matrix-spec-proposals/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](https://github.com/matrix-org/matrix-doc/pull/2241)), 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`).  The QR
   code encodes:
   - Bob's Matrix user ID,
   - Bob's keys that he wants Alice to verify (should contain at least his
     master cross-signing key),
   - what Bob thinks Alice's master cross-signing key is,
   - a random shared secret.
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
   indicating that the code is incorrect, and sends a
   `m.key.verification.cancel` message to Bob's device.

   Otherwise, at this point:
   - Alice's device has now verified Bob's key, and
   - Alice's device knows that Bob has the correct key for her.

   Thus for Bob to verify Alice's key, Alice needs to tell Bob that he has the
   right key.
7. Alice's device displays a message saying that all is well.  This message
   tells Alice that she has the right key for Bob, and tells Bob that he has
   the right key for Alice.
8. Alice's device sends a `m.key.verification.start` message with `method` set
   to `m.reciprocate.v1` to Bob (see below).  The message includes the shared
   secret from the QR code.  This signals to Bob's device that Alice has
   scanned Bob's QR code.

   This message is merely a signal for Bob's device to proceed to the next
   step, and is not used for verification purposes.
9. Upon receipt of the `m.key.verification.start` message, Bob's device ensures
   that the shared secret matches.

   If the shared secret does not match, it should display an error message
   indicating that an attack was attempted.  (This does not affect Alice's
   verification of Bob's keys.)

   If the shared secret does match, it asks Bob to confirm that Alice
   has scanned the QR code.
10. 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.

    Bob's verification of Alice's key hinges on Alice telling Bob the result of
    her scan.  Since the QR code includes what Bob thinks Alice's key is,
    Alice's device can check whether Bob has the right key for her.  Alice has
    no motivation to lie about the result, as getting Bob to trust an incorrect
    key would only affect communications between herself and Bob.  Thus Alice
    telling Bob that the code was scanned successfully is sufficient for Bob to
    trust Alice's key, under the assumption that this communication is done
    over a trusted medium (such as in-person).
11. Both devices send an `m.key.verification.done` message.

This flow allows Alice to verify Bob's key, and Bob to verify Alice's key.
Alice verifies Bob's key because she can trust the QR code that Bob displays
for her, as this is done over a trusted medium.  Bob verifies Alice's key
because Alice can trust the QR code that Bob displays, and Bob can trust Alice
to tell him the result of the verification.

### Verification methods

This proposal defines three verification methods that can be used in
`m.key.verification.request` messages (see
[MSC2241](https://github.com/matrix-org/matrix-doc/pull/2241)).

- `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](#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>...&secret=<shared-secret>&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 QR code has been scanned.

message contents:

- `method`: `m.reciprocate.v1`
- `m.relates_to`: as per [key verification framework](https://github.com/matrix-org/matrix-doc/pull/2241)
- `secret`: the shared secret from the QR code

Example:

```json
{
  "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`](https://matrix.org/docs/spec/client_server/r0.5.0#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](https://github.com/matrix-org/matrix-doc/issues/1267).

Rather than embedding the keys in the QR codes directly, the two clients could
perform an exchange similar to
[MSC1267](https://github.com/matrix-org/matrix-doc/issues/1267), and encoding
the Short Authentication String code in the QR code.  However, this means that
the clients must exchange several messages before they can verify each other,
which would delay showing the QR codes.  This proposal is also simpler to
implement.

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 10 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.