# MSC4108: Mechanism to allow OIDC sign in and E2EE set up via QR code We propose a method to allow an existing authenticated Matrix client to sign in a new client by scanning a QR code. The new client will be a fully bootstrapped Matrix cryptographic device, possessing all the necessary secrets, namely the cryptographic user identity ("cross-signing") and the server-side key backup decryption key (if used). This MSC supersedes [MSC3906](https://github.com/matrix-org/matrix-spec-proposals/pull/3906), [MSC3903](https://github.com/matrix-org/matrix-spec-proposals/pull/3903) and [MSC3886](https://github.com/matrix-org/matrix-spec-proposals/pull/3886) which achieved a similar feature but did not work with a homeserver using the delegated OIDC mechanism proposed by [MSC3861](https://github.com/matrix-org/matrix-spec-proposals/pull/3861). ## Proposal Depending on the pair of devices used, it may be preferable to scan the QR code on either the new or existing device, based on the availability of a camera. As such, this proposal allows for the generation of the QR on either device. In order for the new device to be fully set up, it needs to exchange information with an existing device such that: - The new device knows which homeserver to use - The existing device can facilitate the new device in getting an access token - The existing device shares the secrets necessary to set up end-to-end encryption ### Insecure rendezvous session It is proposed that an HTTP-based protocol be used to establish an ephemeral bi-directional communication session over which the two devices can exchange the necessary data. This session is described as "insecure" as it provides no end-to-end confidentiality nor authenticity by itself---these are layered on top of it. #### High-level description Suppose that Device A wants to establish communications with Device B. Device A can do so by creating a _rendezvous session_ via a `POST /_matrix/client/v1/rendezvous` call to an appropriate homeserver. Its response includes an HTTP _rendezvous URL_ which should be shared out-of-band with Device B. (This URL may be located on a different domain to the initial `POST`.) The rendezvous URL points to an arbitrary data resource (the "payload"), which is initially populated using data from A's initial `POST` request. There are no restrictions on the payload itself, but the rendezvous server SHOULD impose a maximum size limit. Anyone who is able to reach the rendezvous URL - including: Device A; Device B; or a third party; - can then "receive" the payload by polling via a `GET` request, and "send" a new a new payload by making a `PUT` request. In this way, Device A and Device B can communicate by repeatedly inspecting and updating the payload at the rendezvous URL. #### The send mechanism Every send request MUST include an `ETag` header, whose value is supplied by the `ETag` header in the last `GET` response seen by the requester. (The initiating device may also use the `ETag` supplied in the initial `POST` response to immediately update the payload.) Updates will succeed only if the supplied `ETag` matches the server's current revision of the payload. This prevents concurrent writes to the payload. The `ETag` header is standard, described by [RFC9110](https://www.rfc-editor.org/rfc/rfc9110.html#name-etag). In this proposal we only accept strong, single-valued `ETag` values; anything else constitutes a malformed request. n.b. Once a new payload has been sent there is no mechanism to retrieve previous payloads. #### Expiry The rendezvous session (i.e. the payload) SHOULD expire after a period of time communicated to clients via the `Expires` header. After this point, any further attempts to query or update the payload MUST fail. The expiry time SHOULD be extended every time the payload is updated. The rendezvous session can be manually expired with a `DELETE` call to the rendezvous session. ####  API ##### Common HTTP response headers - `ETag` - required, ETag for the current payload at the rendezvous session as per [RFC7232](https://httpwg.org/specs/rfc7232.html#header.etag) - `Expires` - required, the expiry time of the rendezvous as per [RFC7234](https://httpwg.org/specs/rfc7234.html#header.expires) - `Last-Modified` - required, the last modified date of the payload as per [RFC7232](https://httpwg.org/specs/rfc7232.html#header.last-modified) - `Cache-Control` - required, `no-store` as per [RFC7234](https://httpwg.org/specs/rfc7234.html#header.cache-control) - `Pragma` - required, `no-cache` as per [RFC7234](https://httpwg.org/specs/rfc7234.html#header.pragma) ##### Create a rendezvous session and send initial payload: `POST /_matrix/client/v1/rendezvous` This would be part of the Client-Server API. HTTP request headers: - `Content-Length` - required - `Content-Type` - required HTTP request body: - any data up to maximum size allowed by the server HTTP response codes, and Matrix error codes: - `201 Created` - rendezvous session created - `400 Bad Request` (``M_MISSING_PARAM``) - no `Content-Length` was provided. - `403 Forbidden` (``M_FORBIDDEN``) - forbidden by server policy - `413 Payload Too Large` (``M_TOO_LARGE``) - the supplied payload is too large - `429 Too Many Requests` (``M_UNKNOWN``) - the request has been rate limited - `307 Temporary Redirect` - if the request should be served from somewhere else specified in the `Location` response header n.b. the [`307 Temporary Redirect`](https://developer.mozilla.org/en-US/docs/Web/HTTP/Status/307) response code has been chosen explicitly for the behaviour of ensuring that the method and body will not change whilst the user-agent follows the redirect. For this reason, no other `30x` response codes are allowed. HTTP response headers for `201 Created`: - `Content-Type`- required, application/json - common headers as defined above HTTP response body for `201 Created`: - a JSON object with a single key `url` whose value is the absolute URL of the rendezvous session Example response: ```http HTTP 201 Created Content-Type: application/json ETag: VmbxF13QDusTgOCt8aoa0d2PQcnBOXeIxEqhw5aQ03o= Expires: Wed, 07 Sep 2022 14:28:51 GMT Last-Modified: Wed, 07 Sep 2022 14:27:51 GMT Cache-Control: no-store Pragma: no-cache { "url": "http://example.org/abcdEFG12345" } ``` ##### Send a payload to the rendezvous session: `PUT ` HTTP request headers: - `Content-Length` - required - `Content-Type` - required - `If-Match` - required. The ETag of the last payload seen by the requesting device. HTTP request body: - any data up to maximum size allowed by the server HTTP response codes, and Matrix error codes: - `202 Accepted` - payload updated - `400 Bad Request` (`M_MISSING_PARAM`) - a required header was not provided. - `400 Bad Request` (`M_INVALID_PARAM`) - a malformed [`ETag`](https://github.com/matrix-org/matrix-spec-proposals/blob/hughns/simple-rendezvous-capability/proposals/3886-simple-rendezvous-capability.md#the-update-mechanism) header was provided. - `404 Not Found` (`M_NOT_FOUND`) - rendezvous session ID is not valid (it could have expired) - `412 Precondition Failed` (`M_CONCURRENT_WRITE`, a new error code) - when the ETag does not match - `413 Payload Too Large` (`M_TOO_LARGE`) - the supplied payload is too large - `429 Too Many Requests` (`M_UNKNOWN`) - the request has been rate limited HTTP response headers for `202 Accepted` and `412 Precondition Failed`: - common headers as defined above ##### Receive a payload from the rendezvous session: `GET ` HTTP request headers: - `If-None-Match` - optional, as per [RFC7232](https://httpwg.org/specs/rfc7232.html#header.if-none-match) server will only return data if given ETag does not match HTTP response codes, and Matrix error codes: - `200 OK` - payload returned - `304 Not Modified` - when `If-None-Match` is supplied and the ETag does not match - `404 Not Found` (`M_NOT_FOUND`) - rendezvous session URL is not valid (it could have expired) - `429 Too Many Requests` (`M_UNKNOWN`) - the request has been rate