Move redaction algorithm into room version specification

We stick it in a client section of v1 as the earliest version to define the algorithm is v1, and the client-server spec tells clients to use this algorithm.

specification/client_server_api.rst

@@ -1779,39 +1779,7 @@ redacted include a ``redacted_because`` key whose value is the event that caused
 it to be redacted, which may include a reason.
 
 
-Upon receipt of a redaction event, the server should 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``
-
-.. Note:
-   Some of the keys, such as ``hashes``, will appear on the federation-formatted
-   event and therefore the client may not be aware of them.
-
-The content object should also be stripped of all keys, unless it is one of
-one of the following event types:
-
-- ``m.room.member`` allows key ``membership``.
-- ``m.room.create`` allows key ``creator``.
-- ``m.room.join_rules`` allows key ``join_rule``.
-- ``m.room.power_levels`` allows keys ``ban``, ``events``, ``events_default``,
-  ``kick``, ``redact``, ``state_default``, ``users``, ``users_default``.
-- ``m.room.aliases`` allows key ``aliases``.
-- ``m.room.history_visibility`` allows key ``history_visibility``.
+The exact algorithm to apply against an event is defined in the `room version specification`_.
 
 The server should add the event causing the redaction to the ``unsigned``
 property of the redacted event, under the ``redacted_because`` key. When a

specification/rooms/v1.rst

@@ -1,4 +1,5 @@
 .. Copyright 2017,2019 New Vector Ltd
+.. Copyright 2020 The Matrix.org Foundation C.I.C.
 ..
 .. Licensed under the Apache License, Version 2.0 (the "License");
 .. you may not use this file except in compliance with the License.
@@ -21,13 +22,57 @@ blocks for other room versions.
 .. contents:: Table of Contents
 .. sectnum::
 
+Client considerations
+---------------------
+
+Clients may need to consider some algorithms performed by the server for their own
+implementation.
+
+Redactions
+~~~~~~~~~~
+
+Upon receipt of a redaction event, the server should 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``
+
+.. Note:
+   Some of the keys, such as ``hashes``, will appear on the federation-formatted
+   event and therefore the client may not be aware of them.
+
+The content object should also be stripped of all keys, unless it is one of
+one of the following event types:
+
+- ``m.room.member`` allows key ``membership``.
+- ``m.room.create`` allows key ``creator``.
+- ``m.room.join_rules`` allows key ``join_rule``.
+- ``m.room.power_levels`` allows keys ``ban``, ``events``, ``events_default``,
+  ``kick``, ``redact``, ``state_default``, ``users``, ``users_default``.
+- ``m.room.aliases`` allows key ``aliases``.
+- ``m.room.history_visibility`` allows key ``history_visibility``.
+
 Server implementation components
 --------------------------------
 
 .. WARNING::
    The information contained in this section is strictly for server implementors.
    Applications which use the Client-Server API are generally unaffected by the
-   details contained here, and can safely ignore their presence.
+   intricacies contained here. The section above regarding client considerations
+   is the resource that Client-Server API use cases should reference.
 
 
 The algorithms defined here should only apply to version 1 rooms. Other algorithms
@@ -112,7 +157,7 @@ The types of state events that affect authorization are:
 .. NOTE::
 
   Power levels are inferred from defaults when not explicitly supplied.
-  For example, mentions of the ``sender``'s power level can also refer 
+  For example, mentions of the ``sender``'s power level can also refer
   to the default power level for users in the room.
 
 The rules are as follows:
@@ -250,7 +295,7 @@ The rules are as follows:
    #. If there is no previous ``m.room.power_levels`` event in the room, allow.
 
    #. For the keys ``users_default``, ``events_default``,
-      ``state_default``, ``ban``, ``redact``, ``kick``, ``invite`` check if they 
+      ``state_default``, ``ban``, ``redact``, ``kick``, ``invite`` check if they
       were added, changed or removed. For each found alteration:
 
       i. If the current value is higher than the ``sender``'s current power level,
@@ -258,13 +303,13 @@ The rules are as follows:
 
       #. If the new value is higher than the ``sender``'s current power level,
          reject.
-         
-   #. For each entry being added, changed or removed in both the ``events`` and 
+
+   #. For each entry being added, changed or removed in both the ``events`` and
       ``users`` keys:
-      
+
       i. If the current value is higher than the ``sender``'s current power level,
          reject.
-         
+
       #. If the new value is higher than the ``sender``'s current power level,
          reject.