Merge branch 'master' into arc4-support

* master:
  Typo
  Be really explicit about what's good and bad
  Mention return types.
  Consistently use e.g.
  Module documentation.
  Single space.
  Add a new Mode interface to document mode.name and start on some prose docs for interfaces.
  Proper name for the iv thing.
  Actually note the properties for cipher modes types on their ABCs.

Conflicts:
	docs/hazmat/primitives/symmetric-encryption.rst

1 files changed, 26 insertions, 3 deletions

diff --git a/docs/hazmat/primitives/symmetric-encryption.rst b/docs/hazmat/primitives/symmetric-encryption.rst
index 4d0703bb..9d18ce50 100644
--- a/docs/hazmat/primitives/symmetric-encryption.rst
+++ b/docs/hazmat/primitives/symmetric-encryption.rst
@@ -159,6 +159,9 @@ Weak Ciphers
                       ``192``, or ``256`` bits in length.  This must be kept
                       secret.
 
+
+.. _symmetric-encryption-modes:
+
 Modes
 ~~~~~
 
@@ -173,9 +176,29 @@ Modes
                                         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. Do not
-                                        reuse an ``initialization_vector`` with
-                                        a given ``key``.
+                                        ``block_size`` of the cipher. Each time
+                                        something is encrypted a new
+                                        ``initialization_vector`` should be
+                                        generated. Do not reuse an
+                                        ``initialization_vector`` with
+                                        a given ``key``, and particularly do
+                                        not use a constant
+                                        ``initialization_vector``.
+
+    A good construction looks like:
+
+    .. code-block:: pycon
+
+        >>> import os
+        >>> iv = os.urandom(16)
+        >>> mode = CBC(iv)
+
+    While the following is bad and will leak information:
+
+    .. code-block:: pycon
+
+        >>> iv = "a" * 16
+        >>> mode = CBC(iv)
 
 
 .. class:: CTR(nonce)