diff --git a/src/xycrypto/padding.py b/src/xycrypto/padding.py
index 9e00411..ef4df06 100644
--- a/src/xycrypto/padding.py
+++ b/src/xycrypto/padding.py
@@ -6,6 +6,8 @@
 
 
 class PadderContext(metaclass=abc.ABCMeta):
+    """Abstract base class for padder context."""
+
     def __init__(self, block_size):
         """Initialize the current context."""
 
@@ -31,6 +33,8 @@ def _pad(padded_size):
 
 
 class UnpadderContext(metaclass=abc.ABCMeta):
+    """Abstract base class for unpadder context."""
+
     def __init__(self, block_size):
         """Initialize the current context."""