Add config option to sign remote key query responses with a separate key.

changelog.d/5895.feature

@@ -0,0 +1 @@
+Add config option to sign remote key query responses with a separate key.

docs/sample_config.yaml

@@ -1027,6 +1027,14 @@ signing_key_path: "CONFDIR/SERVERNAME.signing.key"
 #
 #trusted_key_servers:
 #  - server_name: "matrix.org"
+#
+
+# The signing keys to use when acting as a trusted key server. If not specified
+# defaults to the server signing key.
+#
+# Can contain multiple keys, one per line.
+#
+#key_server_signing_keys_path: "key_server_signing_keys.key"
 
 
 # Enable SAML2 for registration and login. Uses pysaml2.

synapse/config/key.py

@@ -76,7 +76,7 @@ def read_config(self, config, config_dir_path, **kwargs):
                     config_dir_path, config["server_name"] + ".signing.key"
                 )
 
-            self.signing_key = self.read_signing_key(signing_key_path)
+            self.signing_key = self.read_signing_keys(signing_key_path, "signing_key")
 
         self.old_signing_keys = self.read_old_signing_keys(
             config.get("old_signing_keys", {})
@@ -85,6 +85,14 @@ def read_config(self, config, config_dir_path, **kwargs):
             config.get("key_refresh_interval", "1d")
         )
 
+        key_server_signing_keys_path = config.get("key_server_signing_keys_path")
+        if key_server_signing_keys_path:
+            self.key_server_signing_keys = self.read_signing_keys(
+                key_server_signing_keys_path, "key_server_signing_keys_path"
+            )
+        else:
+            self.key_server_signing_keys = list(self.signing_key)
+
         # if neither trusted_key_servers nor perspectives are given, use the default.
         if "perspectives" not in config and "trusted_key_servers" not in config:
             key_servers = [{"server_name": "matrix.org"}]
@@ -210,16 +218,34 @@ def generate_config_section(
         #
         #trusted_key_servers:
         #  - server_name: "matrix.org"
+        #
+
+        # The signing keys to use when acting as a trusted key server. If not specified
+        # defaults to the server signing key.
+        #
+        # Can contain multiple keys, one per line.
+        #
+        #key_server_signing_keys_path: "key_server_signing_keys.key"
         """
             % locals()
         )
 
-    def read_signing_key(self, signing_key_path):
-        signing_keys = self.read_file(signing_key_path, "signing_key")
+    def read_signing_keys(self, signing_key_path, name):
+        """Read the signing keys in the given path.
+
+        Args:
+            signing_key_path (str)
+            name (str): Associated config key name
+
+        Returns:
+            list[SigningKey]
+        """
+
+        signing_keys = self.read_file(signing_key_path, name)
         try:
             return read_signing_keys(signing_keys.splitlines(True))
         except Exception as e:
-            raise ConfigError("Error reading signing_key: %s" % (str(e)))
+            raise ConfigError("Error reading %s: %s" % (name, str(e)))
 
     def read_old_signing_keys(self, old_signing_keys):
         keys = {}

synapse/crypto/keyring.py

@@ -30,7 +30,6 @@
 from signedjson.sign import (
     SignatureVerifyException,
     encode_canonical_json,
-    sign_json,
     signature_ids,
     verify_signed_json,
 )
@@ -540,13 +539,7 @@ def process_v2_response(self, from_server, response_json, time_added_ms):
                     verify_key=verify_key, valid_until_ts=key_data["expired_ts"]
                 )
 
-        # 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)
+        key_json_bytes = encode_canonical_json(response_json)
 
         yield make_deferred_yieldable(
             defer.gatherResults(
@@ -558,7 +551,7 @@ def process_v2_response(self, from_server, response_json, time_added_ms):
                         from_server=from_server,
                         ts_now_ms=time_added_ms,
                         ts_expires_ms=ts_valid_until_ms,
-                        key_json_bytes=signed_key_json_bytes,
+                        key_json_bytes=key_json_bytes,
                     )
                     for key_id in verify_keys
                 ],

synapse/rest/key/v2/remote_key_resource.py

@@ -13,7 +13,9 @@
 # limitations under the License.
 
 import logging
-from io import BytesIO
+
+from canonicaljson import encode_canonical_json, json
+from signedjson.sign import sign_json
 
 from twisted.internet import defer
 
@@ -95,6 +97,7 @@ def __init__(self, hs):
         self.store = hs.get_datastore()
         self.clock = hs.get_clock()
         self.federation_domain_whitelist = hs.config.federation_domain_whitelist
+        self.config = hs.config
 
     @wrap_json_request_handler
     async def _async_render_GET(self, request):
@@ -214,15 +217,14 @@ def query_keys(self, request, query, query_remote_on_cache_miss=False):
             yield self.fetcher.get_keys(cache_misses)
             yield self.query_keys(request, query, query_remote_on_cache_miss=False)
         else:
-            result_io = BytesIO()
-            result_io.write(b'{"server_keys":')
-            sep = b"["
-            for json_bytes in json_results:
-                result_io.write(sep)
-                result_io.write(json_bytes)
-                sep = b","
-            if sep == b"[":
-                result_io.write(sep)
-            result_io.write(b"]}")
-
-            respond_with_json_bytes(request, 200, result_io.getvalue())
+            signed_keys = []
+            for key_json in json_results:
+                key_json = json.loads(key_json)
+                for signing_key in self.config.key_server_signing_keys:
+                    key_json = sign_json(key_json, self.config.server_name, signing_key)
+
+                signed_keys.append(key_json)
+
+            results = {"server_keys": signed_keys}
+
+            respond_with_json_bytes(request, 200, encode_canonical_json(results))