MSC4326: Device masquerading for appservices (#4326)

Co-authored-by: Tulir Asokan <tulir@maunium.net>

proposals/4326-appservice-device-masquerading.md

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+# MSC4326: Device masquerading for appservices
+
+*History*: This proposal is split off from [MSC3202: Encrypted Appservices](https://github.com/matrix-org/matrix-spec-proposals/pull/3202).
+
+Appservices today can make requests as any (local) user in their namespace through [identity assertion](https://spec.matrix.org/v1.15/application-service-api/#identity-assertion).
+To support end-to-end encryption and other similar device-centric functionality, appservices need to
+be able to also pick the device ID they are speaking as.
+
+This proposal adds device ID to the identity assertion appservices can already perform, leaving other
+aspects of end-to-end encryption support to other MSCs like MSC3202 (mentioned above).
+
+
+## Proposal
+
+To complement the (optional) `user_id` query string parameter during identity assertion, an also-optional
+`device_id` parameter is also supported. The new `device_id` parameter is only available when `user_id`
+is available to the caller - when authenticating using an `as_token`.
+
+When both a `user_id` and `device_id` are provided, and both are known/registered, the server uses those
+details for the remainder of the request. For many endpoints this means updating the "last seen IP"
+and "last seen timestamp" for the device, though for some endpoints it may mean interacting with the
+device specifically (such as when uploading one-time keys).
+
+If the `device_id` does not already exist on the `user_id`, the server returns a `400 M_UNKNOWN_DEVICE`
+standard error response.
+
+If the `device_id` is present without a `user_id`, the `user_id` is assumed to be the appservice's
+default sender (the user implied by `sender_localpart` in its registration). This is the same behaviour
+as today when the appservice makes such requests.
+
+If the `device_id` is present and the requester is not able to use identity assertion, the request
+continues as though the `device_id` parameter was never present. This copies the behaviour of `user_id`.
+
+### Examples
+
+*All examples assume the `user_id` is within the appservice's scope.*
+
+User ID asserted, but not device ID:
+
+```text
+GET /_matrix/client/v3/account/whoami?user_id=@alice:example.org
+Authorization: Bearer as_token_here
+
+{
+  "user_id": "@alice:example.org",
+  "is_guest": false
+}
+```
+
+User ID and device ID asserted:
+
+```text
+GET /_matrix/client/v3/account/whoami?user_id=@alice:example.org&device_id=ABC123
+Authorization: Bearer as_token_here
+
+{
+  "user_id": "@alice:example.org",
+  "is_guest": false,
+  "device_id": "ABC123"
+}
+```
+
+Just device ID asserted:
+
+```text
+GET /_matrix/client/v3/account/whoami?device_id=ABC123
+Authorization: Bearer as_token_here
+
+{
+  "user_id": "@the_appservice_sender:example.org",
+  "is_guest": false,
+  "device_id": "ABC123"
+}
+```
+
+Nothing asserted:
+
+```text
+GET /_matrix/client/v3/account/whoami
+Authorization: Bearer as_token_here
+
+{
+  "user_id": "@the_appservice_sender:example.org",
+  "is_guest": false
+}
+```
+
+## Potential issues
+
+Appservices will need to create and manage their users' devices using another proposal or system. An
+example is [MSC4190](https://github.com/matrix-org/matrix-spec-proposals/pull/4190).
+
+
+## Alternatives
+
+None relevant.
+
+
+## Security considerations
+
+The behaviour of `device_id` is largely copied from `user_id`, so should not increase or decrease an
+appservice's capabilities beyond what it could already do. This is especially true for appservices
+which cover "real" users in their namespaces: while they couldn't (and still can't) access data encrypted
+before using something like [MSC3202](https://github.com/matrix-org/matrix-spec-proposals/pull/3202),
+they could log out whatever devices they don't want and register new ones accordingly.
+
+
+## Unstable prefix
+
+For historical reasons, unstable implementations of this proposal should use `org.matrix.msc3202.device_id`
+instead of `device_id`.
+
+`ORG.MATRIX.MSC4326.M_UNKNOWN_DEVICE` is used as the error code instead of `M_UNKNOWN_DEVICE`.
+
+## Dependencies
+
+None relevant. Some MSCs depend on this MSC's functionality, however.