matrix-spec-proposals/proposals/4108-oidc-qr-login.md

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, MSC3903 and MSC3886 which achieved a similar feature but did not work with a homeserver using the delegated OIDC mechanism proposed by MSC3861.

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. 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
• Expires - required, the expiry time of the rendezvous as per RFC7234
• Last-Modified - required, the last modified date of the payload as per RFC7232
• Cache-Control - required, no-store as per RFC7234
• Pragma - required, no-cache as per RFC7234

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 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 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 <rendezvous session URL>

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 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 <rendezvous session URL>

HTTP request headers:

• If-None-Match - optional, as per RFC7232 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 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 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 <rendezvous session URL>

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 headers, the ETag response header should also be allowed by exposing the following CORS header:

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 and response header safelists:

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.
2. If the client is not logged in and it is known which homeserver the user wants to connect to: try and use that homeserver.
3. Otherwise use a default server.

Example API usage

sequenceDiagram
  participant A as Device A
  participant HS as Homeserver
  participant R as Rendezvous Server<br>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<br>Content-Type: text/plain<br>"Hello from A"
  HS->>-A: 307 https://rz.example.com/foo
  A->>+R: POST /foo<br>Content-Type: text/plain<br>"Hello from A"
  R->>-A: 201 Created<br>ETag: 1<br>{"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<br>rendezvous session using the returned ETag
  activate A

  B->>+R: GET /abc-def-123-456
  R->>-B: 200 OK<br>ETag: 1<br>Content-Type: text/plain<br>"Hello from A"

  loop Device A polls the rendezvous session for a new payload
    A->>+R: GET /abc-def-123-456<br>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<br>If-Match: 1<br>Content-Type: text/plain<br>"Hello from B"
  R->>-B: 202 Accepted<br>ETag: 2

  Note over B: Device B starts polling for new payloads at the<br>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<br>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<br>ETag: 2<br>Content-Type: text/plain<br>"Hello from B"
  end
  deactivate A

  note over A: Device A sends a new payload
    A->>+R: PUT /abc-def-123-456<br>If-None-Match: 2<br>Content-Type: text/plain<br>"Hello again from A"
    R->>-A: 202 Accepted<br>ETag: 3

  note over B: Device B then receives the new payload
  opt modified
      R->>B: 200 OK<br>ETag: 3<br>Content-Type: text/plain<br>...
  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 instantiated with X25519, HKDF-SHA256 for the KDF and ChaCha20-Poly1305 (as specified by RFC8439) 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.

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:

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<br>Location: /_matrix/client/rendezvous/abc-def<br>ETag: 1

    note over G: 3) Device G generates and displays a QR code containing<br>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<br>ETag: 1

    note over S: 4) Device S computes SH, EncKey_S, EncKey_G and LoginInitiateMessage.<br>It sends LoginInitiateMessage via the rendezvous session
    S->>+Z: PUT /_matrix/client/rendezvous/abc-def<br>If-Match: 1<br>Body: LoginInitiateMessage
    Z->>-S: 202 Accepted<br>ETag: 2
    deactivate S

    G->>+Z: GET /_matrix/client/rendezvous/abc-def<br>If-None-Match: 1
    activate G
    Z->>-G: 200 OK<br>ETag: 2<br>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<br>If-Match: 2<br>Body: LoginOkMessage
    Z->>-G: 202 Accepted<br>ETag: 3
    deactivate G

    activate S
    S->>+Z: GET /_matrix/client/rendezvous/abc-def<br>If-None-Match: 2
    Z->>-S: 200 OK<br>ETag: 3<br>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

{
    "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:

New device => Homeserver via HTTP

GET /_matrix/client/v1/auth_issuer HTTP/1.1
Host: synapse-oidc.lab.element.dev
Accept: application/json

With response like:

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:

New device => OIDC Provider via HTTP

GET /.well-known/openid-configuration HTTP/1.1
Host: auth-oidc.lab.element.dev
Accept: application/json

With response like:

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 or uses a static OIDC client_id. We will use my_client_id as an example client_id.