limited HTTP response headers for `200 OK`: - `Content-Type` - required - common headers as defined above HTTP response headers for `304 Not Modified`: - common headers as defined above HTTP response body for `200 OK`:: - The payload last set for this rendezvous session, either via the creation POST request or a subsequent PUT request, up to the maximum size allowed by the server. Example responses: ```http HTTP 200 OK Content-Type: text/plain ETag: VmbxF13QDusTgOCt8aoa0d2PQcnBOXeIxEqhw5aQ03o= Expires: Wed, 07 Sep 2022 14:28:51 GMT Last-Modified: Wed, 07 Sep 2022 14:27:51 GMT Cache-Control: no-store Pragma: no-cache foo ``` ```http HTTP 304 Not Modified ETag: VmbxF13QDusTgOCt8aoa0d2PQcnBOXeIxEqhw5aQ03o= Expires: Wed, 07 Sep 2022 14:28:51 GMT Last-Modified: Wed, 07 Sep 2022 14:27:51 GMT Cache-Control: no-store Pragma: no-cache ``` ##### Cancel a rendezvous session: `DELETE ` HTTP response codes: - `204 No Content` - rendezvous session cancelled - `404 Not Found` (`M_NOT_FOUND`) - rendezvous session ID is not valid (it could have expired) - `429 Too Many Requests` (`M_UNKNOWN`) - the request has been rate limited ##### Authentication These API endpoints do not require authentication because trust is established at the secure channel layer which is described later. ##### Maximum payload size The server should allow a minimum payload size of 10KB and enforce a maximum payload size which is recommended to be 100KB. ###### Maximum duration of a rendezvous The rendezvous session only needs to persist for the duration of the handshake. So a timeout such as 30 seconds is adequate. Clients should handle the case of the rendezvous session being cancelled or timed out by the server. ###### ETags The ETag generated should be unique to the rendezvous session and the last modified time so that two clients can distinguish between identical payloads sent by either client. In order to make sure that no intermediate caches manipulate the ETags, the rendezvous server MUST include the HTTP `Cache-Control` response header with a value of `no-store` and `Pragma` response header with a value of `no-cache`. ###### CORS For the `POST /_matrix/client/rendezvous` API endpoint, in addition to the standard Client-Server API [CORS](https://spec.matrix.org/v1.4/client-server-api/#web-browser-clients) headers, the ETag response header should also be allowed by exposing the following CORS header: ```http Access-Control-Expose-Headers: ETag ``` To support usage from web browsers the rendezvous URLs should allow CORS requests from any origin and expose the headers which aren't on the CORS [request header](https://developer.mozilla.org/en-US/docs/Glossary/CORS-safelisted_request_header) and [response header](https://developer.mozilla.org/en-US/docs/Glossary/CORS-safelisted_response_header) safelists: ```http Access-Control-Allow-Headers: If-Match,If-None-Match Access-Control-Allow-Methods: GET, PUT, DELETE Access-Control-Allow-Origin: * Access-Control-Expose-Headers: ETag ``` ##### Choice of server Ultimately it will be up to the Matrix client implementation to decide which rendezvous server to use. However, it is suggested that the following logic is used by the device/client to choose the rendezvous server in order of preference: 1. If the client is already logged in: try and use the current homeserver. 1. If the client is not logged in and it is known which homeserver the user wants to connect to: try and use that homeserver. 1. Otherwise use a default server. #### Example API usage ```mermaid sequenceDiagram participant A as Device A participant HS as Homeserver participant R as Rendezvous Server
https://rz.example.com participant B as Device B Note over A: Device A determines which rendezvous server to use A->>+HS: POST /_matrix/client/rendezvous
Content-Type: text/plain
"Hello from A" HS->>-A: 307 https://rz.example.com/foo A->>+R: POST /foo
Content-Type: text/plain
"Hello from A" R->>-A: 201 Created
ETag: 1
{"url":"https://rz.example.com/abc-def-123-456"} A-->>B: Rendezvous URL shared out of band as QR code: e.g. https://rz.example.com/abc-def-123-456 Note over A: Device A starts polling for new payloads at the
rendezvous session using the returned ETag activate A B->>+R: GET /abc-def-123-456 R->>-B: 200 OK
ETag: 1
Content-Type: text/plain
"Hello from A" loop Device A polls the rendezvous session for a new payload A->>+R: GET /abc-def-123-456
If-None-Match: 1 alt is not modified R->>-A: 304 Not Modified end end note over B: Device B sends a new payload B->>+R: PUT /abc-def-123-456
If-Match: 1
Content-Type: text/plain
"Hello from B" R->>-B: 202 Accepted
ETag: 2 Note over B: Device B starts polling for new payloads at the
rendezvous session using the new ETag activate B loop Device B polls the rendezvous session for a new payload B->>+R: GET /abc-def-123-456
If-None-Match: 2 alt is not modified R->>-B: 304 Not Modified end end note over A: Device A then receives the new payload opt modified R->>A: 200 OK
ETag: 2
Content-Type: text/plain
"Hello from B" end deactivate A note over A: Device A sends a new payload A->>+R: PUT /abc-def-123-456
If-None-Match: 2
Content-Type: text/plain
"Hello again from A" R->>-A: 202 Accepted
ETag: 3 note over B: Device B then receives the new payload opt modified R->>B: 200 OK
ETag: 3
Content-Type: text/plain
... end deactivate B ``` #### Threat analysis ##### Denial of Service attack surface Because the rendezvous session protocol allows for the creation of arbitrary channels and storage of arbitrary data, it is possible to use it as a denial of service attack surface. As such, the following standard mitigations such as the following may be deemed appropriate by homeserver implementations and administrators: - rate limiting of requests - imposing a low maximum payload size (e.g. kilobytes not megabytes) - limiting the number of concurrent sessions ##### Data exfiltration Because the rendezvous session protocol allows for the storage of arbitrary data, it is possible to use it to circumvent firewalls and other network security measures. Implementation may want to block their production IP addresses from being able to make requests to the rendezvous endpoints in order to avoid attackers using it as a dead-drop for exfiltrating data. ### Secure channel The above rendezvous session is insecure, providing no confidentiality nor authenticity against the rendezvous server or even arbitrary network participants which possess the rendezvous session URL. To provide a secure channel on top of this insecure rendezvous session transport, we propose the following scheme. This scheme is essentially [ECIES](https://en.wikipedia.org/wiki/Integrated_Encryption_Scheme#Formal_description_of_ECIES) instantiated with X25519, HKDF-SHA256 for the KDF and ChaCha20-Poly1305 (as specified by [RFC8439](https://datatracker.ietf.org/doc/html/rfc8439#section-2.8)) for the authenticated encryption. Therefore, existing security analyses of ECIES are applicable in this setting too. Nevertheless we include below a short description of our instantiation of ECIES and discuss some potential pitfalls and attacks. The primary limitation of ECIES is that there is no authentication for the initiating party (the one to send the first payload; Device S in the text below). Thus the recipient party (the one to receive the first payload; Device G in the text below) has no assurance as to who actually sent the payload. In QR code login, we work around this problem by exploiting the fact that both of these devices are physically present during the exchange and offloading the check that they are both in the correct state to the user performing the QR code login process. #### Establishment Participants: - Device G (the device generating the QR code) - Device S (the device scanning the QR code) Regardless of which device generates the QR code, either device can be the existing (already signed in) device. The other device is then the new device (one seeking to be signed in). Symmetric encryption uses a separate encryption key for each sender, both derived from a shared secret using HKDF. A separate deterministic, monotonically-incrementing nonce is used for each sender. Devices initially set both nonces to `0` and increment the corresponding nonce by `1` for each message sent and received. 