matrix-spec/proposals/1543-qr_code_key_verification.md

Key verification using QR codes

Problem/Background

Key verification is essential in ensuring that end-to-end encrypted messages cannot be read by unauthorized parties. Traditionally, key verification is done by comparing long strings. To save users from the tedium of reading out long strings, some systems allow one party to verify the other party by scanning a QR code; by doing this twice, both parties can verify each other. In this proposal, we present a method for both parties to verify each other by only scanning one QR code.

Proposal

When Alice and Bob meet in person to verify keys, Alice will scan a QR code generated by Bob's device. The QR code will encode both Bob's key as well as what Bob thinks Alice's key is. When Alice scans the QR code, she will ensure that the keys match what is expected, in which case, she relays this information to Bob, who can then tell his device that the keys match.

Example flow:

1. Alice and Bob meet in person, and want to verify each other's keys.

2. Alice requests a key verification through her device by sending an m.key.verification.request message (see MSC2241), with m.qr_code.show.v1, m.qr_code.scan.v1, and m.reciprocate.v1 listed in methods.

3. Alice's client displays a QR code that Bob is able to scan, and an option to scan Bob's QR code.

4. Bob's client prompts Bob to verify Alice's key. The prompt includes a QR code that Alice can scan (if the m.key.verification.request message listed m.qr_code.scan.v1), and an option to scan Alice's QR code (if the m.key.verification.request message listed m.qr_code.show.v1).

5. Alice scans Bob's QR code.

6. Alice's device ensures that:

   • the user ID in the QR code is the same as the expected user ID (which it knows because it is the recipient of her m.key.verification.request message),
   • Bob's keys encoded in the QR code match the keys that she already has for Bob, and
   • Alice's cross-signing key matches the cross-signing key encoded in the QR code.

   If any of these checks fail, Alice's device displays an error message. Otherwise, at this point, Alice's device has now verified Bob's key, and her device will display a message saying that all is well.

7. Alice's device sends a m.key.verification.start message with method set to m.reciprocate.v1 to Bob (see below).

8. Upon receipt of the m.key.verification.start message, Bob's device presents a button for him to press /after/ he has checked that Alice's device says that things match.

9. Bob sees Alice's device confirm that the key matches, and presses the button on his device to indicate that Alice's key is verified.

10. Both devices send an m.key.verification.done message.

Verification methods

This proposal defines three verification methods that can be used in m.key.verification.request messages (see MSC2241).

• m.qr_code.show.v1: means that the sender of the m.key.verification.request message can show a QR code that the recipient can scan. If the recipient can scan the QR code, it should allow the user to do so. This method is never sent as part of a m.key.verification.start message.
• m.qr_code.scan.v1: means that the sender of the m.key.verification.request message can scan a QR code displayed by the recipient. If the recipient can display a QR code, it should allow the user to display it so that the sender can scan it. This method is never sent as part of a m.key.verification.start message.
• m.reciprocate.v1: means that the sender can participate in a reciprocal verification, either as initiator or responder, as described in the Message types section below.

QR code format

The QR codes to be displayed and scanned using this format will encode URLs of the form: https://matrix.to/#/<user-id>?request=<event-id>&action=verify&key_<keyid>=<key-in-base64>...&verification_algorithms=<algorithm>&verification_key=<random-key-in-base64>&other_user_key=<master-key-in-base64> (when matrix: URLs are specced, this will be used instead).

• request: is the event ID of the associated verification request event.
• key_<key_id>: each key that the user wants verified will have an entry of this form, where the value is the key in unpadded base64. The QR code should contain at least the user's master cross-signing key.
• secret: is a random single-use shared secret in unpadded base64. It must be at least 256-bits long (43 characters when base64-encoded).
• other_user_key: the other user's master cross-signing key, in unpadded base64. In other words, if Alice is displaying the QR code, this would be the copy of Bob's master cross-signing key that Alice has.

The QR codes to be displayed and scanned, which are not a part of an in-person verification (for example, for printing on business cards), will encode URLs of the form: https://matrix.to/#/<user-id>?&action=verify&key_<keyid>=<key-in-base64>... In this case, only the user scanning the QR code will verify the key of the user whose QR code was scanned; bi-directional verification is not possible.

Message types

m.key.verification.start

Alice's device tells Bob's device that the keys are verified. The request is MAC'ed using the verification algorithm and verification key from the QR code.

message contents:

• method: m.reciprocate.v1
• m.relates_to: as per key verification framework
• secret: the shared secret from the QR code

Example:

{
  "method": "m.reciprocate.v1",
  "m.relates_to": {
    "rel_type": "m.reference",
    "event_id": "!event_id_of_verification_request"
  },
  "secret": "shared+secret"
}

Note that this message could be sent by either the sender or the recipient of the m.key.verification.request message, depending on which user scanned the QR code.

Cancellation

In addition to the cancellation codes specified in the spec for m.key.verification.cancel, the following cancellation codes may be used:

• m.qr_code.invalid: The QR code is invalid (e.g. it is not a URL of the required form)

The verification can also be cancelled with the error codes:

• m.key_mismatch: if the QR code has keys that do not match the expected value
• m.user_mismatch: if the QR code is for a different user from what was expected

Tradeoffs/Alternatives

Other methods of verifying keys, which do not require scanning QR codes, are needed for devices that are unable to scan QR codes. One such method is MSC1267.

Security Considerations

The first check in Step 6 in the example flow is to ensure that Bob does not present a QR code claiming to be Carol's key. Without this check, Bob will be able to trick Alice into verifying a key under his control, and evesdropping on Alice's communications with Carol.

The security of verifying Alice's key depends on Bob not hitting the "Verified" button (step 9 in the example flow) until after Alice's device indicates success or failure. Users have a tendency to click on buttons without reading what the screen says, but this is partially mitigated by the fact that it is unlikely that Bob will be interacting with the device while Alice is scanning and Alice's device will display the verification results immediately upon scanning. Also, Bob's device will not display the button until it receives the m.key.verification.start message that contains the shared secret from the QR code, which means that an attacker would need to be physically present while Alice and Bob verify. This issue can also be addressed by allowing Bob to easily undo the verification if Alice's device displays an error.