Add option to enable encryption by default for new rooms (#7639)

Fixes https://github.com/matrix-org/synapse/issues/2431

Adds config option `encryption_enabled_by_default_for_room_type`, which determines whether encryption should be enabled with the default encryption algorithm in private or public rooms upon creation. Whether the room is private or public is decided based upon the room creation preset that is used.

Part of this PR is also pulling out all of the individual instances of `m.megolm.v1.aes-sha2` into a constant variable to eliminate typos ala https://github.com/matrix-org/synapse/pull/7637

Based on #7637

2 files changed, 57 insertions, 29 deletions

diff --git a/synapse/handlers/federation.py b/synapse/handlers/federation.py
index 3e60774b33..b30f41dc4b 100644
--- a/synapse/handlers/federation.py
+++ b/synapse/handlers/federation.py
@@ -33,7 +33,12 @@ from unpaddedbase64 import decode_base64
 from twisted.internet import defer
 
 from synapse import event_auth
-from synapse.api.constants import EventTypes, Membership, RejectedReason
+from synapse.api.constants import (
+    EventTypes,
+    Membership,
+    RejectedReason,
+    RoomEncryptionAlgorithms,
+)
 from synapse.api.errors import (
     AuthError,
     CodeMessageException,
@@ -742,7 +747,10 @@ class FederationHandler(BaseHandler):
                 if device:
                     keys = device.get("keys", {}).get("keys", {})
 
-                    if event.content.get("algorithm") == "m.megolm.v1.aes-sha2":
+                    if (
+                        event.content.get("algorithm")
+                        == RoomEncryptionAlgorithms.MEGOLM_V1_AES_SHA2
+                    ):
                         # For this algorithm we expect a curve25519 key.
                         key_name = "curve25519:%s" % (device_id,)
                         current_keys = [keys.get(key_name)]
diff --git a/synapse/handlers/room.py b/synapse/handlers/room.py
index 61db3ccc43..46c2739143 100644
--- a/synapse/handlers/room.py
+++ b/synapse/handlers/room.py
@@ -26,7 +26,12 @@ from typing import Tuple
 
 from six import iteritems, string_types
 
-from synapse.api.constants import EventTypes, JoinRules, RoomCreationPreset
+from synapse.api.constants import (
+    EventTypes,
+    JoinRules,
+    RoomCreationPreset,
+    RoomEncryptionAlgorithms,
+)
 from synapse.api.errors import AuthError, Codes, NotFoundError, StoreError, SynapseError
 from synapse.api.room_versions import KNOWN_ROOM_VERSIONS, RoomVersion
 from synapse.events.utils import copy_power_levels_contents
@@ -56,31 +61,6 @@ FIVE_MINUTES_IN_MS = 5 * 60 * 1000
 
 
 class RoomCreationHandler(BaseHandler):
-
-    PRESETS_DICT = {
-        RoomCreationPreset.PRIVATE_CHAT: {
-            "join_rules": JoinRules.INVITE,
-            "history_visibility": "shared",
-            "original_invitees_have_ops": False,
-            "guest_can_join": True,
-            "power_level_content_override": {"invite": 0},
-        },
-        RoomCreationPreset.TRUSTED_PRIVATE_CHAT: {
-            "join_rules": JoinRules.INVITE,
-            "history_visibility": "shared",
-            "original_invitees_have_ops": True,
-            "guest_can_join": True,
-            "power_level_content_override": {"invite": 0},
-        },
-        RoomCreationPreset.PUBLIC_CHAT: {
-            "join_rules": JoinRules.PUBLIC,
-            "history_visibility": "shared",
-            "original_invitees_have_ops": False,
-            "guest_can_join": False,
-            "power_level_content_override": {},
-        },
-    }
-
     def __init__(self, hs):
         super(RoomCreationHandler, self).__init__(hs)
 
@@ -89,6 +69,39 @@ class RoomCreationHandler(BaseHandler):
         self.room_member_handler = hs.get_room_member_handler()
         self.config = hs.config
 
+        # Room state based off defined presets
+        self._presets_dict = {
+            RoomCreationPreset.PRIVATE_CHAT: {
+                "join_rules": JoinRules.INVITE,
+                "history_visibility": "shared",
+                "original_invitees_have_ops": False,
+                "guest_can_join": True,
+                "power_level_content_override": {"invite": 0},
+            },
+            RoomCreationPreset.TRUSTED_PRIVATE_CHAT: {
+                "join_rules": JoinRules.INVITE,
+                "history_visibility": "shared",
+                "original_invitees_have_ops": True,
+                "guest_can_join": True,
+                "power_level_content_override": {"invite": 0},
+            },
+            RoomCreationPreset.PUBLIC_CHAT: {
+                "join_rules": JoinRules.PUBLIC,
+                "history_visibility": "shared",
+                "original_invitees_have_ops": False,
+                "guest_can_join": False,
+                "power_level_content_override": {},
+            },
+        }
+
+        # Modify presets to selectively enable encryption by default per homeserver config
+        for preset_name, preset_config in self._presets_dict.items():
+            encrypted = (
+                preset_name
+                in self.config.encryption_enabled_by_default_for_room_presets
+            )
+            preset_config["encrypted"] = encrypted
+
         self._replication = hs.get_replication_data_handler()
 
         # linearizer to stop two upgrades happening at once
@@ -798,7 +811,7 @@ class RoomCreationHandler(BaseHandler):
             )
             return last_stream_id
 
-        config = RoomCreationHandler.PRESETS_DICT[preset_config]
+        config = self._presets_dict[preset_config]
 
         creator_id = creator.user.to_string()
 
@@ -888,6 +901,13 @@ class RoomCreationHandler(BaseHandler):
                 etype=etype, state_key=state_key, content=content
             )
 
+        if config["encrypted"]:
+            last_sent_stream_id = await send(
+                etype=EventTypes.RoomEncryption,
+                state_key="",
+                content={"algorithm": RoomEncryptionAlgorithms.DEFAULT},
+            )
+
         return last_sent_stream_id
 
     async def _generate_room_id(