1. **Ephemeral key pair generation** Both devices generate an _ephemeral_ Curve25519 key pair: - Device G generates **(Gp, Gs)**, where **Gp** is its public key and **Gs** the private (secret) key. - Device S generates **(Sp, Ss)**, where **Sp** is its public key and **Ss** the private (secret) key. 2. **Create rendezvous session** Device G creates a rendezvous session by making a `POST` request (as described previously) to the nominated homeserver with an empty payload. It parses the **id** and **server** received. 3. **Initial key exchange** Device G displays a QR code containing: - Its public key **Gp** - The insecure rendezvous session **URL** - An indicator (the **intent**) to say if this is a new device which wishes to "initiate" a login, or an existing device that wishes to "reciprocate" a login - If the intent is to reciprocate a login, then the **homeserver base URL** To get a good trade-off between visual compactness and high level of error correction we use a binary mode QR with a similar structure to that of the existing Device Verification QR code encoding described in [Client-Server API](https://spec.matrix.org/v1.9/client-server-api/#qr-code-format). This is defined in detail in a separate section of this proposal. Device S scans and parses the QR code to obtain **Gp**, the rendezvous session **URL**, **intent** and optionally the **homeserver base URL**. At this point Device S should check that the received intent matches what the user has asked to do on the device. 4. **Device S sends the initial payload** Device S computes a shared secret **SH** by performing ECDH between **Ss** and **Gp**. It then discards **Ss** and derives two 32-byte symmetric encryption keys from **SH** using HKDF-SHA256. One of those keys, **EncKey_S** is used for messages encrypted by device S, while the other, **EncKey_G** is used for encryption by device G. The keys are generated with the following HKDF parameters: **EncKey_S** - `MATRIX_QR_CODE_LOGIN_ENCKEY_S|Gp|Sp` as the info, where **Gp** and **Sp** stand for the generating device's and the scanning device's ephemeral public keys, encoded as unpadded base64. - An all-zero salt. **EncKey_G** - `MATRIX_QR_CODE_LOGIN_ENCKEY_G|Gp|Sp` as the info, where **Gp** and **Sp** stand for the generating device's and the scanning device's ephemeral public keys, encoded as unpadded base64. - An all-zero salt. With this, Device S has established its side of the secure channel. Device S then derives a confirmation payload that Device G can use to confirm that the channel is secure. It contains: - The string `MATRIX_QR_CODE_LOGIN_ENCKEY_S`, encrypted and authenticated with ChaCha20-Poly1305. - Its public ephemeral key **Sp**. ``` Nonce_S := 0 SH := ECDH(Ss, Gp) EncKey_S := HKDF_SHA256(SH, "MATRIX_QR_CODE_LOGIN_ENCKEY_S|" || Gp || "|" || Sp, salt=0, size=32) // Stored, but not yet used EncKey_G := HKDF_SHA256(SH, "MATRIX_QR_CODE_LOGIN_ENCKEY_G|" || Gp || "|" || Sp, salt=0, size=32) NonceBytes_S := ToLowEndianBytes(Nonce_S)[..12] TaggedCiphertext := ChaCha20Poly1305_Encrypt(EncKey_S, NonceBytes_S, "MATRIX_QR_CODE_LOGIN_INITIATE") Nonce_S := Nonce_S + 1 LoginInitiateMessage := UnpaddedBase64(TaggedCiphertext) || "|" || UnpaddedBase64(Sp) ``` Device S then sends the **LoginInitiateMessage** as the payload to the rendezvous session using a `PUT` request with `Content-Type` header set to `text/plain`. 5. **Device G confirms** Device G receives **LoginInitiateMessage** (potentially coming from Device S) from the insecure rendezvous session by polling with `GET` requests. It then does the reverse of the previous step, obtaining **Sp**, deriving the shared secret using **Gs** and **Sp**, discarding **Gs**, deriving the two symmetric encryption keys **EncKey_S** and **EncKey_G**, then finally decrypting (and authenticating) the **TaggedCiphertext** using **EncKey_S**, obtaining a plaintext. It checks that the plaintext matches the string `MATRIX_QR_CODE_LOGIN_INITIATE`, failing and aborting if not. It then responds with a dummy payload containing the string `MATRIX_QR_CODE_LOGIN_OK` encrypted with **SH** calculated as follows: ``` Nonce_G := 1 NonceBytes_G := ToLowEndianBytes(Nonce_G)[..12] TaggedCiphertext := ChaCha20Poly1305_Encrypt(EncKey_G, NonceBytes_G, "MATRIX_QR_CODE_LOGIN_OK") Nonce_G := Nonce_G + 1 LoginOkMessage := UnpaddedBase64Encode(TaggedCiphertext) ``` Device G sends **LoginOkMessage** as the payload via `PUT` request with `Content-Type` header set to `text/plain` to the insecure rendezvous session. 6. **Verification by Device S** Device S receives a response over the insecure rendezvous session by polling with `GET` requests, potentially from Device G. It decrypts (and authenticates) it using the previously computed encryption key, which will succeed provided the payload was indeed sent by Device G. It then verifies the plaintext matches `MATRIX_QR_CODE_LOGIN_OK`, failing otherwise. ``` Nonce_G := 1 (TaggedCiphertext, Sp) := Unpack(Message) NonceBytes := ToLowEndianBytes(Nonce)[..12] Plaintext := ChaCha20Poly1305_Decrypt(EncKey_G, NonceBytes, TaggedCiphertext) Nonce_G := Nonce_G + 1 unless Plaintext == "MATRIX_QR_CODE_LOGIN_OK": FAIL ``` If the above was successful, Device S then calculates a two digit **CheckCode** code derived from **SH**, **Gp** and **Sp**: ``` CheckBytes := HKDF_SHA256(SH, "MATRIX_QR_CODE_LOGIN_CHECKCODE|" || Gp "|" || Sp , salt=0, size=2) CheckCode := NumToString(CheckBytes[0] % 10) || NumToString(CheckBytes[1] % 10) ``` Device S then displays an indicator to the user that the secure channel has been established and that the **CheckCode** should be entered on the other device when prompted. Example wording could say "Secure connection established. Enter the code XY on your other device." 7. **Out-of-band confirmation** **Warning**: *This step is crucial for the security of the scheme since it overcomes the aforementioned limitation of ECIES.* Device G asks the user to enter the **CheckCode** that is being displayed on Device S. The purpose of the code being entered is to ensure that the user has actually checked their other device rather than just pressing "continue", and that the Device S has been able to determine that the channel is secure. Device G compares the code that the user has entered with the **CheckCode** that it calculates using the same mechanism as before: ``` CheckBytes := HKDF_SHA256(SH, "MATRIX_QR_CODE_LOGIN_CHECKCODE|" || Gp "|" || Sp , salt=0, size=2) CheckCode := NumToString(CheckBytes[0] % 10) || NumToString(CheckBytes[1] % 10) ``` If the code that the user enters matches then the secure channel is established. Subsequent payloads sent from G should be encrypted using **EncKey_G**, while payloads sent from S should be encrypted with **EncKey_S**, incrementing the corresponding nonce for each message sent/received. #### Sequence diagram The sequence diagram for the above is as follows: ```mermaid sequenceDiagram participant G as Device G participant Z as Rendezvous server participant S as Device S note over G,S: 1) Devices G and S each generate an ephemeral Curve25519 key pair activate G note over G: 2) Device G creates a rendezvous session as follows G->>+Z: POST /_matrix/client/rendezvous Z->>-G: 201 Created
Location: /_matrix/client/rendezvous/abc-def
ETag: 1 note over G: 3) Device G generates and displays a QR code containing
its ephemeral public key and the rendezvous session URL G-->>S: Device S scans the QR code shown by Device G deactivate G activate S note over S: Device S validates QR scanned and the rendezvous session URL S->>+Z: GET /_matrix/client/rendezvous/abc-def Z->>-S: 200 OK
ETag: 1 note over S: 4) Device S computes SH, EncKey_S, EncKey_G and LoginInitiateMessage.
