diff --git a/changelog.d/6807.misc b/changelog.d/6807.misc
new file mode 100644
index 000000000..08aa80bcd
--- /dev/null
+++ b/changelog.d/6807.misc
@@ -0,0 +1 @@
+Refactoring work in preparation for changing the event redaction algorithm.
diff --git a/synapse/crypto/event_signing.py b/synapse/crypto/event_signing.py
index 1f2bccf70..5f733c1cf 100644
--- a/synapse/crypto/event_signing.py
+++ b/synapse/crypto/event_signing.py
@@ -1,6 +1,7 @@
 # -*- coding: utf-8 -*-
-
+#
 # Copyright 2014-2016 OpenMarket 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.
@@ -17,6 +18,7 @@
 import collections.abc
 import hashlib
 import logging
+from typing import Dict
 
 from canonicaljson import encode_canonical_json
 from signedjson.sign import sign_json
@@ -115,18 +117,28 @@ def compute_event_reference_hash(event, hash_algorithm=hashlib.sha256):
     return hashed.name, hashed.digest()
 
 
-def compute_event_signature(event_dict, signature_name, signing_key):
+def compute_event_signature(
+    room_version: RoomVersion,
+    event_dict: JsonDict,
+    signature_name: str,
+    signing_key: SigningKey,
+) -> Dict[str, Dict[str, str]]:
     """Compute the signature of the event for the given name and key.
 
     Args:
-        event_dict (dict): The event as a dict
-        signature_name (str): The name of the entity signing the event
+        room_version: the version of the room that this event is in.
+            (the room version determines the redaction algorithm and hence the
+            json to be signed)
+
+        event_dict: The event as a dict
+
+        signature_name: The name of the entity signing the event
             (typically the server's hostname).
-        signing_key (syutil.crypto.SigningKey): The key to sign with
+
+        signing_key: The key to sign with
 
     Returns:
-        dict[str, dict[str, str]]: Returns a dictionary in the same format of
-        an event's signatures field.
+        a dictionary in the same format of an event's signatures field.
     """
     redact_json = prune_event_dict(event_dict)
     redact_json.pop("age_ts", None)
@@ -161,5 +173,5 @@ def add_hashes_and_signatures(
     event_dict.setdefault("hashes", {})[name] = encode_base64(digest)
 
     event_dict["signatures"] = compute_event_signature(
-        event_dict, signature_name=signature_name, signing_key=signing_key
+        room_version, event_dict, signature_name=signature_name, signing_key=signing_key
     )
diff --git a/synapse/handlers/federation.py b/synapse/handlers/federation.py
index 0f10c3e9b..c86d3177e 100644
--- a/synapse/handlers/federation.py
+++ b/synapse/handlers/federation.py
@@ -1528,7 +1528,10 @@ class FederationHandler(BaseHandler):
 
         event.signatures.update(
             compute_event_signature(
-                event.get_pdu_json(), self.hs.hostname, self.hs.config.signing_key[0]
+                room_version,
+                event.get_pdu_json(),
+                self.hs.hostname,
+                self.hs.config.signing_key[0],
             )
         )