AES Counter support
* vectors from RFC 3686
* Documentation for the mode
diff --git a/docs/primitives/symmetric-encryption.rst b/docs/primitives/symmetric-encryption.rst
index c4bbf0a..52bd6a4 100644
--- a/docs/primitives/symmetric-encryption.rst
+++ b/docs/primitives/symmetric-encryption.rst
@@ -77,6 +77,19 @@
reuse an ``initialization_vector`` with
a given ``key``.
+.. class:: cryptography.primitives.block.modes.CTR(nonce)
+
+ CTR (Counter) is a mode of operation for block ciphers. It is considered
+ cryptographically strong.
+
+ :param bytes nonce: Must be random bytes. They do not need to be kept
+ secret (they can be included in a transmitted
+ message). Must be the same number of bytes as the
+ ``block_size`` of the cipher. It is critical to
+ never reuse a ``nonce`` with a given ``key``. Unlike
+ CBC, reusing a nonce compromises the security of
+ all data encrypted under the key (see: two time pad).
+
.. class:: cryptography.primitives.block.modes.OFB(initialization_vector)
OFB (Output Feedback) is a mode of operation for block ciphers. It