It sends LoginInitiateMessage via the rendezvous session S->>+Z: PUT /_matrix/client/rendezvous/abc-def
If-Match: 1
Body: LoginInitiateMessage Z->>-S: 202 Accepted
ETag: 2 deactivate S G->>+Z: GET /_matrix/client/rendezvous/abc-def
If-None-Match: 1 activate G Z->>-G: 200 OK
ETag: 2
Body: Data note over G: 5) Device G attempts to parse Data as LoginInitiateMessage after calculating SH, EncKey_S and EncKey_G note over G: Device G checks that the plaintext matches MATRIX_QR_CODE_LOGIN_INITIATE note over G: Device G computes LoginOkMessage and sends to the rendezvous session G->>+Z: PUT /_matrix/client/rendezvous/abc-def
If-Match: 2
Body: LoginOkMessage Z->>-G: 202 Accepted
ETag: 3 deactivate G activate S S->>+Z: GET /_matrix/client/rendezvous/abc-def
If-None-Match: 2 Z->>-S: 200 OK
ETag: 3
Body: Data note over S: 6) Device S attempts to parse Data as LoginOkMessage note over S: Device S checks that the plaintext matches MATRIX_QR_CODE_LOGIN_OK note over S: If okay, Device S calculates the CheckCode to be displayed note over S: Device S displays a green checkmark, "secure connection established" and the CheckCode note over S: Device S knows that the channel is secure deactivate S note over G: 7) Device G asks the user to confirm that the other device is showing a green checkmark and enter the CheckCode note over G: If the user enters the correct CheckCode and confirms that a green checkmark is shown then Device G knows that the channel is secure ``` #### Secure operations Conceptually, once established, the secure channel offers two operations, `SecureSend` and `SecureReceive`, which wrap the `Send` and `Receive` operations offered by the rendezvous session API to securely send and receive data between two devices. At the end of the establishment phase, the next nonce for each device should be `1`. Device G sets: ``` Nonce_G := 1 Nonce_S := 1 ``` Device S sets: ``` Nonce_G := 1 Nonce_S := 1 ``` #### Threat analysis In an attack scenario, we add a participant called Specter with the following capabilities: - Specter is present for QR code generation/scanning ("shoulder-surfing") and can scan the code themselves. - Specter has full control over the network (in a Dolev-Yao sense), being able to observe and modify all traffic. - Specter controls both the homeserver and the rendezvous server. ##### Replay protection Due to use of ephemeral key pairs which are immediately discarded after use, each QR code login session derives a unique secret so payloads from earlier sessions cannot be replayed. Each payload in the session is unique and expected only once. Finally, the use of deterministic nonces prevents any possibility of replay. ##### Pure Dolev-Yao attacker An attacker with control over the network but _not_ present for the QR code scanning cannot thwart the process since they are unable to obtain the ephemeral key **Gp** of Device G. ##### Shoulder-surfing attacker (Specter) Since Device G has no way of authenticating Device S, an attacker present for the QR code scanning can learn **Gp** and attempt to mimic Device S in order to get their Device S signed in instead. - In step 3, Specter can shoulder surf the QR code scanning to obtain **Gp**. - In step 4, Specter can intercept S's payload and replace it with a payload of their own, replacing **Sp** with its own key. - The attack is only thwarted in step 7, because Device S won't ever display the indicator of success to the user. The user then must cancel the process on Device G, preventing it from sharing any sensitive material. ### The OIDC login part and set up of E2EE Once the secure channel has been established, the two devices can then communicate securely. #### Login via OIDC Device Authorization Grant In this section the sequence of steps depends on whether the new device generated or scanned the QR code. For example, in the case that the new device scanned the QR code it is the first to do a `SecureSend` whereas if the new device generated the QR then the existing device is the first to do a `SecureSend`. This can make it hard to read what is going on. 1. **Homeserver discovery** The new device needs to know which homeserver it will be authenticating with. In the case that the new device scanned the QR code then the homeserver base URL can be taken from the QR code and the new device proceeds to step 2 immediately. Otherwise the new device waits to be informed by receiving an `m.login.protocols` message from the existing device. The existing device would need to determine which "protocols" are available for the new device to use. Currently this could only be device_authorization_grant meaning the OIDC Provider supports the `urn:ietf:params:oauth:grant-type:device_code` grant type. If it is available then the existing device informs the new device by sending the `m.login.protocols` message with the homeserver specified: *Existing device => New device via secure channel* ```json { "type": "m.login.protocols", "protocols": ["device_authorization_grant"], "homeserver": "https://synapse-oidc.lab.element.dev" } ``` 2. **New device checks if it can use an available protocol** Once the existing device knows which homeserver it is to use it then: - checks that the homeserver is using delegated OIDC by calling `GET /_matrix/client/v1/auth_issuer` from [MSC2965](https://github.com/matrix-org/matrix-spec-proposals/pull/2965): *New device => Homeserver via HTTP* ```http GET /_matrix/client/v1/auth_issuer HTTP/1.1 Host: synapse-oidc.lab.element.dev Accept: application/json ``` With response like: ```http 200 OK Content-Type: application/json { "issuer": "https://auth-oidc.lab.element.dev/" } ``` - parses the OIDC Provider (`issuer`) from the response - fetches the OIDC Provider metadata as per [MSC2965](https://github.com/matrix-org/matrix-spec-proposals/pull/2965): *New device => OIDC Provider via HTTP* ```http GET /.well-known/openid-configuration HTTP/1.1 Host: auth-oidc.lab.element.dev Accept: application/json ``` With response like: ```http 200 OK Content-Type: application/json { "issuer": "https://auth-oidc.lab.element.dev/", "authorization_endpoint": "https://auth-oidc.lab.element.dev/authorize", "token_endpoint": "https://auth-oidc.lab.element.dev/oauth2/token", "jwks_uri": "https://auth-oidc.lab.element.dev/oauth2/keys.json", "registration_endpoint": "https://auth-oidc.lab.element.dev/oauth2/registration", "scopes_supported": ["openid", "email"], "response_types_supported": [...], "response_modes_supported": [...], "grant_types_supported": [ "authorization_code", "refresh_token", "client_credentials", "urn:ietf:params:oauth:grant-type:device_code" ], ... "device_authorization_endpoint": "https://auth-oidc.lab.element.dev/oauth2/device" } ``` - either does Dynamic Client Registration as per [MSC2966](https://github.com/matrix-org/matrix-spec-proposals/pull/2966) or uses a static OIDC client_id. We will use `my_client_id` as an example `client_id`. - sends a [RFC8628 Device Authorization Request](https://datatracker.ietf.org/doc/html/rfc8628#section-3.1) to the OIDC Provider using the `device_authorization_endpoint`: *New device => OIDC Provider via HTTP* ```http POST /oauth2/device HTTP/1.1 Host: auth-oidc.lab.element.dev Content-Type: application/x-www-form-urlencoded client_id=my_client_id&scope=openid%20urn%3Amatrix%3Aclient%3Aapi%3A%2A%20urn%3Amatrix%3Aclient%3Adevice%3AABCDEGH ``` With response like: ```http 200 OK Content-Type: application/json { "device_code": "GmRhmhcxhwAzkoEqiMEg_DnyEysNkuNhszIySk9eS", "user_code": "123456", "verification_uri": "https://auth-oidc.lab.element.dev/link", "verification_uri_complete": "https://auth-oidc.lab.element.dev/link?code=123456", "expires_in": 1800, "interval": 5 } ``` - parses the [Device Authorization Response](https://datatracker.ietf.org/doc/html/rfc8628#section-3.2) above At this point the new device knows that, subject to the user consenting, it should be able to complete the login 3. **New device informs existing device that it wants to use the `device_authorization_grant`** At this point, the new device should ensure it has generated its Olm account, so that it has its Curve25519 and Ed25519 device identity keys. It then sends a `m.login.protocol` message to the existing device, containing: - An indicator that it wants to use protocol `device_authorization_grant` - The `verification_uri` - The `verification_uri_complete`, if present - The device ID it will be using, which MUST equal the unpadded base64-encoded form of the public part of the Curve25519 identity key that the new device has generated - A proof of ownership of the device ID, which is a base64-encoded form of the proof described below The new device proves it controls the Curve25519 key - with public part `Ip` and private part `Is` - by doing an ECDH between the private part of the Curve25519 identity key and the other device's secure channel ephemeral public key (`Ep` ) to derive a shared secret. `Ep` equates to either `Gp` or `Sp` from the secure channel set up depending on which device did the QR code scanning. This derived secret is then used to to construct a proof of ownership based on HMAC-SHA256. Due to the properties of ECDH, the existing device knows that the new device can only do this if it possesses the private part of the Curve25519 identity key. By requiring the device ID to equal the device identity key, we reduce the number of (unnecessarily) free parameters, allowing a user's E2EE devices to be uniquely identified only by their identity key, rather than by a (device ID, identity key) 2-tuple. This paves the way for potentially making this a strict requirement for all E2EE-supporting devices in a future iteration of the Matrix E2EE protocol. This would provide a marked increase in protocol robustness and reduces potential for implementation errors. Separately, the proof of ownership of the identity key ensures that the new device cannot submit a key it does not control, either by accident or maliciously. While this scenario doesn't represent an outright security compromise---because a device cannot decrypt traffic for an identity key it does not control---it further reduces the margin for implementation error. To calculate the proof the new device does: ``` SH := ECDH(Is, Ep) ProofKey := HKDF_SHA256(SH, "MATRIX_QR_CODE_LOGIN_PROOFKEY|" || Ip || "|" || Ep, salt=0, size=32) ProofBytes := HMAC_SHA256(ProofKey, "MATRIX_QR_CODE_PROOF_OF_POSSESSION") Proof := UnpaddedBase64Encode(ProofBytes) ``` *New device => Existing device via secure channel* ```json { "type": "m.login.protocol", "protocol": "device_authorization_grant", "device_authorization_grant": { "verification_uri": "https://auth-oidc.lab.element.dev/link", "verification_uri_complete": "https://auth-oidc.lab.element.dev/link?code=123456" }, "device_id": "3C5BFWi2Y8MaVvjM8M22DBmh24PmgR0nPvJOIArzgyI", "device_id_proof": "$base64_encoded_proof_of_identity_key_ownership" } ``` The sequence for steps 1 to 3 is as follows: (the sequence depending on which device has scanned the code varies for readability) _New device scanned QR code:_ ```mermaid sequenceDiagram title: Variant: New device scanned QR code participant E as Existing device
already signed in participant Z as Rendezvous server participant N as New device
wanting to sign in participant OP as OIDC Provider participant HS as Homeserver rect rgba(255,0,0, 0.1) #alt if New device scanned QR code note over N: New device completes checks from secure channel establishment step 6 - it now trusts the channel note over N: 1) New device got Homeserver base URL from QR code #else if Existing device scanned QR code # note over E: Existing device completes step 6 # note over E: Existing device displays checkmark and CheckCode # note over E: 1) Existing device sends m.login.protocols message # E->>Z: SecureSend({"type":"m.login.protocols", "protocols":["device_authorization_grant],
"homeserver": "https://matrix-client.matrix.org"}) # note over N: New device waits for user to confirm secure channel from step 7 # Z->>N: SecureReceive() => {"type":"m.login.protocols", "protocols":["device_authorization_grant],
"homeserver": "https://matrix-client.matrix.org"} # note over N: If user enters the correct CheckCode and confirms checkmark
then new device now trusts the channel, and uses the homeserver provided end rect rgba(0,255,0, 0.1) note over N: 2) New device checks if it can use an available protocol: N->>+HS: GET /_matrix/client/v1/auth_issuer activate N HS-->>-N: 200 OK {"issuer": "https://id.matrix.org"} Note over N: New device checks that it can communicate
with the issuer (OIDC Provider). Completing dynamic registration if needed N->>+OP: GET /.well-known/openid-configuration OP->>-N: 200 OK {..., "device_authorization_endpoint":
"https://id.matrix.org/auth/device", ...} Note over N: Device now knows the OP and what the endpoint is, so then attempts to start the login N->>+OP: POST /auth/device client_id=xyz&scope=openid+urn:matrix:api:*+urn:matrix:device:ABCDEFGH... OP->>-N: 200 OK {"user_code": "123456",
"verification_uri_complete": "https://id.matrix.org/device/abcde",
"expires_in_ms": 120000, "device_code": "XYZ", "interval": 1} note over N: 3) New device informs existing device of choice of protocol: N->>Z: SecureSend({"type": "m.login.protocol", "protocol": "device_authorization_grant",
"device_authorization_grant":{
"verification_uri_complete": "https://id.matrix.org/device/abcde",
"verification_uri": ...,
"device_id": "3C5BFWi2Y8MaVvjM8M22DBmh24PmgR0nPvJOIArzgyI", "device_id_proof": "$base64_encoded_proof_of_identity_key_ownership"}) deactivate N end rect rgba(255,0,0, 0.1) # alt if New device scanned QR code note over N: New device displays checkmark and CheckCode note over E: Existing device waits for user to enter CheckCode
and confirm secure channel from step 7 end rect rgba(0,255,0, 0.1) Z->>E: SecureReceive() => {"type": "m.login.protocol", "protocol": "device_authorization_grant",
"device_authorization_grant":{
"verification_uri_complete": "https://id.matrix.org/device/abcde",
"verification_uri": ...},
"device_id": "3C5BFWi2Y8MaVvjM8M22DBmh24PmgR0nPvJOIArzgyI", "device_id_proof": "$base64_encoded_proof_of_identity_key_ownership"} end rect rgba(255,0,0, 0.1) # alt if New device scanned QR code note over E: If user entered correct CheckCode
and confirms checkmark then existing device now trusts the channel end rect rgba(0,255,0, 0.1) note over E: Existing device checks that requested protocol is supported alt if requested protocol is not valid E->>N: SecureSend({"type":"m.login.failure", "reason":"unsupported_protocol",
"homeserver": "https://matrix-client.matrix.org}) end end ``` _Existing device scanned QR code:_ ```mermaid sequenceDiagram title: Variant: Existing device scanned QR code participant E as Existing device
already signed in participant Z as Rendezvous server participant N as New device
wanting to sign in participant OP as OIDC Provider participant HS as Homeserver #alt if New device scanned QR code # note over N: New device completes checks from secure channel establishment step 6 - it now trusts the channel # note over N: 1) New device got Homeserver base URL from QR code rect rgba(0,0,255, 0.1) #else if Existing device scanned QR code note over E: Existing device completes step 6 note over E: Existing device displays checkmark and CheckCode note over E: 1) Existing device sends m.login.protocols message E->>Z: SecureSend({"type":"m.login.protocols", "protocols":["device_authorization_grant],
"homeserver": "https://matrix-client.matrix.org"}) note over N: New device waits for user to confirm secure channel from step 7 Z->>N: SecureReceive() => {"type":"m.login.protocols", "protocols":["device_authorization_grant],
"homeserver": "https://matrix-client.matrix.org"} note over N: If user enters the correct CheckCode and confirms checkmark
then new device now trusts the channel, and uses the homeserver provided end rect rgba(0,255,0, 0.1) note over N: 2) New device checks if it can use an available protocol: N->>+HS: GET /_matrix/client/v1/auth_issuer activate N HS-->>-N: 200 OK {"issuer": "https://id.matrix.org"} Note over N: New device checks that it can communicate
with the issuer (OIDC Provider). Completing dynamic registration if needed N->>+OP: GET /.well-known/openid-configuration OP->>-N: 200 OK {..., "device_authorization_endpoint":
"https://id.matrix.org/auth/device", ...} Note over N: Device now knows the OP and what the endpoint is, so then attempts to start the login N->>+OP: POST /auth/device client_id=xyz&scope=openid+urn:matrix:api:*+urn:matrix:device:ABCDEFGH... OP->>-N: 200 OK {"user_code": "123456",
"verification_uri_complete": "https://id.matrix.org/device/abcde",
