Begin designing the KDF interfaces. Fixes #511
diff --git a/docs/exceptions.rst b/docs/exceptions.rst
index 1fbd326..f9e29f3 100644
--- a/docs/exceptions.rst
+++ b/docs/exceptions.rst
@@ -30,3 +30,9 @@
 
     This is raised when a backend doesn't support the requested algorithm (or
     combination of algorithms).
+
+
+.. class:: InvalidKey
+
+    This is raised when the verify method of a key derivation function does not
+    compare equal.