MSC3823: Account Suspension (#3823)

* MSC: A code for account suspension

Signed-off-by: David Teller <davidt@element.io>

* WIP: Adding prefix

* Update proposals/3823-code-for-account-suspension.md

Co-authored-by: Travis Ralston <travpc@gmail.com>

* Apply suggestions from code review

Co-authored-by: Dirk Klimpel <5740567+dklimpel@users.noreply.github.com>

* Update for latest requirements

* Add list of permitted/forbidden actions
* Remove `href` from error code - this will be handled in a later MSC
* Match modern template

* Clarify MSC

* Clarify differences in locking vs suspension, and future scope

---------

Signed-off-by: David Teller <davidt@element.io>
Co-authored-by: Travis Ralston <travpc@gmail.com>
Co-authored-by: Dirk Klimpel <5740567+dklimpel@users.noreply.github.com>
Co-authored-by: Travis Ralston <travisr@matrix.org>

proposals/3823-code-for-account-suspension.md

@@ -0,0 +1,80 @@
+# MSC3823: Account Suspension
+
+Unlike [account locking](https://spec.matrix.org/v1.12/client-server-api/#account-locking), suspension
+allows the user to have a (largely) readonly view of their account. Homeserver administrators and
+moderators may use this functionality to temporarily deactivate an account, or place conditions on
+the account's experience. Critically, like locking, account suspension is reversible, unlike the
+deactivation mechanism currently available in Matrix - a destructive, irreversible, action.
+
+This proposal introduces a concept of suspension to complement locking for server admins to use. Locking
+is typically more used in narrow scenarios, where the server admin wants to prevent the account from
+engaging any further. An example of this may be too many failed password attempts. Suspension is more
+general purpose to create a barrier to further action - a "something weird is going on -> suspend"
+kind of button.
+
+The error code introduced by this proposal is accompanied by guidelines on how servers can implement
+suspension. APIs to invoke or clear suspension are not introduced, and left as an implementation detail.
+These will typically be done through an administrator-only API.
+
+## Proposal
+
+When an account is suspended, any [Client-Server API](https://spec.matrix.org/v1.10/client-server-api/)
+endpoint MAY return a 403 HTTP status code with `errcode` of `M_USER_SUSPENDED`. This indicates to
+the user that the associated action is unavailable.
+
+Clients should note that for more general endpoints, like `/send/:eventType`, suspension MAY only be
+applied to a subset of request parameters. For example, a user may be allowed to *redact* events but
+not send messages.
+
+The specific list of permitted actions during suspension is left as a deliberate implementation
+detail, however a server SHOULD permit the user to:
+
+* Log in/create additional sessions (which should also behave as suspended).
+* See and receive messages, particularly via `/sync` and `/messages`.
+* [Verify their other devices](https://spec.matrix.org/v1.10/client-server-api/#device-verification)
+  and write associated [cross-signing data](https://spec.matrix.org/v1.10/client-server-api/#cross-signing).
+* [Populate their key backup](https://spec.matrix.org/v1.10/client-server-api/#server-side-key-backups).
+* Leave rooms & reject invites.
+* Redacting their own events.
+* Log out/delete any device of theirs, including the current session.
+* Deactivate their account, potentially with a deliberate time delay to discourage making a new
+  account right away.
+* Change or add [admin contacts](https://spec.matrix.org/v1.10/client-server-api/#adding-account-administrative-contact-information),
+  but not remove. Servers are recommended to only permit this if they keep a changelog on contact information
+  to prevent misuse.
+
+The recommendation for users to continue receiving/reading messages is largely so the administrator
+can maintain contact with the user, where applicable. Future MSCs may improve upon the admin<>user
+communication, and account locking may also be used to prevent access to messages.
+
+The suggested set of explicitly forbidden actions is:
+
+* Joining or knocking on rooms, including accepting invites.
+* Sending messages.
+* Sending invites.
+* Changing profile data (display name and avatar).
+* Redacting other users' events (when a moderator/admin of a room, for example).
+
+## Potential issues
+
+This proposal does not communicate *why* a user's account is restricted. The human-readable `error`
+field may contain some information, though anything comprehensive may not be surfaced to the user.
+A future MSC is expected to build a system for both informing the user of the action taken against
+their account and allow the user to appeal that action.
+
+## Alternatives
+
+No significant alternatives are plausible. `M_USER_DEACTIVATED` could be expanded with a `permanent`
+flag, though ideally each error code should provide meaning on its own.
+
+The related concept of locking, as discussed in places like [MSC3939](https://github.com/matrix-org/matrix-spec-proposals/pull/3939)
+and [matrix-org/glossary](https://github.com/matrix-org/glossary), is semantically different from
+suspension. Suspension has the key difference that the user is *not* logged out of their client,
+typically used by safety teams, while locking *does* log the user out and would primarily be used by
+security teams. A future MSC may combine or split the two related error codes into a descriptive set
+of capabilities the user can perform.
+
+## Unstable prefixes
+
+Until this proposal is considered stable, implementations must use
+`ORG.MATRIX.MSC3823.USER_SUSPENDED` instead of `M_USER_SUSPENDED`.