Implement signed event handler

tfw/internals/event_handling/__init__.py

@@ -1,3 +1,4 @@
-from .event_handler_factory_base import EventHandlerFactoryBase
 from .event_handler import EventHandler
+from .event_handler_factory_base import EventHandlerFactoryBase
 from .fsm_aware_event_handler import FSMAwareEventHandler
+from .signed_event_handler import SignedEventHandler

tfw/internals/event_handling/signed_event_handler.py

@@ -0,0 +1,20 @@
+import logging
+
+from tfw.internals.crypto import KeyManager, verify_message
+
+from .event_handler import EventHandler
+
+LOG = logging.getLogger(__name__)
+
+
+# pylint: disable=abstract-method
+class SignedEventHandler(EventHandler):
+    def __init__(self, connector):
+        self._auth_key = KeyManager().auth_key
+        super().__init__(connector)
+
+    def _event_callback(self, message):
+        if verify_message(self._auth_key, message):
+            self.handle_event(message, self.connector)
+        else:
+            LOG.error('Message does not have valid signature: %s', message)