Specify MSC3882: Using an existing session to log in another

changelogs/client_server/newsfragments/1530.feature

@@ -0,0 +1 @@
+Add an ability to use an existing session to log in another, as per [MSC3882](https://github.com/matrix-org/matrix-spec-proposals/pull/3882).

changelogs/client_server/newsfragments/1530.new

@@ -0,0 +1 @@
+[`POST /_matrix/client/v3/login/get_token`](/client-server-api/#post_matrixclientv3loginget_token).

content/client-server-api/_index.md

@@ -1176,6 +1176,9 @@ client supports it, the client should redirect the user to the
 is complete, the client will need to submit a `/login` request matching
 `m.login.token`.
 
+{{< added-in v="1.7" >}} Already-authenticated clients can additionally generate
+a token for their user ID if supported by the homeserver using `POST /login/get_token`.
+
 {{% http-api spec="client-server" api="login" %}}
 
 {{% http-api spec="client-server" api="refresh" %}}

data/api/client-server/login.yaml

@@ -42,7 +42,8 @@ paths:
           examples:
             application/json: {
               "flows": [
-                {"type": "m.login.password"}
+                {"type": "m.login.password"},
+                {"type": "m.login.token", "get_login_token": true}
               ]
             }
           schema:
@@ -54,12 +55,22 @@ paths:
                 items:
                   type: object
                   title: LoginFlow
+                  required: ['type']
                   properties:
                     type:
                       description: |-
                         The login type. This is supplied as the `type` when
                         logging in.
                       type: string
+                    get_login_token:
+                      type: boolean
+                      x-addedInMatrixVersion: "1.7"
+                      description: |-
+                        If `type` is `m.login.token`, an optional field to indicate
+                        to the unauthenticated client that the homeserver supports
+                        the `POST /login/get_token` endpoint. Note that supporting
+                        the endpoint does not necessarily indicate that the user
+                        attempting to log in will be able to generate such a token.
         429:
           description: This request was rate-limited.
           schema:
@@ -240,3 +251,87 @@ paths:
             "$ref": "definitions/errors/rate_limited.yaml"
       tags:
         - Session management
+  "/login/get_token":
+    post:
+      summary: Optional endpoint to generate a single-use, time-limited, `m.login.token` token.
+      description: |-
+        Optional endpoint - the server is not required to implement this endpoint if it does not
+        intend to use or support this functionality.
+
+        This API endpoint uses the [User-Interactive Authentication API](/client-server-api/#user-interactive-authentication-api).
+
+        An already-authenticated client would call this endpoint to generate a single-use, time-limited,
+        token for an unauthenticated client to log in with, becoming logged in as the same user which
+        called this endpoint. The unauthenticated client uses the generated token in a `m.login.token`
+        login flow with the homeserver.
+
+        Clients, both authenticated and unauthenticated, might wish to hide user interface which exposes
+        this feature if the server is not offering it. Authenticated clients can check for support on
+        a per-user basis with the `m.get_login_token` [capability](/client-server-api/#capabilities-negotiation),
+        while unauthenticated clients can detect server support by looking for an `m.login.token` login
+        flow with `get_login_token: true` on [`GET /login`](/client-server-api/#post_matrixclientv3login).
+
+        In v1.7 of the specification, transmission of the generated token to an unauthenticated client is
+        left as an implementation detail. Future MSCs such as [MSC3906](https://github.com/matrix-org/matrix-spec-proposals/pull/3906)
+        might standarise a way to transmit the token between clients.
+
+        The generated token MUST only be valid for a single login, enforced by the server. Clients which
+        intend to log in multiple devices must generate a token for each.
+
+        With other User-Interactive Authentication (UIA)-supporting endpoints, servers sometimes do not re-prompt
+        for verification if the session recently passed UIA. For this endpoint, servers should always re-prompt
+        the user for verification to ensure explicit consent is gained for each additional client.
+
+        Servers are encouraged to apply stricter than normal rate limiting to this endpoint, such as maximum
+        of 1 request per minute.
+      operationId: generateLoginToken
+      x-addedInMatrixVersion: "1.7"
+      security:
+        - accessToken: []
+      parameters:
+        - in: body
+          name: body
+          required: true
+          schema:
+            type: object
+            description: An empty JSON object.
+      responses:
+        200:
+          description: The login token an unauthenticated client can use to log in as the requesting user.
+          examples:
+            application/json: {
+              "login_token": "<opaque string>",
+              "expires_in_ms": 120000
+            }
+          schema:
+            type: object
+            required: ["login_token", "expires_in_ms"]
+            properties:
+              login_token:
+                type: string
+                description: The login token for the `m.login.token` login flow.
+              expires_in_ms:
+                type: integer
+                description: |-
+                  The time remaining in milliseconds until the homeserver will no longer accept the token. `120000`
+                  (2 minutes) is recommended as a default.
+        400:
+          description: |-
+            The request was malformed, or the user does not have an ability to generate tokens for their devices,
+            as implied by the [User-Interactive Authentication API](/client-server-api/#user-interactive-authentication-api).
+
+            Clients should verify whether the user has an ability to call this endpoint with the `m.get_login_token`
+            [capability](/client-server-api/#capabilities-negotiation).
+          schema:
+            "$ref": "definitions/errors/error.yaml"
+        401:
+          description: |-
+            The homeserver requires additional authentication information.
+          schema:
+            "$ref": "definitions/auth_response.yaml"
+        429:
+          description: This request was rate-limited.
+          schema:
+            "$ref": "definitions/errors/rate_limited.yaml"
+      tags:
+        - Session management