Add ECB class + docs + tests
* Slightly refactors test_nist to allow fetching of data that has no IV
* Does not modify create_block_cipher_context (next commit)
diff --git a/docs/primitives/symmetric-encryption.rst b/docs/primitives/symmetric-encryption.rst
index 1b8d1d7..8a9bbbd 100644
--- a/docs/primitives/symmetric-encryption.rst
+++ b/docs/primitives/symmetric-encryption.rst
@@ -67,3 +67,15 @@
``block_size`` of the cipher. Do not
reuse an ``initialization_vector`` with
a given ``key``.
+
+
+Insecure Modes
+--------------
+
+.. class:: cryptography.primitives.block.modes.ECB()
+
+ ECB (Electronic Code Book) is the simplest mode of operation for block
+ ciphers. The data is separated into blocks and each block is encrypted
+ separately. This means identical plaintext blocks will always result in
+ identical encrypted blocks. Due to this property it is not recommended
+ for use. Really, don't use it. Just. Don't.