• sends a RFC8628 Device Authorization Request to the OIDC Provider using the device_authorization_endpoint:

New device => OIDC Provider via 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:

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 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

{
    "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:

sequenceDiagram
    title: Variant: New device scanned QR code
    participant E as Existing device <br>already signed in
    participant Z as Rendezvous server
    participant N as New device <br>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],<br> "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],<br> "homeserver": "https://matrix-client.matrix.org"}
    #    note over N: If user enters the correct CheckCode and confirms checkmark<br>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<br> with the issuer (OIDC Provider). Completing dynamic registration if needed 
        N->>+OP: GET /.well-known/openid-configuration
        OP->>-N: 200 OK {..., "device_authorization_endpoint":<br> "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",<br>"verification_uri_complete": "https://id.matrix.org/device/abcde",<br>"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",<br> "device_authorization_grant":{<br>"verification_uri_complete": "https://id.matrix.org/device/abcde",<br>"verification_uri": ...,<br>"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<br>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",<br> "device_authorization_grant":{<br>"verification_uri_complete": "https://id.matrix.org/device/abcde",<br>"verification_uri": ...},<br>"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<br>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",<br>"homeserver": "https://matrix-client.matrix.org})
    end
    end

Existing device scanned QR code:

sequenceDiagram
    title: Variant: Existing device scanned QR code
    participant E as Existing device <br>already signed in
    participant Z as Rendezvous server
    participant N as New device <br>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],<br> "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],<br> "homeserver": "https://matrix-client.matrix.org"}
        note over N: If user enters the correct CheckCode and confirms checkmark<br>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<br> with the issuer (OIDC Provider). Completing dynamic registration if needed 
        N->>+OP: GET /.well-known/openid-configuration
        OP->>-N: 200 OK {..., "device_authorization_endpoint":<br> "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",<br>"verification_uri_complete": "https://id.matrix.org/device/abcde",<br>"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",<br> "device_authorization_grant":{<br>"verification_uri_complete": "https://id.matrix.org/device/abcde",<br>"verification_uri": ...},<br>"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<br>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",<br> "device_authorization_grant":{<br>"verification_uri_complete": "https://id.matrix.org/device/abcde",<br>"verification_uri": ...},<br>"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<br>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",<br>"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 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 using the interval and bounded by expires_in.

New device => OIDC Provider via 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 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/<device_id> 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

{
    "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:

sequenceDiagram
    participant E as Existing device <br>already signed in
    participant UA as Web Browser
    participant N as New device <br>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 <interval> seconds
                N->>OP: POST /token client_id=xyz<br>&grant_type=urn:ietf:params:oauth:grant-type:device_code<br>&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 <br>verification_uri_complete (with fallback to verification_uri)<br> in the system web browser/ASWebAuthenticationSession:
            Note over E: n.b. in the case of a Web Browser the user needs to have<br> 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: <html/> 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/<device_id> 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:

{
    "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:

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:

sequenceDiagram
    participant E as Existing device <br>already signed in
    participant N as New device <br>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

{
    "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:

{
    "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:

{
    "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_failed The proof of device key ownership failed device_not_found The new device is not present in the device list as returned by the Homeserver unexpected_message_received Sent by either device to indicate that they received a message of a type that they weren't expecting unsupported_protocol Sent by a device where no suitable protocol is available or the requested protocol requested is not supported user_cancelled Sent 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:

{
    "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:

{
    "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:

{
    "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

Example:

{
    "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.

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 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:

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
2. Check that the homeserver is using the OIDC architecture (/auth_issuer) from MSC2965
3. 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.

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 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.

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:

{
    "versions": ["..."],
    "unstable_features": {
        "org.matrix.msc4108": true
    }
}

Dependencies

This MSC builds on MSC3861 (and its dependencies) which proposes the adoption of OIDC for authentication in Matrix.