change OFB iv to nonce to reflect dstufft nomenclature pitch

* Namely, we should try to call things IV if reuse leaks a small amount
  of data and nonce if reuse can result in a complete break. This can
  be somewhat ambiguous, but we'll track in #58

cryptography/bindings/openssl/api.py

@@ -76,6 +76,8 @@ class API(object):
         assert evp_cipher != self._ffi.NULL
         if isinstance(mode, interfaces.ModeWithInitializationVector):
             iv_nonce = mode.initialization_vector
+        elif isinstance(mode, interfaces.ModeWithNonce):
+            iv_nonce = mode.nonce
         else:
             iv_nonce = self._ffi.NULL
 

cryptography/primitives/block/modes.py

@@ -31,10 +31,10 @@ class ECB(object):
 class OFB(object):
     name = "OFB"
 
-    def __init__(self, initialization_vector):
+    def __init__(self, nonce):
         super(OFB, self).__init__()
-        self.initialization_vector = initialization_vector
+        self.nonce = nonce
 
 
 interfaces.ModeWithInitializationVector.register(CBC)
-interfaces.ModeWithInitializationVector.register(OFB)
+interfaces.ModeWithNonce.register(OFB)

cryptography/primitives/interfaces.py

@@ -20,3 +20,7 @@ import six
 
 class ModeWithInitializationVector(six.with_metaclass(abc.ABCMeta)):
     pass
+
+
+class ModeWithNonce(six.with_metaclass(abc.ABCMeta)):
+    pass

docs/primitives/symmetric-encryption.rst

@@ -68,18 +68,16 @@ Modes
                                         reuse an ``initialization_vector`` with
                                         a given ``key``.
 
-.. class:: cryptography.primitives.block.modes.OFB(initialization_vector)
+.. class:: cryptography.primitives.block.modes.OFB(nonce)
 
     OFB (Output Feedback) is a mode of operation for block ciphers. It
     transforms a block cipher into a stream cipher.
 
-    :param bytes initialization_vector: 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. Do not
-                                        reuse an ``initialization_vector`` with
-                                        a given ``key``.
+    :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.  Reuse of a ``nonce`` with a given
+                        ``key`` can allow recovery of the original plaintext.
 
 
 Insecure Modes