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

3 files changed, 86 insertions, 3 deletions

diff --git a/docs/hazmat/primitives/index.rst b/docs/hazmat/primitives/index.rst
index c81018ae..614c414a 100644
--- a/docs/hazmat/primitives/index.rst
+++ b/docs/hazmat/primitives/index.rst
@@ -10,3 +10,4 @@ Primitives
     hmac
     symmetric-encryption
     padding
+    interfaces
diff --git a/docs/hazmat/primitives/interfaces.rst b/docs/hazmat/primitives/interfaces.rst
new file mode 100644
index 00000000..7068316e
--- /dev/null
+++ b/docs/hazmat/primitives/interfaces.rst
@@ -0,0 +1,59 @@
+.. hazmat::
+
+Interfaces
+==========
+
+
+``cryptography`` uses `Abstract Base Classes`_ as interfaces to describe the
+properties and methods of most primitive constructs. Backends may also use
+this information to influence their operation. Interfaces should also be used
+to document argument and return types.
+
+.. _`Abstract Base Classes`: http://docs.python.org/3.2/library/abc.html
+
+
+Cipher Modes
+~~~~~~~~~~~~
+
+.. currentmodule:: cryptography.hazmat.primitives.interfaces
+
+Interfaces used by the symmetric cipher modes described in
+:ref:`Symmetric Encryption Modes <symmetric-encryption-modes>`.
+
+.. class:: Mode
+
+    A named cipher mode.
+
+    .. attribute:: name
+
+        :type: str
+
+        This should be the standard shorthand name for the mode, for example
+        Cipher-Block Chaining mode is "CBC".
+
+        The name may be used by a backend to influence the operation of a
+        cipher in conjunction with the algorithm's name.
+
+
+.. class:: ModeWithInitializationVector
+
+    A cipher mode with an initialization vector.
+
+    .. attribute:: initialization_vector
+
+        :type: bytes
+
+        Exact requirements of the initialization are described by the
+        documentation of individual modes.
+
+
+.. class:: ModeWithNonce
+
+    A cipher mode with a nonce.
+
+    .. attribute:: nonce
+
+        :type: bytes
+
+        Exact requirements of the nonce are described by the documentation of
+        individual modes.
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)