Merge pull request #192 from alex/pkcs7-padding

PKCS7 padding

1 files changed, 69 insertions, 0 deletions

diff --git a/docs/hazmat/primitives/padding.rst b/docs/hazmat/primitives/padding.rst
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+.. danger::
+
+    This is a "Hazardous Materials" module. You should **ONLY** use it if
+    you're 100% absolutely sure that you know what you're doing because this
+    module is full of land mines, dragons, and dinosaurs with laser guns.
+
+
+Padding
+=======
+
+.. currentmodule:: cryptography.hazmat.primitives.padding
+
+Padding is a way to take data that may or may not be be a multiple of the block
+size for a cipher and extend it out so that it is. This is required for many
+block cipher modes as they require the data to be encrypted to be an exact
+multiple of the block size.
+
+
+.. class:: PKCS7(block_size)
+
+    PKCS7 padding is a generalization of PKCS5 padding (also known as standard
+    padding). PKCS7 padding works by appending ``N`` bytes with the value of
+    ``chr(N)``, where ``N`` is the number of bytes required to make the final
+    block of data the same size as the block size. A simple example of padding
+    is:
+
+    .. doctest::
+
+        >>> from cryptography.hazmat.primitives import padding
+        >>> padder = padding.PKCS7(128).padder()
+        >>> padder.update(b"1111111111")
+        ''
+        >>> padder.finalize()
+        '1111111111\x06\x06\x06\x06\x06\x06'
+
+    :param block_size: The size of the block in bits that the data is being
+                       padded to.
+
+    .. method:: padder()
+
+        :returns: A padding
+            :class:`~cryptography.hazmat.primitives.interfaces.PaddingContext`
+            provider.
+
+    .. method:: unpadder()
+
+        :returns: An unpadding
+            :class:`~cryptography.hazmat.primitives.interfaces.PaddingContext`
+            provider.
+
+
+.. currentmodule:: cryptography.hazmat.primitives.interfaces
+
+.. class:: PaddingContext
+
+    When calling ``padder()`` or ``unpadder()`` you will receive an a return
+    object conforming to the ``PaddingContext`` interface. You can then call
+    ``update(data)`` with data until you have fed everything into the context.
+    Once that is done call ``finalize()`` to finish the operation and obtain
+    the remainder of the data.
+
+    .. method:: update(data)
+
+        :param bytes data: The data you wish to pass into the context.
+        :return bytes: Returns the data that was padded or unpadded.
+
+    .. method:: finalize()
+
+        :return bytes: Returns the remainder of the data.