"expires_in_ms": 120000, "device_code": "XYZ", "interval": 1} note over N: 3) New device informs existing device of choice of protocol: N->>Z: SecureSend({"type": "m.login.protocol", "protocol": "device_authorization_grant",
"device_authorization_grant":{
"verification_uri_complete": "https://id.matrix.org/device/abcde",
"verification_uri": ...},
"device_id": "3C5BFWi2Y8MaVvjM8M22DBmh24PmgR0nPvJOIArzgyI", "device_id_proof": "$base64_encoded_proof_of_identity_key_ownership"}) deactivate N end # alt if New device scanned QR code # note over N: New device displays checkmark and CheckCode # note over E: Existing device waits for user to enter CheckCode
and confirm secure channel from step 7 #end rect rgba(0,255,0, 0.1) Z->>E: SecureReceive() => {"type": "m.login.protocol", "protocol": "device_authorization_grant",
"device_authorization_grant":{
"verification_uri_complete": "https://id.matrix.org/device/abcde",
"verification_uri": ...},
"device_id": "3C5BFWi2Y8MaVvjM8M22DBmh24PmgR0nPvJOIArzgyI", "device_id_proof": "$base64_encoded_proof_of_identity_key_ownership"} end # alt if New device scanned QR code # note over E: If user entered correct CheckCode
and confirms checkmark then existing device now trusts the channel #end rect rgba(0,255,0, 0.1) note over E: Existing device checks that requested protocol is supported alt if requested protocol is not valid E->>N: SecureSend({"type":"m.login.failure", "reason":"unsupported_protocol",
"homeserver": "https://matrix-client.matrix.org}) end end ``` Then we continue with the actual login: 4. **New device waits for approval from OIDC Provider** On receipt of the `m.login.protocol_accepted` message: - In accordance with [RFC8628](https://datatracker.ietf.org/doc/html/rfc8628#section-3.3.1) the new device must display the `user_code` in order that the user can confirm it on the OIDC Provider if required. - The new device then starts to poll the OIDC Provider by making [Device Access Token Requests](https://datatracker.ietf.org/doc/html/rfc8628#section-3.4) using the interval and bounded by `expires_in`. *New device => OIDC Provider via HTTP* ```http POST /oauth2/token HTTP/1.1 Host: auth-oidc.lab.element.dev Content-Type: application/x-www-form-urlencoded grant_type=urn%3Aietf%3Aparams%3Aoauth%3Agrant-type%3Adevice_code &device_code=GmRhmhcxhwAzkoEqiMEg_DnyEysNkuNhszIySk9eS &client_id=my_client_id ``` - It then parses the [Device Access Token Response](https://datatracker.ietf.org/doc/html/rfc8628#section-3.5) and handles the different responses - If the user consents in the next step then the new device will receive an `access_token` and `refresh_token` etc. as normal for OIDC with MSC3861. 5. **User is asked by OIDC Provider to consent on existing device** On receipt of the `m.login.protocol` message above, and having completed step 7 of the secure channel establishment, the existing device then verifies the `device_id_proof` and asserts that there is no existing device corresponding to the `device_id` from the `m.login.protocol` message. The existing device does the following to verify the proof: ``` ProofBytes := UnpaddedBase64_Decode(device_id_proof) SH := ECDH(Es, Ip) ProofKey := HKDF_SHA256(SH, "MATRIX_QR_CODE_LOGIN_PROOFKEY|" || Ip || "|" || Ep, salt=0, size=32) unless HMAC_SHA256(ProofKey, "MATRIX_QR_CODE_PROOF_OF_POSSESSION") == ProofBytes: FAIL ``` If the proof does not match then the login request should be rejected with an `m.login.failure` and reason `device_proof_failed`. To asset the device does not already exist it calls [GET /_matrix/client/v3/devices/](https://spec.matrix.org/v1.9/client-server-api/#get_matrixclientv3devicesdeviceid) and expects to receive an HTTP 404 response. If the device already exists then the login request should be rejected with an `m.login.failure` and reason `device_already_exists`. If no existing device was found then the existing device opens the `verification_uri_complete` - falling back to the `verification_uri`, if `verification_uri_complete` isn't present - in a system browser. Ideally this is in a trusted/secure environment where the cookie jar and password manager features are available. e.g. on iOS this could be a `ASWebAuthenticationSession` The existing device then sends an acknowledgement message to let the other device know that the consent process is in progress: *Existing device => New device via secure channel* ```json { "type": "m.login.protocol_accepted" } ``` The user is then prompted to consent by the OIDC Provider. They may be prompted to undertake additional actions by the OIDC Provider such as 2FA, but this is all handled within the browser. Note that the existing device does not see the new access token. This is one of the benefits of the OIDC architecture. The sequence diagram for steps 4 and 5 is as follows: ```mermaid sequenceDiagram participant E as Existing device
already signed in participant UA as Web Browser participant N as New device
wanting to sign in participant OP as OIDC Provider participant HS as Homeserver rect rgba(0,255,0, 0.1) note over E: Existing device validates the proof of device key ownership alt proof invalid E->>N: SecureSend({ "type": "m.login.failure", "reason": "device_proof_failed" }) end E->>HS: GET /_matrix/client/v3/devices/{device_id} alt device already exists HS->>E: 200 OK E->>N: SecureSend({ "type": "m.login.failure", "reason": "device_already_exists" }) else device not found HS->>E: 404 Not Found end par E->>N: SecureSend({"type":"m.login.protocol_accepted"}) note over N: 4) New device polls the OIDC Provider awaiting the outcome as per RFC8628 OIDC loop Poll for result at interval seconds N->>OP: POST /token client_id=xyz
&grant_type=urn:ietf:params:oauth:grant-type:device_code
&device_code=XYZ alt pending OP-->>N: 400 Bad Request {"error": "authorization_pending"} else granted OP-->>N: 200 OK {"access_token": "...", "token_type": "Bearer", ...} N->>E: SecureSend({ "type": "m.login.success" }) Note over N: Device now has an access_token and can start to talk to the homeserver else denied OP-->>N: 400 Bad Request {"error": "authorization_declined"} N->>E: SecureSend({"type":"m.login.declined"}) else expired OP-->>N: 400 Bad Request {"error": "expired_token"} N->>E: SecureSend({"type":"m.login.failure", "reason": "authorization_expired"}) end end and E->>UA: 5) Existing device opens
verification_uri_complete (with fallback to verification_uri)
in the system web browser/ASWebAuthenticationSession: Note over E: n.b. in the case of a Web Browser the user needs to have
clicked a button in order for the navigation to happen rect rgba(240,240,240,0.5) UA->>OP: GET https://id.matrix.org/device/abcde OP->>UA: consent screen showing the user_code UA->>OP: POST /allow or /deny end Note over UA: User closes browser end end ``` #### Secret sharing and device verification Once the new device has logged in and obtained an access token it will want to obtain the secrets necessary to set up end-to-end encryption on the device and make itself cross-signed. Before sharing the end-to-end encryption secrets the existing device should validate that the new device has successfully obtained an access token from the OIDC Provider. The purpose of this is so that, if the user or OIDC Provider has disallowed the login, the secrets are not leaked. If checked successfully then the existing device sends the following secrets to the new device: - The private cross-signing key triplet: MSK, SSK, USK - The backup recovery key and the currently used backup version. This is achieved as following: 1. **Existing device confirms that a device with the previously committed-to device ID has indeed logged in successfully** On receipt of an `m.login.success` message the existing device queries the homeserver to check that the is a device online with the corresponding device_id (from the `m.login.protocol` message). It does so by calling [GET /_matrix/client/v3/devices/](https://spec.matrix.org/v1.9/client-server-api/#get_matrixclientv3devicesdeviceid) and expecting to receive an HTTP 200 response. If the device isn't immediately visible it can repeat the `GET` request for up to, say, 10 seconds to allow for some latency. If no device is found then the process should be stopped. 