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.

1 files changed, 74 insertions, 1 deletions

diff --git a/tests/events/test_utils.py b/tests/events/test_utils.py
index 02f0800a31..e40eac2eb0 100644
--- a/tests/events/test_utils.py
+++ b/tests/events/test_utils.py
@@ -394,7 +394,7 @@ class PruneEventTestCase(stdlib_unittest.TestCase):
         )
 
     def test_member(self) -> None:
-        """Member events have changed behavior starting with MSC3375."""
+        """Member events have changed behavior in MSC3375 and MSC3821."""
         self.run_test(
             {
                 "type": "m.room.member",
@@ -437,6 +437,79 @@ class PruneEventTestCase(stdlib_unittest.TestCase):
             room_version=RoomVersions.V9,
         )
 
+        # After MSC3821, the signed key under third_party_invite is protected
+        # from redaction.
+        THIRD_PARTY_INVITE = {
+            "display_name": "alice",
+            "signed": {
+                "mxid": "@alice:example.org",
+                "signatures": {
+                    "magic.forest": {
+                        "ed25519:3": "fQpGIW1Snz+pwLZu6sTy2aHy/DYWWTspTJRPyNp0PKkymfIsNffysMl6ObMMFdIJhk6g6pwlIqZ54rxo8SLmAg"
+                    }
+                },
+                "token": "abc123",
+            },
+        }
+
+        self.run_test(
+            {
+                "type": "m.room.member",
+                "content": {
+                    "membership": "invite",
+                    "third_party_invite": THIRD_PARTY_INVITE,
+                    "other_key": "stripped",
+                },
+            },
+            {
+                "type": "m.room.member",
+                "content": {
+                    "membership": "invite",
+                    "third_party_invite": {"signed": THIRD_PARTY_INVITE["signed"]},
+                },
+                "signatures": {},
+                "unsigned": {},
+            },
+            room_version=RoomVersions.MSC3821,
+        )
+
+        # Ensure this doesn't break if an invalid field is sent.
+        self.run_test(
+            {
+                "type": "m.room.member",
+                "content": {
+                    "membership": "invite",
+                    "third_party_invite": {},
+                    "other_key": "stripped",
+                },
+            },
+            {
+                "type": "m.room.member",
+                "content": {"membership": "invite", "third_party_invite": {}},
+                "signatures": {},
+                "unsigned": {},
+            },
+            room_version=RoomVersions.MSC3821,
+        )
+
+        self.run_test(
+            {
+                "type": "m.room.member",
+                "content": {
+                    "membership": "invite",
+                    "third_party_invite": "stripped",
+                    "other_key": "stripped",
+                },
+            },
+            {
+                "type": "m.room.member",
+                "content": {"membership": "invite"},
+                "signatures": {},
+                "unsigned": {},
+            },
+            room_version=RoomVersions.MSC3821,
+        )
+
     def test_relations(self) -> None:
         """Event relations get redacted until MSC3389."""
         # Normally the m._relates_to field is redacted.