GCM support
diff --git a/cryptography/exceptions.py b/cryptography/exceptions.py
index c2e7149..f2a731f 100644
--- a/cryptography/exceptions.py
+++ b/cryptography/exceptions.py
@@ -18,3 +18,7 @@
 
 class AlreadyFinalized(Exception):
     pass
+
+
+class NotFinalized(Exception):
+    pass