Try to document CBC
diff --git a/docs/primitives/symmetric-encryption.rst b/docs/primitives/symmetric-encryption.rst
index 750ab88..4675ebf 100644
--- a/docs/primitives/symmetric-encryption.rst
+++ b/docs/primitives/symmetric-encryption.rst
@@ -39,10 +39,17 @@
choice for encryption.
:param bytes key: The secret key, either ``128``, ``192``, or ``256`` bits.
+ This must be kept secret.
Modes
~~~~~
-CBC
-+++
+.. class:: cryptographically.primitives.CBC(initialization_vector)
+
+ CBC (Cipher block chaining) is a mode of operation for block ciphers. It is
+ considered cryptographically strong.
+
+ :param bytes initialization_vector: Must be random bytes. They do not need
+ to be kept secret (they can be included
+ in a transmitted message).