Ensure that server_keys fetched via a notary server are correctly signed.

synapse/crypto/keyring.py

@@ -394,8 +394,7 @@ def process_v2_response(
         POST /_matrix/key/v2/query.
 
         Checks that each signature in the response that claims to come from the origin
-        server is valid. (Does not check that there actually is such a signature, for
-        some reason.)
+        server is valid, and that there is at least one such signature.
 
         Stores the json in server_keys_json so that it can be used for future responses
         to /_matrix/key/v2/query.
@@ -430,16 +429,25 @@ def process_v2_response(
                     verify_key=verify_key, valid_until_ts=ts_valid_until_ms
                 )
 
-        # TODO: improve this signature checking
         server_name = response_json["server_name"]
+        verified = False
         for key_id in response_json["signatures"].get(server_name, {}):
-            if key_id not in verify_keys:
+            # each of the keys used for the signature must be present in the response
+            # json.
+            key = verify_keys.get(key_id)
+            if not key:
                 raise KeyLookupError(
-                    "Key response must include verification keys for all signatures"
+                    "Key response is signed by key id %s:%s but that key is not "
+                    "present in the response" % (server_name, key_id)
                 )
 
-            verify_signed_json(
-                response_json, server_name, verify_keys[key_id].verify_key
+            verify_signed_json(response_json, server_name, key.verify_key)
+            verified = True
+
+        if not verified:
+            raise KeyLookupError(
+                "Key response for %s is not signed by the origin server"
+                % (server_name,)
             )
 
         for key_id, key_data in response_json["old_verify_keys"].items():
@@ -677,12 +685,6 @@ def get_server_verify_key_v2_direct(self, server_name, key_ids):
             except HttpResponseException as e:
                 raise_from(KeyLookupError("Remote server returned an error"), e)
 
-            if (
-                u"signatures" not in response
-                or server_name not in response[u"signatures"]
-            ):
-                raise KeyLookupError("Key response not signed by remote server")
-
             if response["server_name"] != server_name:
                 raise KeyLookupError(
                     "Expected a response for server %r not %r"

tests/crypto/test_keyring.py

@@ -55,11 +55,11 @@ def get_signed_key(self, server_name, verify_key):
                 key_id: {"key": signedjson.key.encode_verify_key_base64(verify_key)}
             },
         }
-        return self.get_signed_response(res)
+        self.sign_response(res)
+        return res
 
-    def get_signed_response(self, res):
+    def sign_response(self, res):
         signedjson.sign.sign_json(res, self.server_name, self.key)
-        return res
 
 
 class KeyringTestCase(unittest.HomeserverTestCase):
@@ -238,7 +238,7 @@ def test_get_keys_from_server(self):
         testkey = signedjson.key.generate_signing_key("ver1")
         testverifykey = signedjson.key.get_verify_key(testkey)
         testverifykey_id = "ed25519:ver1"
-        VALID_UNTIL_TS = 1000
+        VALID_UNTIL_TS = 200 * 1000
 
         # valid response
         response = {
@@ -326,9 +326,10 @@ def test_get_keys_from_perspectives(self):
             },
         }
 
-        persp_resp = {
-            "server_keys": [self.mock_perspective_server.get_signed_response(response)]
-        }
+        # the response must be signed by both the origin server and the perspectives
+        # server.
+        signedjson.sign.sign_json(response, SERVER_NAME, testkey)
+        self.mock_perspective_server.sign_response(response)
 
         def post_json(destination, path, data, **kwargs):
             self.assertEqual(destination, self.mock_perspective_server.server_name)
@@ -337,7 +338,7 @@ def post_json(destination, path, data, **kwargs):
             # check that the request is for the expected key
             q = data["server_keys"]
             self.assertEqual(list(q[SERVER_NAME].keys()), ["key1"])
-            return persp_resp
+            return {"server_keys": [response]}
 
         self.http_client.post_json.side_effect = post_json
 
@@ -365,9 +366,74 @@ def post_json(destination, path, data, **kwargs):
 
         self.assertEqual(
             bytes(res["key_json"]),
-            canonicaljson.encode_canonical_json(persp_resp["server_keys"][0]),
+            canonicaljson.encode_canonical_json(response),
         )
 
+    def test_invalid_perspectives_responses(self):
+        """Check that invalid responses from the perspectives server are rejected"""
+        # arbitrarily advance the clock a bit
+        self.reactor.advance(100)
+
+        SERVER_NAME = "server2"
+        testkey = signedjson.key.generate_signing_key("ver1")
+        testverifykey = signedjson.key.get_verify_key(testkey)
+        testverifykey_id = "ed25519:ver1"
+        VALID_UNTIL_TS = 200 * 1000
+
+        def build_response():
+            # valid response
+            response = {
+                "server_name": SERVER_NAME,
+                "old_verify_keys": {},
+                "valid_until_ts": VALID_UNTIL_TS,
+                "verify_keys": {
+                    testverifykey_id: {
+                        "key": signedjson.key.encode_verify_key_base64(testverifykey)
+                    }
+                },
+            }
+
+            # the response must be signed by both the origin server and the perspectives
+            # server.
+            signedjson.sign.sign_json(response, SERVER_NAME, testkey)
+            self.mock_perspective_server.sign_response(response)
+            return response
+
+        def get_key_from_perspectives(response):
+            fetcher = PerspectivesKeyFetcher(self.hs)
+            server_name_and_key_ids = [(SERVER_NAME, ("key1",))]
+
+            def post_json(destination, path, data, **kwargs):
+                self.assertEqual(destination, self.mock_perspective_server.server_name)
+                self.assertEqual(path, "/_matrix/key/v2/query")
+                return {"server_keys": [response]}
+
+            self.http_client.post_json.side_effect = post_json
+
+            return self.get_success(
+                fetcher.get_keys(server_name_and_key_ids)
+            )
+
+        # start with a valid response so we can check we are testing the right thing
+        response = build_response()
+        keys = get_key_from_perspectives(response)
+        k = keys[SERVER_NAME][testverifykey_id]
+        self.assertEqual(k.verify_key, testverifykey)
+
+        # remove the perspectives server's signature
+        response = build_response()
+        del response["signatures"][self.mock_perspective_server.server_name]
+        self.http_client.post_json.return_value = {"server_keys": [response]}
+        keys = get_key_from_perspectives(response)
+        self.assertEqual(keys, {}, "Expected empty dict with missing persp server sig")
+
+        # remove the origin server's signature
+        response = build_response()
+        del response["signatures"][SERVER_NAME]
+        self.http_client.post_json.return_value = {"server_keys": [response]}
+        keys = get_key_from_perspectives(response)
+        self.assertEqual(keys, {}, "Expected empty dict with missing origin server sig")
+
 
 @defer.inlineCallbacks
 def run_in_context(f, *args, **kwargs):