Simplify process_v2_response (#5236)

* Pass time_added_ms into process_v2_response

* Simplify process_v2_response

We can merge old_verify_keys into verify_keys, and reduce the number of dicts
flying around.

changelog.d/5236.misc

@@ -0,0 +1 @@
+Simplify Keyring.process_v2_response.

synapse/crypto/keyring.py

@@ -489,7 +489,7 @@ def get_server_verify_key_v2_indirect(
                 )
 
             processed_response = yield self.process_v2_response(
-                perspective_name, response
+                perspective_name, response, time_added_ms=time_now_ms
             )
             server_name = response["server_name"]
 
@@ -541,6 +541,7 @@ def get_server_verify_key_v2_direct(self, server_name, key_ids):
                 from_server=server_name,
                 requested_ids=[requested_key_id],
                 response_json=response,
+                time_added_ms=time_now_ms,
             )
             yield self.store.store_server_verify_keys(
                 server_name,
@@ -552,7 +553,9 @@ def get_server_verify_key_v2_direct(self, server_name, key_ids):
         defer.returnValue({server_name: keys})
 
     @defer.inlineCallbacks
-    def process_v2_response(self, from_server, response_json, requested_ids=[]):
+    def process_v2_response(
+        self, from_server, response_json, time_added_ms, requested_ids=[]
+    ):
         """Parse a 'Server Keys' structure from the result of a /key request
 
         This is used to parse either the entirety of the response from
@@ -573,6 +576,8 @@ def process_v2_response(self, from_server, response_json, requested_ids=[]):
 
             response_json (dict): the json-decoded Server Keys response object
 
+            time_added_ms (int): the timestamp to record in server_keys_json
+
             requested_ids (iterable[str]): a list of the key IDs that were requested.
                 We will store the json for these key ids as well as any that are
                 actually in the response
@@ -581,8 +586,9 @@ def process_v2_response(self, from_server, response_json, requested_ids=[]):
             Deferred[dict[str, nacl.signing.VerifyKey]]:
                 map from key_id to key object
         """
-        time_now_ms = self.clock.time_msec()
-        response_keys = {}
+
+        # start by extracting the keys from the response, since they may be required
+        # to validate the signature on the response.
         verify_keys = {}
         for key_id, key_data in response_json["verify_keys"].items():
             if is_signing_algorithm_supported(key_id):
@@ -591,36 +597,38 @@ def process_v2_response(self, from_server, response_json, requested_ids=[]):
                 verify_key = decode_verify_key_bytes(key_id, key_bytes)
                 verify_keys[key_id] = verify_key
 
-        old_verify_keys = {}
+        # TODO: improve this signature checking
+        server_name = response_json["server_name"]
+        for key_id in response_json["signatures"].get(server_name, {}):
+            if key_id not in verify_keys:
+                raise KeyLookupError(
+                    "Key response must include verification keys for all signatures"
+                )
+
+            verify_signed_json(
+                response_json, server_name, verify_keys[key_id]
+            )
+
         for key_id, key_data in response_json["old_verify_keys"].items():
             if is_signing_algorithm_supported(key_id):
                 key_base64 = key_data["key"]
                 key_bytes = decode_base64(key_base64)
                 verify_key = decode_verify_key_bytes(key_id, key_bytes)
-                old_verify_keys[key_id] = verify_key
-
-        server_name = response_json["server_name"]
-        for key_id in response_json["signatures"].get(server_name, {}):
-            if key_id not in response_json["verify_keys"]:
-                raise KeyLookupError(
-                    "Key response must include verification keys for all" " signatures"
-                )
-            if key_id in verify_keys:
-                verify_signed_json(response_json, server_name, verify_keys[key_id])
+                verify_keys[key_id] = verify_key
 
+        # re-sign the json with our own key, so that it is ready if we are asked to
+        # give it out as a notary server
         signed_key_json = sign_json(
             response_json, self.config.server_name, self.config.signing_key[0]
         )
 
         signed_key_json_bytes = encode_canonical_json(signed_key_json)
         ts_valid_until_ms = signed_key_json[u"valid_until_ts"]
 
+        # for reasons I don't quite understand, we store this json for the key ids we
+        # requested, as well as those we got.
         updated_key_ids = set(requested_ids)
         updated_key_ids.update(verify_keys)
-        updated_key_ids.update(old_verify_keys)
-
-        response_keys.update(verify_keys)
-        response_keys.update(old_verify_keys)
 
         yield logcontext.make_deferred_yieldable(
             defer.gatherResults(
@@ -630,7 +638,7 @@ def process_v2_response(self, from_server, response_json, requested_ids=[]):
                         server_name=server_name,
                         key_id=key_id,
                         from_server=from_server,
-                        ts_now_ms=time_now_ms,
+                        ts_now_ms=time_added_ms,
                         ts_expires_ms=ts_valid_until_ms,
                         key_json_bytes=signed_key_json_bytes,
                     )
@@ -640,7 +648,7 @@ def process_v2_response(self, from_server, response_json, requested_ids=[]):
             ).addErrback(unwrapFirstError)
         )
 
-        defer.returnValue(response_keys)
+        defer.returnValue(verify_keys)
 
 
 @defer.inlineCallbacks