_device events to Bob's devices with the type set to m.key.verification.request, as described below. The m.key.verification.request messages should all have the same transaction_id, and are considered to be a single request. Thus, for example, if Bob rejects the request on one device, then the entire request should be considered as rejected across all of his devices. Similarly, if Bob accepts the request on one device, that device is now in charge of completing the key verification, and Bob's other devices no longer need to be involved.

The m.key.verification.request event lists the verification methods that Alice's device supports, and upon receipt of this message, Bob's client should prompt him to verify keys with Alice using one of the applicable methods. In order to avoid displaying stale key verification prompts, if Bob does not interact with the prompt, it should be automatically hidden 10 minutes after the message is sent (according to the timestamp field), or 2 minutes after the client receives the message, whichever comes first. The prompt should also be hidden if an appropriate m.key.verification.cancel message is received.

If Bob chooses to reject the key verification request, Bob's client should send a m.key.verification.cancel message to Alice's device. This indicates to Alice that Bob does not wish to verify keys with her. In this case, Alice's device should send an m.key.verification.cancel message to all of Bob's devices to notify them that the request has been rejected.

If one of Bob's clients does not understand any of the methods offered, it should display a message to Bob saying so. However, it should not send a m.key.verification.cancel message to Alice's device unless Bob chooses to reject the verification request, as Bob may have another device that is capable of verifying using one of the given methods.

To initiate a key verification process, Bob's device sends a to_device event to one of Alice's devices with the type set to m.key.verification.start. This may either be done in response to an m.key.verification.request message, or can be done independently. If it is done in response to an m.key.verification.request messsage, it should use the same transaction_id as the m.key.verification.request message. If Alice's device receives an m.key.verification.start message in response to an m.key.verification.request message, it should send an m.key.verification.cancel message to Bob's other devices that it had originally sent an m.key.verification.request to, in order to cancel the key verification request.

Verification methods can define other events required to complete the verification. Event types for verification methods defined in the Matrix spec should be in the m.key.verification namespace. Event types that are not defined in the Matrix spec must be namespaced following the Java package naming convention.

Alice's or Bob's devices can cancel a key verification process or a key verification request by sending a to_device event with type set to m.key.verification.cancel.

Event Definitions

m.key.verification.request

Requests a key verification.

Properties:

• from_device (string): Required. The device ID of the device requesting verification.
• transaction_id (string): Required. An identifier for the verification request. Must be unique with respect to the pair of devices.
• methods ([string]): Required. The verification methods supported by the sender.
• timestamp (integer): Required. The time when the request was made. If the timestamp is in the future (by more than 5 minutes, to allow for clock skew), or more than 10 minutes in the past, then the message must be ignored.

m.key.verification.start

Begins a key verification process.

Properties:

• method (string): Required. The verification method to use.
• from_device (string): Required. The device ID of the device starting the verification process.
• transaction_id (string): Required. An identifier for the verification process. If this message is sent in reponse to an m.key.verification.request event, then it must use the same transaction_id as the one given in the m.key.verification.request.
• next_method (string): Optional. If the selected verification method only verifies one user's key, then this property can be used to indicate the method to use to verify the other user's key, which will be started immediately after after the current key verification is complete.

Key verification methods can define additional properties to be included.

m.key.verification.cancel

Cancels a key verification process or a key verification request. Upon receiving an m.key.verification.cancel message, the receiving device must cancel the verification or the request. If it is a verification process that is cancelled, or a verification request initiated by the recipient of the cancellation message, the device should inform the user of the reason.

Properties:

• transaction_id (string): the identifier for the request or key verification to cancel.
• code (string): machine-readable reason for cancelling. Possible reasons are:
  • m.user: the user cancelled the verification.
  • m.timeout: the verification process has timed out. Different verification methods may define their own timeouts.
  • m.unknown_transaction: the device does not know about the given transaction ID.
  • m.unknown_method: the device does not know how to handle the given method. This can be sent in response to an m.key.verification.start message, or can be sent in response to other verification method-specific messages.
  • m.unexpected_message: the device received an unexpected message. For example, a message for a verification method may have been received when it was not expected.
  • m.key_mismatch: the key was not verified.
  • m.user_mismatch: the expected user did not match the user verified.
  • m.invalid_message: an invalid message was received.
  • m.accepted: when an m.key.verification.request is accepted by one device, an m.key.verification.cancel message with code set to m.accepted is sent to the other devices
• reason (string): human-readable reason for cancelling. This should only be used if the recieving client does not understand the code given in the code property.

Verification methods may define their own additional cancellation codes. Cancellation codes defined in the Matrix spec will begin with m.; other cancellation codes must be namespaced following the Java package naming convention.

Tradeoffs

Rather than broadcasting verification requests to Bob's devices, Alice could simply send an m.key.verification.start request to a single device. However, this would require Alice to choose the right device to send to, which may be hard for Alice to do if, for example, Bob has many devices, or if his devices have similar names.

Security considerations

An attacker could try to spam a user with verification requests. Clients should take care that such requests do not interfere with a user's use of the client.

Conclusion

This proposal presents common event definitions for use by key verification methods and defines a way for users to request key verification.