2. **Existing device shares secrets with new device** The existing device sends a `m.login.secrets` message via the secure channel: ```json { "type": "m.login.secrets", "cross_signing": { "master_key": "$base64_of_the_key", "self_signing_key": "$base64_of_the_key", "user_signing_key": "$base64_of_the_key" }, "backup": { "algorithm": "foobar", "key": "$base64_of_the_backup_recovery_key", "backup_version": "version_string" } } ``` 3. **New device cross-signs itself and uploads device keys** On receipt of the `m.login.secrets` message the new device can store the secrets locally The new device can then generate the cross-signing signature for itself. It can then use a single request to upload the device keys and cross signing signature. This removes the chance of other devices seeing the new device as unverified, incorrectly prompting the user to verify the already verified device. The request would look just like any other `/keys/upload` request, it would just include one additional signature, the one from the self-signing key. The request would look like follows: ```http POST /_matrix/client/v3/keys/upload HTTP/1.1 Host: synapse-oidc.lab.element.dev Content-Type: application/json { "device_keys": { "algorithms": [ "m.olm.v1.curve25519-aes-sha2", "m.megolm.v1.aes-sha2" ], "device_id": "3C5BFWi2Y8MaVvjM8M22DBmh24PmgR0nPvJOIArzgyI", "keys": { "curve25519:3C5BFWi2Y8MaVvjM8M22DBmh24PmgR0nPvJOIArzgyI": "3C5BFWi2Y8MaVvjM8M22DBmh24PmgR0nPvJOIArzgyI", "ed25519:3C5BFWi2Y8MaVvjM8M22DBmh24PmgR0nPvJOIArzgyI": "b8gROFh+UIHLD/obY0+IlxoWiGtYVhKdqixvw4QHcN8" }, "signatures": { "@testing_35:morpheus.localhost": { "ed25519:3C5BFWi2Y8MaVvjM8M22DBmh24PmgR0nPvJOIArzgyI": "ziHEUIsHnrYBH4CqYpN1JC/ex3t4VG3zvo16D8ORqN6yAErpsKsnd/5LDdZERIOB1MGffKGfCL6ny5V7rT9FCQ", "ed25519:bkYgAVUNqvuyy8b1w09utJNJxBvK3hZB65xxoLPVzFol": "p257k0tfPF98OIDuXnFSJS2DmVlxO4sgTHdF41DTdZBCpTZfPwok6iASo3xMRKdyy3WMEgkQ6lzhEyRKKZBGBQ" } }, "user_id": "@testing_35:morpheus.localhost" } } ``` The sequence diagram for this would look as follows: ```mermaid sequenceDiagram participant E as Existing device
already signed in participant N as New device
wanting to sign in participant OP as OIDC Provider participant HS as Homeserver rect rgba(0,255,0, 0.1) rect rgb(191, 223, 255) note over N,E: This step is duplicated from the previous section for readability N-->>+E: { "type": "m.login.success" } end Note over E: 1) Existing device checks that the device is actually online E->>HS: GET /_matrix/client/v3/devices/{device_id} activate HS alt is device not found note over E: We should wait and retry for 10 seconds HS->>E: 404 Not Found E->>N: { "type": "m.login.failure", "reason": "device_not_found" } else is device found HS->>E: 200 OK deactivate HS E->>-N: 2) { "type": "m.login.secrets", "cross_signing": {...}, "backup": {...} } activate N note over N: 3) New device stores the secrets locally alt is cross_signing present in m.login.secrets? note over N: New device signs itself note over N: New device uploads device keys and cross-signing signature: N->>+HS: POST /_matrix/client/v3/keys/upload HS->>-N: 200 OK else note over N: New device uploads device keys only: N->>+HS: POST /_matrix/client/v3/keys/upload HS->>-N: 200 OK end alt is backup present in m.login.secrets? note over N: New device connects to room-key backup end note over N: All done! deactivate N end end ``` #### Message reference These are the messages that are exchanged between the devices via the secure channel to negotiate the sign in and set up of E2EE. ##### `m.login.protocols` Sent by: existing device Purpose: to state the available protocols for signing in. At the moment only "`device_authorization_grant` is supported Fields: |Field|Type|| |--- |--- |--- | |`type`|required `string`|`m.login.protocols`| |`protocols`|required `string[]`|Array of: one of: `device_authorization_grant` | |`homeserver`|required `string`|The base URL of the homeserver| ```json { "type": "m.login.protocols", "protocols": ["device_authorization_grant"], "homeserver": "https://matrix-client.matrix.org" } ``` ##### `m.login.protocol` Sent by: new device Purpose: the new device sends this to indicate which protocol it intends to use Fields: |Field|Type|| |--- |--- |--- | |`type`|required `string`|`m.login.protocol`| |`protocol`|required `string`|One of: `device_authorization_grant`| |`device_authorization_grant`|Required `object` where `protocol` is `device_authorization_grant`|These values are taken from the RFC8628 Device Authorization Response that the new device received from the OIDC Provider:
Field Type
verification_uri required string
verification_uri_complete string
| |`device_id`|required `string`|The device ID that the new device will use| |`device_id_proof`|required `string`|New device's proof of identity key ownership, base64-encoded| Example: ```json { "type": "m.login.protocol", "protocol": "device_authorization_grant", "device_authorization_grant": { "verification_uri_complete": "https://id.matrix.org/device/abcde", "verification_uri": "..." }, "device_id": "3C5BFWi2Y8MaVvjM8M22DBmh24PmgR0nPvJOIArzgyI", "device_id_proof": "$base64_encoded_proof_of_identity_key_ownership" } ``` ##### `m.login.protocol_accepted` Sent by: existing device Purpose: Indicates that the existing device has accepted the protocol request and will open the `verification_uri` (or `verification_uri_complete`) for the user to grant consent Example: ```json { "type":"m.login.protocol_accepted" } ``` ##### `m.login.failure` Sent by: either device Purpose: used to indicate a failure Fields: |Field|Type|| |--- |--- |--- | |`type`|required `string`|`m.login.failure`| |`reason`|required `string`| One of:
Value Description
authorization_expired The Device Authorization Grant expired
device_already_exists The device ID specified by the new client already exists in the Homeserver provided device list
device_proof_failedThe proof of device key ownership failed
device_not_foundThe new device is not present in the device list as returned by the Homeserver
unexpected_message_receivedSent by either device to indicate that they received a message of a type that they weren't expecting
unsupported_protocolSent by a device where no suitable protocol is available or the requested protocol requested is not supported
user_cancelledSent by either new or existing device to indicate that the user has cancelled the login
| |`homeserver`|`string`| When the existing device is sending this it can include the Base URL of the homeserver so that the new device can at least save the user the hassle of typing it in| Example: ```json { "type":"m.login.failure", "reason": "unsupported", "homeserver": "https://matrix-client.matrix.org" } ``` ##### `m.login.declined` Sent by: existing device Purpose: Indicates that the user declined the request Fields: |Field|Type|| |--- |--- |--- | |`type`|required `string`|`m.login.declined`| Example: ```json { "type":"m.login.declined" } ``` ##### `m.login.success` Sent by: new device Purpose: to inform the existing device that it has successfully obtained an access token. Fields: |Field|Type|| |--- |--- |--- | |`type`|required `string`|`m.login.success`| Example: ```json { "type": "m.login.success" } ``` ##### `m.login.secrets` Sent by: existing device Purpose: Shares the secrets used for cross-signing and room key backups Fields: |Field|Type|| |--- |--- |--- | |`type`|required `string`|`m.login.secrets`| |`cross_signing`|`object`|
Field Type
master_key required string Unpadded base64 encoded private key
self_signing_key required string Unpadded base64 encoded private key
user_signing_key required string Unpadded base64 encoded private key
| |`backup`|`object`|
Field Type
algorithm required string One of the algorithms listed at https://spec.matrix.org/v1.9/client-server-api/#server-side-key-backups
key required string Unpadded base64 encoded private/secret key
backup_version required string The backup version as returned by [`POST /_matrix/client/v3/room_keys/version`](https://spec.matrix.org/v1.10/client-server-api/#post_matrixclientv3room_keysversion)
