Implement MSC3821 to update redaction rules (`third_party_invite.signed`) (#15563)

Updates the redaction rules to protect enough information that the
event can still be properly verified.

2 files changed, 50 insertions, 0 deletions

diff --git a/synapse/api/room_versions.py b/synapse/api/room_versions.py
index e65b9a0287..7030b133d3 100644
--- a/synapse/api/room_versions.py
+++ b/synapse/api/room_versions.py
@@ -103,6 +103,8 @@ class RoomVersion:
     msc3787_knock_restricted_join_rule: bool
     # MSC3667: Enforce integer power levels
     msc3667_int_only_power_levels: bool
+    # MSC3821: Do not redact the third_party_invite content field for membership events.
+    msc3821_redaction_rules: bool
     # MSC3931: Adds a push rule condition for "room version feature flags", making
     # some push rules room version dependent. Note that adding a flag to this list
     # is not enough to mark it "supported": the push rule evaluator also needs to
@@ -133,6 +135,7 @@ class RoomVersions:
         msc3389_relation_redactions=False,
         msc3787_knock_restricted_join_rule=False,
         msc3667_int_only_power_levels=False,
+        msc3821_redaction_rules=False,
         msc3931_push_features=(),
         msc3989_redaction_rules=False,
     )
@@ -155,6 +158,7 @@ class RoomVersions:
         msc3389_relation_redactions=False,
         msc3787_knock_restricted_join_rule=False,
         msc3667_int_only_power_levels=False,
+        msc3821_redaction_rules=False,
         msc3931_push_features=(),
         msc3989_redaction_rules=False,
     )
@@ -177,6 +181,7 @@ class RoomVersions:
         msc3389_relation_redactions=False,
         msc3787_knock_restricted_join_rule=False,
         msc3667_int_only_power_levels=False,
+        msc3821_redaction_rules=False,
         msc3931_push_features=(),
         msc3989_redaction_rules=False,
     )
@@ -199,6 +204,7 @@ class RoomVersions:
         msc3389_relation_redactions=False,
         msc3787_knock_restricted_join_rule=False,
         msc3667_int_only_power_levels=False,
+        msc3821_redaction_rules=False,
         msc3931_push_features=(),
         msc3989_redaction_rules=False,
     )
@@ -221,6 +227,7 @@ class RoomVersions:
         msc3389_relation_redactions=False,
         msc3787_knock_restricted_join_rule=False,
         msc3667_int_only_power_levels=False,
+        msc3821_redaction_rules=False,
         msc3931_push_features=(),
         msc3989_redaction_rules=False,
     )
@@ -243,6 +250,7 @@ class RoomVersions:
         msc3389_relation_redactions=False,
         msc3787_knock_restricted_join_rule=False,
         msc3667_int_only_power_levels=False,
+        msc3821_redaction_rules=False,
         msc3931_push_features=(),
         msc3989_redaction_rules=False,
     )
@@ -265,6 +273,7 @@ class RoomVersions:
         msc3389_relation_redactions=False,
         msc3787_knock_restricted_join_rule=False,
         msc3667_int_only_power_levels=False,
+        msc3821_redaction_rules=False,
         msc3931_push_features=(),
         msc3989_redaction_rules=False,
     )
@@ -287,6 +296,7 @@ class RoomVersions:
         msc3389_relation_redactions=False,
         msc3787_knock_restricted_join_rule=False,
         msc3667_int_only_power_levels=False,
+        msc3821_redaction_rules=False,
         msc3931_push_features=(),
         msc3989_redaction_rules=False,
     )
@@ -309,6 +319,7 @@ class RoomVersions:
         msc3389_relation_redactions=False,
         msc3787_knock_restricted_join_rule=False,
         msc3667_int_only_power_levels=False,
+        msc3821_redaction_rules=False,
         msc3931_push_features=(),
         msc3989_redaction_rules=False,
     )
@@ -331,6 +342,7 @@ class RoomVersions:
