MSC 1915 - Add unbind 3PID APIs

Note that this is a simplified version of MSC1194.

Motivation

Currently we do not have a reasonable route for a user to unbind/remove a 3PID from their account, particularly when deactivating their account. Users have an expectation to be able to do this, and thus we should have an API to provide it.

This is meant as a simple extension to the current APIs, and so this explicitly does not try and solve any existing usability concerns.

New APIs

Client-Server API

Add POST /_matrix/client/r0/account/3pid/delete API, which expects a JSON body with medium, address and id_server fields (as per existing APIs).

The id_server parameter is optional and if missing the server will attempt to unbind from a suitable identity server (e.g. its default identity server or the server used when originally binding the 3pid).

The 200 response is a JSON object with an id_server_unbind_result field whose value is either success or no-support, where the latter indicates that the identity server (IS) does not support unbinding 3PIDs directly.

Example:

POST /_matrix/client/r0/account/3pid/delete HTTP/1.1

{
    "medium": "email",
    "address": "foobar@example.com",
    "id_server": "https://matrix.org
}

HTTP/1.1 200 OK
{
    "id_server_unbind_result": "success"
}

Identity Server API

Add POST /_matrix/identity/api/v1/unbind with mxid and threepid fields. The mxid is the user's user_id and threepid is a dict with the usual medium and address fields.

If the server returns a 400, 404 or 501 HTTP error code then the homeserver should assume that the identity server doesn't support the /unbind API, unless it returns a specific matrix error response (i.e. the body is a JSON object with error and errcode fields).

The identity server should authenticate the request in one of two ways:

The request is signed by the homeserver which controls the user_id.
The request includes the sid and client_server params (as per /bind), which proves ownership of the given 3PID.

Example:

POST /_matrix/identity/api/v1/unbind HTTP/1.1

{
    "mxid": "@foobar:example.com",
    "threepid": {
        "medium": "email",
        "address": "foobar@example.com"
    }
}

HTTP/1.1 200 OK

{}

Trade-offs

A homeserver can unbind any 3PID associated with one of its users, and specifically does not require a re-validation of control of the 3PID. This means that users have to trust that their homeserver will not arbitrarily remove valid 3PIDs, however users must already trust their homeserver to a large extent. The flip side is that this provides a mechanism for homeservers and users to remove 3PIDs directed at their user IDs that they no longer (or never did) have control over.

Removing a 3PID does not require user interactive auth (UIA), which opens a potential attack whereby a logged in device can remove all associated 3PIDs and then log out all devices. If the user has forgotten their password they would no longer be able to reset their password via a 3PID (e.g. email), resulting in losing access to their account. However, given that clients and servers have implemented these APIs in the wild this is considered a sufficient edge case that adding UIA is unlikely to be worthwhile.

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