MSC2778: Providing authentication method for appservice users

proposals/2778-appservice-login.md

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+# MSC2778: Providing authentication method for appservice users
+
+Appservices within Matrix are increasingly attempting to support End-to-End Encryption. As such, they
+need a way to generate devices for their users so that they can participate in E2E rooms. In order to
+do so, this proposal suggests implementing an appservice extension to the 
+[`POST /login` endpoint](https://matrix.org/docs/spec/client_server/r0.6.0#post-matrix-client-r0-login).
+
+Appservice users do not usually need to login as they do not need their own access token, and do not
+traditionally need a "device". However E2E encryption demands that all users in a room have a device
+which means bridge users need to be able to generate a device on demand.
+
+## Proposal
+
+A new `type` is to be added to `POST /login`.
+
+`m.login.application_service`
+
+The `/login` endpoint may now take an `access_token` in the same way that other
+authenticated endpoints do. No additional parameters should be specified in the request body.
+
+Example request
+
+```json
+{
+  "type": "m.login.application_service",
+  "identifier": {
+    "type": "m.id.user",
+    "user": "alice"
+  }
+}
+```
+
+The response body should be unchanged from the existing `/login` specification.
+
+If:
+
+- The access token is not provided
+- The access token does not correspond to a appservice
+- The access token does not correspond to a appservice that manages this user
+- Or the user has not previously been registered
+
+Then the servers should reject with HTTP 403, with an `errcode` of `"M_FORBIDDEN"`. 
+
+Homeservers should ignore the `access_token` parameter if a type other than
+`m.login.application_service` has been provided.
+
+The expected flow for appservices would be to `/register` their users, and
+then `/login` to generate the appropriate device.
+
+## Potential issues
+
+This proposal means that there will be more calls to make when setting up a appservice user, when
+using encryption. While this could be done during the registration step, this would prohibit creating
+new devices should the appservice intentionally or inadvertently lost the client-side device data.
+
+## Alternatives
+
+One minor tweak to the current proposal could be to include the token as part of the auth data, rather than
+being part of the header/params to the request. An argument could be made for either, but since the specification
+expects the appservice to pass the token this way in all requests, including `/register`, it seems wise to keep
+it that way.
+
+Some community members have used implementation details such as a "shared secret" authentication method to
+log into the accounts without having to use the /login process at all. Synapse provides such a function,
+but also means the appservice can now authenticate as any user on the homeserver. This seems undesirable from a
+security standpoint.
+
+A third option could be to create a new endpoint that simply creates a new device for an appservice user on demand.
+Given the rest of the matrix eco-system does this with /login, and /login is already extensible with `type`, it would
+create more work for all parties involved for little benefit.
+
+## Security considerations
+
+The /login endpoint will generate an access token which can be used to control the appservice user, which
+is superflous as the appservice `as_token` should be used to authenticate all requests on behalf of ghosts.
+This can safely be ignored or used, but is an extra token hanging around.
+
+## Unstable prefix
+
+Implementations should use `uk.half-shot.msc2778.login.application_service` for `type` given in the
+`POST /login` until this lands in a released version of the specification.