Add config option to hide device names over federation (#9945)

Now that cross signing exists there is much less of a need for other people to look at devices and verify them individually. This PR adds a config option to allow you to prevent device display names from being shared with other servers.

Signed-off-by: Aaron Raimist <aaron@raim.ist>

changelog.d/9945.feature

@@ -0,0 +1 @@
+Add a config option to allow you to prevent device display names from being shared over federation. Contributed by @aaronraimist.

docs/sample_config.yaml

@@ -741,6 +741,12 @@ acme:
 #
 #allow_profile_lookup_over_federation: false
 
+# Uncomment to disable device display name lookup over federation. By default, the
+# Federation API allows other homeservers to obtain device display names of any user
+# on this homeserver. Defaults to 'true'.
+#
+#allow_device_name_lookup_over_federation: false
+
 
 ## Caching ##
 

synapse/config/federation.py

@@ -44,6 +44,10 @@ class FederationConfig(Config):
             "allow_profile_lookup_over_federation", True
         )
 
+        self.allow_device_name_lookup_over_federation = config.get(
+            "allow_device_name_lookup_over_federation", True
+        )
+
     def generate_config_section(self, config_dir_path, server_name, **kwargs):
         return """\
         ## Federation ##
@@ -75,6 +79,12 @@ class FederationConfig(Config):
         # on this homeserver. Defaults to 'true'.
         #
         #allow_profile_lookup_over_federation: false
+
+        # Uncomment to disable device display name lookup over federation. By default, the
+        # Federation API allows other homeservers to obtain device display names of any user
+        # on this homeserver. Defaults to 'true'.
+        #
+        #allow_device_name_lookup_over_federation: false
         """
 
 

synapse/storage/databases/main/end_to_end_keys.py

@@ -84,7 +84,9 @@ class EndToEndKeyWorkerStore(EndToEndKeyBackgroundStore):
                 if keys:
                     result["keys"] = keys
 
-                device_display_name = device.display_name
+                device_display_name = None
+                if self.hs.config.allow_device_name_lookup_over_federation:
+                    device_display_name = device.display_name
                 if device_display_name:
                     result["device_display_name"] = device_display_name