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Padding.py

#
# Util/Padding.py :  Functions to manage padding
#
# ===================================================================
#
# Copyright (c) 2014, Legrandin <helderijs@gmail.com>
# All rights reserved.
#
# Redistribution and use in source and binary forms, with or without
# modification, are permitted provided that the following conditions
# are met:
#
# 1. Redistributions of source code must retain the above copyright
#    notice, this list of conditions and the following disclaimer.
# 2. Redistributions in binary form must reproduce the above copyright
#    notice, this list of conditions and the following disclaimer in
#    the documentation and/or other materials provided with the
#    distribution.
#
# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
# "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
# LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
# FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
# COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
# INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
# BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
# LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
# CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
# LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
# ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
# POSSIBILITY OF SUCH DAMAGE.
# ===================================================================

__all__ = [ 'pad', 'unpad' ]

from Crypto.Util.py3compat import *

def pad(data_to_pad, block_size, style='pkcs7'):
    """Apply standard padding.

Args:
      data_to_pad (byte string):
        The data that needs to be padded.
      block_size (integer):
        The block boundary to use for padding. The output length is guaranteed
        to be a multiple of :data:`block_size`.
      style (string):
        Padding algorithm. It can be *'pkcs7'* (default), *'iso7816'* or *'x923'*.

Return:
      byte string : the original data with the appropriate padding added at the end.
    """

padding_len = block_size-len(data_to_pad)%block_size
    if style == 'pkcs7':
        padding = bchr(padding_len)*padding_len
    elif style == 'x923':
        padding = bchr(0)*(padding_len-1) + bchr(padding_len)
    elif style == 'iso7816':
        padding = bchr(128) + bchr(0)*(padding_len-1)
    else:
        raise ValueError("Unknown padding style")
    return data_to_pad + padding

def unpad(padded_data, block_size, style='pkcs7'):
    """Remove standard padding.

Args:
      padded_data (byte string):
        A piece of data with padding that needs to be stripped.
      block_size (integer):
        The block boundary to use for padding. The input length
        must be a multiple of :data:`block_size`.
      style (string):
        Padding algorithm. It can be *'pkcs7'* (default), *'iso7816'* or *'x923'*.
    Return:
        byte string : data without padding.
    Raises:
      ValueError: if the padding is incorrect.
    """

pdata_len = len(padded_data)
    if pdata_len == 0:
        raise ValueError("Zero-length input cannot be unpadded")
    if pdata_len % block_size:
        raise ValueError("Input data is not padded")
    if style in ('pkcs7', 'x923'):
        padding_len = bord(padded_data[-1])
        if padding_len<1 or padding_len>min(block_size, pdata_len):
            raise ValueError("Padding is incorrect.")
        if style == 'pkcs7':
            if padded_data[-padding_len:]!=bchr(padding_len)*padding_len:
                raise ValueError("PKCS#7 padding is incorrect.")
        else:
            if padded_data[-padding_len:-1]!=bchr(0)*(padding_len-1):
                raise ValueError("ANSI X.923 padding is incorrect.")
    elif style == 'iso7816':
        padding_len = pdata_len - padded_data.rfind(bchr(128))
        if padding_len<1 or padding_len>min(block_size, pdata_len):
            raise ValueError("Padding is incorrect.")
        if padding_len>1 and padded_data[1-padding_len:]!=bchr(0)*(padding_len-1):
            raise ValueError("ISO 7816-4 padding is incorrect.")
    else:
        raise ValueError("Unknown padding style")
    return padded_data[:-padding_len]

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