Implement a ZMQ prefix-match countermeasure in EHBase

lib/tfw/event_handler_base.py

@@ -31,10 +31,25 @@ class EventHandlerBase(ABC):
         a response back in case the handler returned something.
         This is subscribed in __init__().
         """
+        if not self.check_key(message):
+            return
+
         response = self.dispatch_handling(message)
         if response:
             self.server_connector.send(response)
 
+    def check_key(self, message):
+        """
+        Checks whether the message is intended for this
+        EventHandler.
+
+        This is necessary because ZMQ handles PUB - SUB
+        connetions with pattern matching (e.g. someone
+        subscribed to 'fsm' will receive 'fsm_update'
+        messages as well.
+        """
+        return self.key == message['key']
+
     def dispatch_handling(self, message):
         """
         Used to dispatch messages to their specific handlers.