Add registration token UIA type (#3616)

* Add registration token UIA type

MSC: #3231

**Note**: This introduces the endpoint as v1 rather than r0 given the global versioning changes landed between the acceptance of the MSC and now.

* Fix swagger

* Changelogs

* Update data/api/client-server/registration_tokens.yaml

Co-authored-by: Hubert Chathi <hubert@uhoreg.ca>

Co-authored-by: Hubert Chathi <hubert@uhoreg.ca>

changelogs/client_server/newsfragments/3616.feature

@@ -0,0 +1 @@
+Add token-authenticated registration support as per [MSC3231](https://github.com/matrix-org/matrix-doc/pull/3231).

changelogs/client_server/newsfragments/3616.new

@@ -0,0 +1 @@
+Add `/register/m.login.registration_token/validity` as per [MSC3231](https://github.com/matrix-org/matrix-doc/pull/3231).

content/client-server-api/_index.md

@@ -618,6 +618,7 @@ This specification defines the following auth types:
 -   `m.login.email.identity`
 -   `m.login.msisdn`
 -   `m.login.dummy`
+-   `m.login.registration_token`
 
 ##### Password-based
 
@@ -789,6 +790,49 @@ just the type and session, if provided:
 }
 ```
 
+##### Token-authenticated registration
+
+{{% added-in v="1.2" %}}
+
+| Type                          | Description                                                       |
+|-------------------------------|-------------------------------------------------------------------|
+| `m.login.registration_token`  | Registers an account with a pre-shared token for authentication   |
+
+{{% boxes/note %}}
+The `m.login.registration_token` authentication type is only valid on the
+[`/register`](#post_matrixclientv3register) endpoint.
+{{% /boxes/note %}}
+
+This authentication type provides homeservers the ability to allow registrations
+to a limited set of people instead of either offering completely open registrations
+or completely closed registration (where the homeserver administrators create
+and distribute accounts).
+
+The token required for this authentication type is shared out of band from
+Matrix and is an opaque string with maximum length of 64 characters in the
+range `[A-Za-z0-9._~-]`. The server can keep any number of tokens for any
+length of time/validity. Such cases might be a token limited to 100 uses or
+for the next 2 hours - after the tokens expire, they can no longer be used
+to create accounts.
+
+To use this authentication type, clients should submit an auth dict with just
+the type, token, and session:
+
+```json
+{
+  "type": "m.login.registration_token",
+  "token": "fBVFdqVE",
+  "session": "<session ID>"
+}
+```
+
+To determine if a token is valid before attempting to use it, the client can
+use the `/validity` API defined below. The API doesn't guarantee that a token
+will be valid when used, but does avoid cases where the user finds out late
+in the registration process that their token has expired.
+
+{{% http-api spec="client-server" api="registration_tokens" %}}
+
 #### Fallback
 
 Clients cannot be expected to be able to know how to process every

data/api/client-server/registration_tokens.yaml

@@ -0,0 +1,80 @@
+# Copyright 2022 The Matrix.org Foundation C.I.C.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+swagger: '2.0'
+info:
+  title: "Matrix Client-Server Registration Token API"
+  version: "1.0.0"
+host: localhost:8008
+schemes:
+  - https
+  - http
+basePath: /_matrix/client/v1
+consumes:
+  - application/json
+produces:
+  - application/json
+paths:
+  "/register/m.login.registration_token/validity":
+    get:
+      x-addedInMatrixVersion: "1.2"
+      summary: Query if a given registration token is still valid.
+      description: |-
+        Queries the server to determine if a given registration token is still
+        valid at the time of request. This is a point-in-time check where the
+        token might still expire by the time it is used.
+
+        Servers should be sure to rate limit this endpoint to avoid brute force
+        attacks.
+      operationId: registrationTokenValidity
+      parameters:
+        - in: query
+          name: token
+          type: string
+          x-example: "fBVFdqVE"
+          required: true
+          description: The token to check validity of.
+      responses:
+        200:
+          description: The check has a result.
+          examples:
+            application/json: {
+              "valid": true
+            }
+          schema:
+            type: object
+            properties:
+              valid:
+                type: boolean
+                description: |-
+                  True if the token is still valid, false otherwise. This should
+                  additionally be false if the token is not a recognised token by
+                  the server.
+            required: ['valid']
+        403:
+          description: |-
+            The homeserver does not permit registration and thus all tokens are
+            considered invalid.
+          examples:
+            application/json: {
+                  "errcode": "M_FORBIDDEN",
+                  "error": "Registration is not enabled on this homeserver."
+              }
+          schema:
+            "$ref": "definitions/errors/error.yaml"
+        429:
+          description: This request was rate-limited.
+          schema:
+            "$ref": "definitions/errors/rate_limited.yaml"
+      tags:
+        - Account management