| Example: ```json { "type": "m.login.secrets", "cross_signing": { "master_key": "$base64_of_the_key", "self_signing_key": "$base64_of_the_key", "user_signing_key": "$base64_of_the_key" }, "backup": { "algorithm": "foobar", "key": "$base64_of_the_backup_recovery_key", "backup_version": "version_string" } } ``` ### QR code format The proposed format of the QR code intends to be similar to that which is already described in the Client-Server API for [device verification](https://spec.matrix.org/v1.9/client-server-api/#qr-code-format). Additional modes are added to the byte used for "QR code verification mode" to allow for the two login intents: initiate on a new device; reciprocate on an existing device; The QR codes to be displayed and scanned using this format will encode binary strings in the general form: - the ASCII string `MATRIX` - one byte indicating the QR code version (must be `0x02`) - one byte indicating the QR code intent/mode. Should be one of the following values: - `0x03` a new device wishing to initiate a login and self-verify - `0x04` an existing device wishing to reciprocate the login of a new device and self-verify that other device - the ephemeral Curve25519 public key, as 32 bytes - the rendezvous session URL encoded as: - two bytes in network byte order (big-endian) indicating the length in bytes of the rendezvous session URL as a UTF-8 string - the rendezvous session URL as a UTF-8 string - If the QR code intent/mode is `0x04` then the homeserver base URL encode as: - two bytes in network byte order (big-endian) indicating the length in bytes of the homeserver base URL as a UTF-8 string - the homeserver base URL as a UTF-8 string For example, if Alice displays a QR code encoding the following binary string: This indicates that Alice is a new device that wishes to initiate a login using her ephemeral public key of `0001020304050607...` (which is `AAECAwQFBg…` in base64), via the rendezvous session at URL `https:/…`. #### Example for QR code generated on new device A full example for a new device using ephemeral public key `2IZoarIZe3gOMAqdSiFHSAcA15KfOasxueUUNwJI7Ws` (base64 encoded) at rendezvous session `https://rendezvous.lab.element.dev/e8da6355-550b-4a32-a193-1619d9830668` is as follows: (Whitespace is for readability only) ``` 4D 41 54 52 49 58 02 03 d8 86 68 6a b2 19 7b 78 0e 30 0a 9d 4a 21 47 48 07 00 d7 92 9f 39 ab 31 b9 e5 14 37 02 48 ed 6b 00 47 68 74 74 70 73 3a 2f 2f 72 65 6e 64 65 7a 76 6f 75 73 2e 6c 61 62 2e 65 6c 65 6d 65 6e 74 2e 64 65 76 2f 65 38 64 61 36 33 35 35 2d 35 35 30 62 2d 34 61 33 32 2d 61 31 39 33 2d 31 36 31 39 64 39 38 33 30 36 36 38 ``` Which looks as follows as a QR with error correction level Q: ![Example QR for mode 0x03](images/4108-qr-mode03.png) #### Example for QR code generated on existing device A full example for an existing device using ephemeral public key `2IZoarIZe3gOMAqdSiFHSAcA15KfOasxueUUNwJI7Ws` (base64 encoded), at rendezvous session `https://rendezvous.lab.element.dev/e8da6355-550b-4a32-a193-1619d9830668` on homeserver `https://matrix-client.matrix.org` is as follows: (Whitespace is for readability only) ``` 4D 41 54 52 49 58 02 04 d8 86 68 6a b2 19 7b 78 0e 30 0a 9d 4a 21 47 48 07 00 d7 92 9f 39 ab 31 b9 e5 14 37 02 48 ed 6b 00 47 68 74 74 70 73 3a 2f 2f 72 65 6e 64 65 7a 76 6f 75 73 2e 6c 61 62 2e 65 6c 65 6d 65 6e 74 2e 64 65 76 2f 65 38 64 61 36 33 35 35 2d 35 35 30 62 2d 34 61 33 32 2d 61 31 39 33 2d 31 36 31 39 64 39 38 33 30 36 36 38 00 20 68 74 74 70 73 3a 2f 2f 6d 61 74 72 69 78 2d 63 6c 69 65 6e 74 2e 6d 61 74 72 69 78 2e 6f 72 67 ``` Which looks as follows as a QR with error correction level Q: ![Example QR for mode 0x04](images/4108-qr-mode04.png) ### Discoverability of the capability Before offering this capability it would make sense that the device can check the availability of the feature. Where the homeserver is known: 1. Check if the homeserver has a rendezvous session API available (/versions) from this MSC 1. Check that the homeserver is using the OIDC architecture (/auth_issuer) from MSC2965 1. Check that the Device Authorization Grant is available on the OIDC Provider from MSC2965 For a new device it would need to know the homeserver ahead of time in order to do these checks. Additionally the new device needs to either have an existing (i.e. static) OIDC client registered with the OIDC Provider already, or the OIDC Provider must support and allow dynamic client registration as described in [MSC2966](https://github.com/matrix-org/matrix-spec-proposals/pull/2966). ## Potential issues Because this is an entirely new set of functionality it should not cause issue with any existing Matrix functions or capabilities. The proposed protocol requires the devices to have IP connectivity to the server which might not be the case in P2P scenarios. ## Alternatives ### Alternative to the rendezvous session protocol #### Send-to-Device messaging If you squint then this proposal looks similar in some regards to the existing [Send-to-device messaging](https://spec.matrix.org/v1.9/client-server-api/#send-to-device-messaging) capability. Whilst to-device messaging already provides a mechanism for secure communication between two Matrix clients/devices, a key consideration for the anticipated login with QR capability is that one of the clients is not yet authenticated with a homeserver. Furthermore the client might not know which homeserver the user wishes to connect to. Conceptually, one could create a new type of "guest" login that would allow the unauthenticated client to connect to a homeserver for the purposes of communicating with an existing authenticated client via to-device messages. Some considerations for this: Where the "actual" homeserver is not known then the "guest" homeserver nominated by the new client would need to be federated with the "actual" homeserver. The "guest" homeserver would probably want to automatically clean up the "guest" accounts after a short period of time. The "actual" homeserver operator might not want to open up full "guest" access so a second type of "guest" account might be required. Does the new device/client need to accept the T&Cs of the "guest" homeserver? #### Other existing protocols One could try and do something with STUN or TURN or [COAP](https://datatracker.ietf.org/doc/html/rfc7252). #### Implementation details Rather than requiring the devices to poll for updates, "long-polling" could be used instead similar to `/sync`. Or WebSockets. ### Alternative method of secret sharing Instead of the existing device sharing the secrets bundle instead the existing device could cross-sign the new device and then use to-device messaging for sharing the secrets. For: - You re-use existing secret sharing Against: - The existing device needs to wait for the new device to upload the device keys for it to sign the new device. - Takes several round-trips for the secrets to be be shared which will add latency to the overall flow - The backup cannot be immediately enabled since we received the backup version as well, something the `m.secret.send` mechanism does not offer. - The new device cannot upload the cross-signing signature with the device keys in a single request. This introduces a chance of other devices seeing the new device as unverified, incorrectly prompting the user to verify the device that will soon be verified. ## Security considerations This proposed mechanism has been designed to protects users and their devices from the following threats: - A malicious actor who is able to scan the QR code generated by the legitimate user. - A malicious actor who can intercept and modify traffic on the application layer, even if protected by encryption like TLS. - Both of the above at the same time. Additionally, the OIDC Provider is able to define and enforce policies that can prevent a sign in on a new device. Such policies depend on the OIDC Provider in use and could include, but are not limited to, time of day, day of the week, source IP address and geolocation. A threat analysis has been done within each of the key layers in the proposal above. ## Unstable prefix While this feature is in development the new `POST` endpoint should be exposed using the following unstable prefix: - `/_matrix/client/unstable/org.matrix.msc4108/rendezvous` Additionally, the feature is to be advertised as unstable feature in the GET /_matrix/client/versions response, with the key org.matrix.msc4108 set to true. So, the response could look then as following: ```json { "versions": ["..."], "unstable_features": { "org.matrix.msc4108": true } } ``` ## Dependencies This MSC builds on [MSC3861](https://github.com/matrix-org/matrix-spec-proposals/pull/3861) (and its dependencies) which proposes the adoption of OIDC for authentication in Matrix.