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matrix-spec/content/rooms/v9.md

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---
title: Room Version 9
type: docs
weight: 60
---
This room version builds on [version 8](/rooms/v8) to add additional redaction
rules that were unintentionally missed when incorporating v8.
## Client considerations
See [room version 8](/rooms/v8) for specific details regarding the addition of
restricted rooms.
Clients which implement the redaction algorithm locally should refer to the
[redactions](#redactions) section below for a full overview.
### Redactions
{{% added-in this=true %}} `m.room.member` now keep `join_authorised_via_users_server`
in addition to other keys in `content` when being redacted.
{{% boxes/rationale %}}
Without the `join_authorised_via_users_server` property, redacted join events
can become invalid when verifying the auth chain of a given event, thus creating
a split-brain scenario where the user is able to speak from one server's
perspective but most others will continually reject their events.
This can theoretically be worked around with a rejoin to the room, being careful
not to use the faulty events as `prev_events`, though instead it is encouraged
to use v9 rooms over v8 rooms to outright avoid the situation.
[Issue #3373](https://github.com/matrix-org/matrix-doc/issues/3373) has further
information.
{{% /boxes/rationale %}}
The full redaction algorithm follows.
{{% rver-fragment name="v3-handling-redactions" %}}
Upon receipt of a redaction event, the server must strip off any keys
not in the following list:
- `event_id`
- `type`
- `room_id`
- `sender`
- `state_key`
- `content`
- `hashes`
- `signatures`
- `depth`
- `prev_events`
- `prev_state`
- `auth_events`
- `origin`
- `origin_server_ts`
- `membership`
The content object must also be stripped of all keys, unless it is one
of one of the following event types:
- `m.room.member` allows keys `membership`, `join_authorised_via_users_server`.
- `m.room.create` allows key `creator`.
- `m.room.join_rules` allows keys `join_rule`, `allow`.
- `m.room.power_levels` allows keys `ban`, `events`, `events_default`,
`kick`, `redact`, `state_default`, `users`, `users_default`.
- `m.room.history_visibility` allows key `history_visibility`.
## Server implementation components
{{% boxes/warning %}}
The information contained in this section is strictly for server
implementors. Applications which use the Client-Server API are generally
unaffected by the intricacies contained here. The section above
regarding client considerations is the resource that Client-Server API
use cases should reference.
{{% /boxes/warning %}}
Room version 8 added a new `restricted` join rule to allow members of a room
to join another room without invite. Room version 9 is based upon v8 with the
following considerations.
### Redactions
[See above](#redactions).
## Unchanged from v8
The following sections have not been modified since v8, but are included for
completeness.
### Handling redactions
{{% rver-fragment name="v3-handling-redactions" %}}
### Event IDs
{{% rver-fragment name="v4-event-ids" %}}
### Event format
{{% rver-fragment name="v4-event-format" %}}
### Authorization rules
{{% rver-fragment name="v8-auth-rules" %}}
### State resolution
{{% rver-fragment name="v2-state-res" %}}
### Canonical JSON
{{% rver-fragment name="v6-canonical-json" %}}
### Signing key validity period
{{% rver-fragment name="v5-signing-requirements" %}}