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. This easily allows Alice to verify Bob's key, but does not give Bob any information about Alice's key in order to verify it. We can add a secret key to the QR code, which Alice's device can use to MAC her key to send to Bob. In order to ensure that an attacker, who manages to also scan the QR code, is unable to send a false device key to Bob, Bob's device now sends to Alice's device what it thinks is her key, signed by his key. Since Alice has verified Bob's key via the QR code, Alice's device verifies that the key send by Bob matches her key, and that his signature is valid. 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`). 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). At this point, Alice's device has now verified Bob's key. 7. Alice's device sends a `m.key.verification.start` message with `method` set to `m.reciprocate.v1` to Bob (see below). 8. Bob's device fetches Alice's public key, checks it against what was received in the `m.key.verification.start` message, signs it, and sends it to Alice in a `m.key.verification.check_own_key` message (see below). Bob's device displays a message saying that Alice wants him to verify her key, and presents a button for him to press /after/ Alice's device says that things match. 9. Alice's device receives the `m.key.verification.check_own_key` message, checks Bob's signature, and checks that the key is the same as her device key, as well as checking that the rest of the contents match the expected values. Alice's device displays whether the verification was successful or not. 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. ### 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/#/?request=&action=verify&key_=...&verification_algorithms=&verification_key=` (when `matrix:` URLs are specced, this will be used instead). The `request`, `verification_algorithm`, and `verification_key` parameters are only present if this QR code is related to a key verification request event. `verification_algorithms` is a comma-separated list of hashing algorithms that can be used for verifying the keys of the user who scanned the QR code; currently, only `hmac-sha256` is defined, which is HMAC using SHA-256 as the hash. `verification_key` is a random single-use shared secret, with a length depending on the `verification_algorithm`; for `hmac-sha256`, it must be at least 256-bits long (43 characters when base64-encoded). ### Message types #### `m.key.verification.start` Alice's device tells Bob's device that his key is verified, and asks it to verify her keys. 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](https://github.com/matrix-org/matrix-doc/pull/2241) - `keys`: a map of key ID to key in unpadded base64 - `signatures`: MAC of the message contents, formed as in [Signing JSON](https://matrix.org/docs/spec/appendices#signing-json), with the chosen verification algorithm as the signing algorithm. The key ID depends on the verification algorithm; for `hmac-sha256`, it is the SHA-256 hash of the verification key. The MAC is calculated similarly to Signed JSON: 1. The `unsigned` and `signatures` keys are removed, and the contents are encoded as canonical JSON. 2. The encoded object is then MAC'ed using the verification key according to the selected algorithm, and the MAC is encoded in unpadded base64. Example: ```json { "method": "m.reciprocate.v1", "m.relates_to": { "rel_type": "m.reference", "event_id": "!event_id_of_verification_request" }, "keys": { "ed25519:ODRMFSSXPK": "5YaK7EA3HvtPWr+B0jXFXJ9UidyJ4I9PWpT03xCCJrY", }, "signatures": { "@alice:example.com": { "hmac-sha256:key+id": "mac+of+message+in+unpadded+base64" } } } ``` Note that this message could be sent by either Alice or Bob. That is, it can be sent by either the sender or the recipient of the `m.key.verification.request` message. #### `m.key.verification.check_own_key` Tells Alice's device what Bob's device thinks her key is. message contents: - `m.relates_to`: as per [key verification framework](https://github.com/matrix-org/matrix-doc/pull/2241) - `keys`: A map of key IDs to the key that Bob's device has. Must be the same as the `keys` property from the `m.key.verification.start` event. - `signatures`: signature of the mesage contents, signed using Bob's key Example: ```json { "m.relates_to": { "rel_type": "m.reference", "event_id": "!event_id_of_verification_request" }, "keys": { "ed25519:ODRMFSSXPK": "5YaK7EA3HvtPWr+B0jXFXJ9UidyJ4I9PWpT03xCCJrY", }, "signatures": { "@bob:example.com": { "ed25519:bobs+key+id": "signature+of+message" } } } ``` ### 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) - `m.invalid_signature`: The signature of the `m.key.verification.check_own_key` message was incorrect. 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). Security Considerations ----------------------- 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. However, users have a tendency to click on buttons without reading what the screen says. This is partially mitigated by having Alice's device send her keys MAC'ed with a shared secret. But this relies on the shared secret actually being secret, which may not be the case if an attacker is able to view the QR code, which limits the possible attackers to people who are physically present when Alice and Bob verify. This can also be addressed by allowing Bob to easily undo the verification if Alice's device subsequently gives an error. One potential attack involves an attacker preventing the `m.key.verification.check_own_key` message from reaching Alice, and hoping that Bob blindly clicks on the "Verify" button without waiting for Alice's device to check that the key is correct. In this case, Alice's device will not display an error message saying that the key is incorrect, the users may assume that the absence of an error message means that everything is OK. To prevent this, Alice's device should display an error message if it does not receive a `m.key.verification.check_own_key` message as a response to its `m.key.verification.start` message after a reasonable amount of time.