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.. hazmat:: Symmetric Padding ================= .. module:: cryptography.hazmat.primitives.padding Padding is a way to take data that may or may not 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() >>> padded_data = padder.update(b"11111111111111112222222222") >>> padded_data b'1111111111111111' >>> padded_data += padder.finalize() >>> padded_data b'11111111111111112222222222\x06\x06\x06\x06\x06\x06' >>> unpadder = padding.PKCS7(128).unpadder() >>> data = unpadder.update(padded_data) >>> data b'1111111111111111' >>> data + unpadder.finalize() b'11111111111111112222222222' :param block_size: The size of the block in :term:`bits` that the data is being padded to. :raises ValueError: Raised if block size is not a multiple of 8 or is not between 0 and 2040 inclusive. .. method:: padder() :returns: A padding :class:`~cryptography.hazmat.primitives.padding.PaddingContext` instance. .. method:: unpadder() :returns: An unpadding :class:`~cryptography.hazmat.primitives.padding.PaddingContext` instance. .. class:: ANSIX923(block_size) .. versionadded:: 1.3 `ANSI X9.23`_ padding works by appending ``N-1`` bytes with the value of ``0`` and a last byte 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:: >>> padder = padding.ANSIX923(128).padder() >>> padded_data = padder.update(b"11111111111111112222222222") >>> padded_data b'1111111111111111' >>> padded_data += padder.finalize() >>> padded_data b'11111111111111112222222222\x00\x00\x00\x00\x00\x06' >>> unpadder = padding.ANSIX923(128).unpadder() >>> data = unpadder.update(padded_data) >>> data b'1111111111111111' >>> data + unpadder.finalize() b'11111111111111112222222222' :param block_size: The size of the block in :term:`bits` that the data is being padded to. :raises ValueError: Raised if block size is not a multiple of 8 or is not between 0 and 2040 inclusive. .. method:: padder() :returns: A padding :class:`~cryptography.hazmat.primitives.padding.PaddingContext` instance. .. method:: unpadder() :returns: An unpadding :class:`~cryptography.hazmat.primitives.padding.PaddingContext` instance. .. class:: PaddingContext When calling ``padder()`` or ``unpadder()`` the result will conform 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 data: The data you wish to pass into the context. :type data: :term:`bytes-like` :return bytes: Returns the data that was padded or unpadded. :raises TypeError: Raised if data is not bytes. :raises cryptography.exceptions.AlreadyFinalized: See :meth:`finalize`. :raises TypeError: This exception is raised if ``data`` is not ``bytes``. .. method:: finalize() Finalize the current context and return the rest of the data. After ``finalize`` has been called this object can no longer be used; :meth:`update` and :meth:`finalize` will raise an :class:`~cryptography.exceptions.AlreadyFinalized` exception. :return bytes: Returns the remainder of the data. :raises TypeError: Raised if data is not bytes. :raises ValueError: When trying to remove padding from incorrectly padded data. .. _`ANSI X9.23`: https://en.wikipedia.org/wiki/Padding_%28cryptography%29#ANSI_X9.23