         msc3389_relation_redactions=False,
         msc3787_knock_restricted_join_rule=False,
         msc3667_int_only_power_levels=False,
+        msc3821_redaction_rules=False,
         msc3931_push_features=(),
         msc3989_redaction_rules=False,
     )
@@ -353,6 +365,30 @@ class RoomVersions:
         msc3389_relation_redactions=False,
         msc3787_knock_restricted_join_rule=True,
         msc3667_int_only_power_levels=False,
+        msc3821_redaction_rules=False,
+        msc3931_push_features=(),
+        msc3989_redaction_rules=False,
+    )
+    MSC3821 = RoomVersion(
+        "org.matrix.msc3821.opt1",
+        RoomDisposition.UNSTABLE,
+        EventFormatVersions.ROOM_V4_PLUS,
+        StateResolutionVersions.V2,
+        enforce_key_validity=True,
+        special_case_aliases_auth=False,
+        strict_canonicaljson=True,
+        limit_notifications_power_levels=True,
+        msc2175_implicit_room_creator=False,
+        msc2176_redaction_rules=False,
+        msc3083_join_rules=True,
+        msc3375_redaction_rules=True,
+        msc2403_knocking=True,
+        msc2716_historical=False,
+        msc2716_redactions=False,
+        msc3389_relation_redactions=False,
+        msc3787_knock_restricted_join_rule=False,
+        msc3667_int_only_power_levels=False,
+        msc3821_redaction_rules=True,
         msc3931_push_features=(),
         msc3989_redaction_rules=False,
     )
@@ -375,6 +411,7 @@ class RoomVersions:
         msc3389_relation_redactions=False,
         msc3787_knock_restricted_join_rule=True,
         msc3667_int_only_power_levels=True,
+        msc3821_redaction_rules=False,
         msc3931_push_features=(),
         msc3989_redaction_rules=False,
     )
@@ -397,6 +434,7 @@ class RoomVersions:
         msc3389_relation_redactions=False,
         msc3787_knock_restricted_join_rule=False,
         msc3667_int_only_power_levels=False,
+        msc3821_redaction_rules=False,
         msc3931_push_features=(),
         msc3989_redaction_rules=False,
     )
@@ -420,6 +458,7 @@ class RoomVersions:
         msc3389_relation_redactions=False,
         msc3787_knock_restricted_join_rule=True,
         msc3667_int_only_power_levels=True,
+        msc3821_redaction_rules=False,
         msc3931_push_features=(PushRuleRoomFlag.EXTENSIBLE_EVENTS,),
         msc3989_redaction_rules=False,
     )
@@ -442,6 +481,7 @@ class RoomVersions:
         msc3389_relation_redactions=False,
         msc3787_knock_restricted_join_rule=True,
         msc3667_int_only_power_levels=True,
+        msc3821_redaction_rules=False,
         msc3931_push_features=(),
         msc3989_redaction_rules=True,
     )
diff --git a/synapse/events/utils.py b/synapse/events/utils.py
index e540f1582a..e6d040176b 100644
--- a/synapse/events/utils.py
+++ b/synapse/events/utils.py
@@ -130,6 +130,16 @@ def prune_event_dict(room_version: RoomVersion, event_dict: JsonDict) -> JsonDic
         add_fields("membership")
         if room_version.msc3375_redaction_rules:
             add_fields(EventContentFields.AUTHORISING_USER)
+        if room_version.msc3821_redaction_rules:
+            # Preserve the signed field under third_party_invite.
+            third_party_invite = event_dict["content"].get("third_party_invite")
+            if isinstance(third_party_invite, collections.abc.Mapping):
+                new_content["third_party_invite"] = {}
+                if "signed" in third_party_invite:
+                    new_content["third_party_invite"]["signed"] = third_party_invite[
+                        "signed"
+                    ]
+
     elif event_type == EventTypes.Create:
         # MSC2176 rules state that create events cannot be redacted.
         if room_version.msc2176_redaction_rules: