python实现的DES加密算法和3DES加密算法实例

#############################################################################
#         Documentation          #
#############################################################################
# Author:  Todd Whiteman
# Date:   16th March, 2009
# Verion:  2.0.0
# License: Public Domain - free to do as you wish
# Homepage: http://twhiteman.netfirms.com/des.html
#
# This is a pure python implementation of the DES encryption algorithm.
# It's pure python to avoid portability issues, since most DES 
# implementations are programmed in C (for performance reasons).
#
# Triple DES class is also implemented, utilising the DES base. Triple DES
# is either DES-EDE3 with a 24 byte key, or DES-EDE2 with a 16 byte key.
#
# See the README.txt that should come with this python module for the
# implementation methods used.
#
# Thanks to:
# * David Broadwell for ideas, comments and suggestions.
# * Mario Wolff for pointing out and debugging some triple des CBC errors.
# * Santiago Palladino for providing the PKCS5 padding technique.
# * Shaya for correcting the PAD_PKCS5 triple des CBC errors.
#
"""A pure python implementation of the DES and TRIPLE DES encryption algorithms.
Class initialization
--------------------
pyDes.des(key, [mode], [IV], [pad], [padmode])
pyDes.triple_des(key, [mode], [IV], [pad], [padmode])
key   -> Bytes containing the encryption key. 8 bytes for DES, 16 or 24 bytes
    for Triple DES
mode  -> Optional argument for encryption type, can be either
    pyDes.ECB (Electronic Code Book) or pyDes.CBC (Cypher Block Chaining)
IV   -> Optional Initial Value bytes, must be supplied if using CBC mode.
    Length must be 8 bytes.
pad   -> Optional argument, set the pad character (PAD_NORMAL) to use during
    all encrypt/decrpt operations done with this instance.
padmode -> Optional argument, set the padding mode (PAD_NORMAL or PAD_PKCS5)
    to use during all encrypt/decrpt operations done with this instance.
I recommend to use PAD_PKCS5 padding, as then you never need to worry about any
padding issues, as the padding can be removed unambiguously upon decrypting
data that was encrypted using PAD_PKCS5 padmode.
Common methods
--------------
encrypt(data, [pad], [padmode])
decrypt(data, [pad], [padmode])
data  -> Bytes to be encrypted/decrypted
pad   -> Optional argument. Only when using padmode of PAD_NORMAL. For
    encryption, adds this characters to the end of the data block when
    data is not a multiple of 8 bytes. For decryption, will remove the
    trailing characters that match this pad character from the last 8
    bytes of the unencrypted data block.
padmode -> Optional argument, set the padding mode, must be one of PAD_NORMAL
    or PAD_PKCS5). Defaults to PAD_NORMAL.

Example
-------
from pyDes import *
data = "Please encrypt my data"
k = des("DESCRYPT", CBC, "", pad=None, padmode=PAD_PKCS5)
# For Python3, you'll need to use bytes, i.e.:
#  data = b"Please encrypt my data"
#  k = des(b"DESCRYPT", CBC, b"", pad=None, padmode=PAD_PKCS5)
d = k.encrypt(data)
print "Encrypted: %r" % d
print "Decrypted: %r" % k.decrypt(d)
assert k.decrypt(d, padmode=PAD_PKCS5) == data

See the module source (pyDes.py) for more examples of use.
You can also run the pyDes.py file without and arguments to see a simple test.
Note: This code was not written for high-end systems needing a fast
   implementation, but rather a handy portable solution with small usage.
"""
import sys
# _pythonMajorVersion is used to handle Python2 and Python3 differences.
_pythonMajorVersion = sys.version_info[0]
# Modes of crypting / cyphering
ECB =  0
CBC =  1
# Modes of padding
PAD_NORMAL = 1
PAD_PKCS5 = 2
# PAD_PKCS5: is a method that will unambiguously remove all padding
#      characters after decryption, when originally encrypted with
#      this padding mode.
# For a good description of the PKCS5 padding technique, see:
# http://www.faqs.org/rfcs/rfc1423.html
# The base class shared by des and triple des.
class _baseDes(object):
  def __init__(self, mode=ECB, IV=None, pad=None, padmode=PAD_NORMAL):
    if IV:
      IV = self._guardAgainstUnicode(IV)
    if pad:
      pad = self._guardAgainstUnicode(pad)
    self.block_size = 8
    # Sanity checking of arguments.
    if pad and padmode == PAD_PKCS5:
      raise ValueError("Cannot use a pad character with PAD_PKCS5")
    if IV and len(IV) != self.block_size:
      raise ValueError("Invalid Initial Value (IV), must be a multiple of " + str(self.block_size) + " bytes")
    # Set the passed in variables
    self._mode = mode
    self._iv = IV
    self._padding = pad
    self._padmode = padmode
  def getKey(self):
    """getKey() -> bytes"""
    return self.__key
  def setKey(self, key):
    """Will set the crypting key for this object."""
    key = self._guardAgainstUnicode(key)
    self.__key = key
  def getMode(self):
    """getMode() -> pyDes.ECB or pyDes.CBC"""
    return self._mode
  def setMode(self, mode):
    """Sets the type of crypting mode, pyDes.ECB or pyDes.CBC"""
    self._mode = mode
  def getPadding(self):
    """getPadding() -> bytes of length 1. Padding character."""
    return self._padding
  def setPadding(self, pad):
    """setPadding() -> bytes of length 1. Padding character."""
    if pad is not None:
      pad = self._guardAgainstUnicode(pad)
    self._padding = pad
  def getPadMode(self):
    """getPadMode() -> pyDes.PAD_NORMAL or pyDes.PAD_PKCS5"""
    return self._padmode
  def setPadMode(self, mode):
    """Sets the type of padding mode, pyDes.PAD_NORMAL or pyDes.PAD_PKCS5"""
    self._padmode = mode
  def getIV(self):
    """getIV() -> bytes"""
    return self._iv
  def setIV(self, IV):
    """Will set the Initial Value, used in conjunction with CBC mode"""
    if not IV or len(IV) != self.block_size:
      raise ValueError("Invalid Initial Value (IV), must be a multiple of " + str(self.block_size) + " bytes")
    IV = self._guardAgainstUnicode(IV)
    self._iv = IV
  def _padData(self, data, pad, padmode):
    # Pad data depending on the mode
    if padmode is None:
      # Get the default padding mode.
      padmode = self.getPadMode()
    if pad and padmode == PAD_PKCS5:
      raise ValueError("Cannot use a pad character with PAD_PKCS5")
    if padmode == PAD_NORMAL:
      if len(data) % self.block_size == 0:
        # No padding required.
        return data
      if not pad:
        # Get the default padding.
        pad = self.getPadding()
      if not pad:
        raise ValueError("Data must be a multiple of " + str(self.block_size) + " bytes in length. Use padmode=PAD_PKCS5 or set the pad character.")
      data += (self.block_size - (len(data) % self.block_size)) * pad
    elif padmode == PAD_PKCS5:
      pad_len = 8 - (len(data) % self.block_size)
      if _pythonMajorVersion < 3:
        data += pad_len * chr(pad_len)
      else:
        data += bytes([pad_len] * pad_len)
    return data
  def _unpadData(self, data, pad, padmode):
    # Unpad data depending on the mode.
    if not data:
      return data
    if pad and padmode == PAD_PKCS5:
      raise ValueError("Cannot use a pad character with PAD_PKCS5")
    if padmode is None:
      # Get the default padding mode.
      padmode = self.getPadMode()
    if padmode == PAD_NORMAL:
      if not pad:
        # Get the default padding.
        pad = self.getPadding()
      if pad:
        data = data[:-self.block_size] + 
            data[-self.block_size:].rstrip(pad)
    elif padmode == PAD_PKCS5:
      if _pythonMajorVersion < 3:
        pad_len = ord(data[-1])
      else:
        pad_len = data[-1]
      data = data[:-pad_len]
    return data
  def _guardAgainstUnicode(self, data):
    # Only accept byte strings or ascii unicode values, otherwise
    # there is no way to correctly decode the data into bytes.
    if _pythonMajorVersion < 3:
      if isinstance(data, unicode):
        raise ValueError("pyDes can only work with bytes, not Unicode strings.")
    else:
      if isinstance(data, str):
        # Only accept ascii unicode values.
        try:
          return data.encode('ascii')
        except UnicodeEncodeError:
          pass
        raise ValueError("pyDes can only work with encoded strings, not Unicode.")
    return data
#############################################################################
#           DES            #
#############################################################################
class des(_baseDes):
  """DES encryption/decrytpion class
  Supports ECB (Electronic Code Book) and CBC (Cypher Block Chaining) modes.
  pyDes.des(key,[mode], [IV])
  key -> Bytes containing the encryption key, must be exactly 8 bytes
  mode -> Optional argument for encryption type, can be either pyDes.ECB
    (Electronic Code Book), pyDes.CBC (Cypher Block Chaining)
  IV  -> Optional Initial Value bytes, must be supplied if using CBC mode.
    Must be 8 bytes in length.
  pad -> Optional argument, set the pad character (PAD_NORMAL) to use
    during all encrypt/decrpt operations done with this instance.
  padmode -> Optional argument, set the padding mode (PAD_NORMAL or
    PAD_PKCS5) to use during all encrypt/decrpt operations done
    with this instance.
  """

  # Permutation and translation tables for DES
  __pc1 = [56, 48, 40, 32, 24, 16, 8,
, 57, 49, 41, 33, 25, 17,
, 1, 58, 50, 42, 34, 26,
, 10, 2, 59, 51, 43, 35,
, 54, 46, 38, 30, 22, 14,
, 61, 53, 45, 37, 29, 21,
, 5, 60, 52, 44, 36, 28,
, 12, 4, 27, 19, 11, 3
  ]
  # number left rotations of pc1
  __left_rotations = [
, 1, 2, 2, 2, 2, 2, 2, 1, 2, 2, 2, 2, 2, 2, 1
  ]
  # permuted choice key (table 2)
  __pc2 = [
, 16, 10, 23, 0, 4,
, 27, 14, 5, 20, 9,
, 18, 11, 3, 25, 7,
, 6, 26, 19, 12, 1,
, 51, 30, 36, 46, 54,
, 39, 50, 44, 32, 47,
, 48, 38, 55, 33, 52,
, 41, 49, 35, 28, 31
  ]
  # initial permutation IP
  __ip = [57, 49, 41, 33, 25, 17, 9, 1,
, 51, 43, 35, 27, 19, 11, 3,
, 53, 45, 37, 29, 21, 13, 5,
, 55, 47, 39, 31, 23, 15, 7,
, 48, 40, 32, 24, 16, 8, 0,
, 50, 42, 34, 26, 18, 10, 2,
, 52, 44, 36, 28, 20, 12, 4,
, 54, 46, 38, 30, 22, 14, 6
  ]
  # Expansion table for turning 32 bit blocks into 48 bits
  __expansion_table = [
, 0, 1, 2, 3, 4,
, 4, 5, 6, 7, 8,
, 8, 9, 10, 11, 12,
, 12, 13, 14, 15, 16,
, 16, 17, 18, 19, 20,
, 20, 21, 22, 23, 24,
, 24, 25, 26, 27, 28,
, 28, 29, 30, 31, 0
  ]
  # The (in)famous S-boxes
  __sbox = [
    # S1
    [14, 4, 13, 1, 2, 15, 11, 8, 3, 10, 6, 12, 5, 9, 0, 7,
, 15, 7, 4, 14, 2, 13, 1, 10, 6, 12, 11, 9, 5, 3, 8,
, 1, 14, 8, 13, 6, 2, 11, 15, 12, 9, 7, 3, 10, 5, 0,
, 12, 8, 2, 4, 9, 1, 7, 5, 11, 3, 14, 10, 0, 6, 13],
    # S2
    [15, 1, 8, 14, 6, 11, 3, 4, 9, 7, 2, 13, 12, 0, 5, 10,
, 13, 4, 7, 15, 2, 8, 14, 12, 0, 1, 10, 6, 9, 11, 5,
, 14, 7, 11, 10, 4, 13, 1, 5, 8, 12, 6, 9, 3, 2, 15,
, 8, 10, 1, 3, 15, 4, 2, 11, 6, 7, 12, 0, 5, 14, 9],
    # S3
    [10, 0, 9, 14, 6, 3, 15, 5, 1, 13, 12, 7, 11, 4, 2, 8,
, 7, 0, 9, 3, 4, 6, 10, 2, 8, 5, 14, 12, 11, 15, 1,
, 6, 4, 9, 8, 15, 3, 0, 11, 1, 2, 12, 5, 10, 14, 7,
, 10, 13, 0, 6, 9, 8, 7, 4, 15, 14, 3, 11, 5, 2, 12],
    # S4
    [7, 13, 14, 3, 0, 6, 9, 10, 1, 2, 8, 5, 11, 12, 4, 15,
, 8, 11, 5, 6, 15, 0, 3, 4, 7, 2, 12, 1, 10, 14, 9,
, 6, 9, 0, 12, 11, 7, 13, 15, 1, 3, 14, 5, 2, 8, 4,
, 15, 0, 6, 10, 1, 13, 8, 9, 4, 5, 11, 12, 7, 2, 14],
    # S5
    [2, 12, 4, 1, 7, 10, 11, 6, 8, 5, 3, 15, 13, 0, 14, 9,
, 11, 2, 12, 4, 7, 13, 1, 5, 0, 15, 10, 3, 9, 8, 6,
, 2, 1, 11, 10, 13, 7, 8, 15, 9, 12, 5, 6, 3, 0, 14,
, 8, 12, 7, 1, 14, 2, 13, 6, 15, 0, 9, 10, 4, 5, 3],
    # S6
    [12, 1, 10, 15, 9, 2, 6, 8, 0, 13, 3, 4, 14, 7, 5, 11,
, 15, 4, 2, 7, 12, 9, 5, 6, 1, 13, 14, 0, 11, 3, 8,
, 14, 15, 5, 2, 8, 12, 3, 7, 0, 4, 10, 1, 13, 11, 6,
, 3, 2, 12, 9, 5, 15, 10, 11, 14, 1, 7, 6, 0, 8, 13],
    # S7
    [4, 11, 2, 14, 15, 0, 8, 13, 3, 12, 9, 7, 5, 10, 6, 1,
, 0, 11, 7, 4, 9, 1, 10, 14, 3, 5, 12, 2, 15, 8, 6,
, 4, 11, 13, 12, 3, 7, 14, 10, 15, 6, 8, 0, 5, 9, 2,
, 11, 13, 8, 1, 4, 10, 7, 9, 5, 0, 15, 14, 2, 3, 12],
    # S8
    [13, 2, 8, 4, 6, 15, 11, 1, 10, 9, 3, 14, 5, 0, 12, 7,
, 15, 13, 8, 10, 3, 7, 4, 12, 5, 6, 11, 0, 14, 9, 2,
, 11, 4, 1, 9, 12, 14, 2, 0, 6, 10, 13, 15, 3, 5, 8,
, 1, 14, 7, 4, 10, 8, 13, 15, 12, 9, 0, 3, 5, 6, 11],
  ]

  # 32-bit permutation function P used on the output of the S-boxes
  __p = [
, 6, 19, 20, 28, 11,
, 16, 0, 14, 22, 25,
, 17, 30, 9, 1, 7,
,13, 31, 26, 2, 8,
, 12, 29, 5, 21, 10,
, 24
  ]
  # final permutation IP^-1
  __fp = [
, 7, 47, 15, 55, 23, 63, 31,
, 6, 46, 14, 54, 22, 62, 30,
, 5, 45, 13, 53, 21, 61, 29,
, 4, 44, 12, 52, 20, 60, 28,
, 3, 43, 11, 51, 19, 59, 27,
, 2, 42, 10, 50, 18, 58, 26,
, 1, 41, 9, 49, 17, 57, 25,
, 0, 40, 8, 48, 16, 56, 24
  ]
  # Type of crypting being done
  ENCRYPT =  0x00
  DECRYPT =  0x01
  # Initialisation
  def __init__(self, key, mode=ECB, IV=None, pad=None, padmode=PAD_NORMAL):
    # Sanity checking of arguments.
    if len(key) != 8:
      raise ValueError("Invalid DES key size. Key must be exactly 8 bytes long.")
    _baseDes.__init__(self, mode, IV, pad, padmode)
    self.key_size = 8
    self.L = []
    self.R = []
    self.Kn = [ [0] * 48 ] * 16  # 16 48-bit keys (K1 - K16)
    self.final = []
    self.setKey(key)
  def setKey(self, key):
    """Will set the crypting key for this object. Must be 8 bytes."""
    _baseDes.setKey(self, key)
    self.__create_sub_keys()
  def __String_to_BitList(self, data):
    """Turn the string data, into a list of bits (1, 0)'s"""
    if _pythonMajorVersion < 3:
      # Turn the strings into integers. Python 3 uses a bytes
      # class, which already has this behaviour.
      data = [ord(c) for c in data]
    l = len(data) * 8
    result = [0] * l
    pos = 0
    for ch in data:
      i = 7
      while i >= 0:
        if ch & (1 << i) != 0:
          result[pos] = 1
        else:
          result[pos] = 0
        pos += 1
        i -= 1
    return result
  def __BitList_to_String(self, data):
    """Turn the list of bits -> data, into a string"""
    result = []
    pos = 0
    c = 0
    while pos < len(data):
      c += data[pos] << (7 - (pos % 8))
      if (pos % 8) == 7:
        result.append(c)
        c = 0
      pos += 1
    if _pythonMajorVersion < 3:
      return ''.join([ chr(c) for c in result ])
    else:
      return bytes(result)
  def __permutate(self, table, block):
    """Permutate this block with the specified table"""
    return list(map(lambda x: block[x], table))
  # Transform the secret key, so that it is ready for data processing
  # Create the 16 subkeys, K[1] - K[16]
  def __create_sub_keys(self):
    """Create the 16 subkeys K[1] to K[16] from the given key"""
    key = self.__permutate(des.__pc1, self.__String_to_BitList(self.getKey()))
    i = 0
    # Split into Left and Right sections
    self.L = key[:28]
    self.R = key[28:]
    while i < 16:
      j = 0
      # Perform circular left shifts
      while j < des.__left_rotations[i]:
        self.L.append(self.L[0])
        del self.L[0]
        self.R.append(self.R[0])
        del self.R[0]
        j += 1
      # Create one of the 16 subkeys through pc2 permutation
      self.Kn[i] = self.__permutate(des.__pc2, self.L + self.R)
      i += 1
  # Main part of the encryption algorithm, the number cruncher :)
  def __des_crypt(self, block, crypt_type):
    """Crypt the block of data through DES bit-manipulation"""
    block = self.__permutate(des.__ip, block)
    self.L = block[:32]
    self.R = block[32:]
    # Encryption starts from Kn[1] through to Kn[16]
    if crypt_type == des.ENCRYPT:
      iteration = 0
      iteration_adjustment = 1
    # Decryption starts from Kn[16] down to Kn[1]
    else:
      iteration = 15
      iteration_adjustment = -1
    i = 0
    while i < 16:
      # Make a copy of R[i-1], this will later become L[i]
      tempR = self.R[:]
      # Permutate R[i - 1] to start creating R[i]
      self.R = self.__permutate(des.__expansion_table, self.R)
      # Exclusive or R[i - 1] with K[i], create B[1] to B[8] whilst here
      self.R = list(map(lambda x, y: x ^ y, self.R, self.Kn[iteration]))
      B = [self.R[:6], self.R[6:12], self.R[12:18], self.R[18:24], self.R[24:30], self.R[30:36], self.R[36:42], self.R[42:]]
      # Optimization: Replaced below commented code with above
      #j = 0
      #B = []
      #while j < len(self.R):
      #  self.R[j] = self.R[j] ^ self.Kn[iteration][j]
      #  j += 1
      #  if j % 6 == 0:
      #    B.append(self.R[j-6:j])
      # Permutate B[1] to B[8] using the S-Boxes
      j = 0
      Bn = [0] * 32
      pos = 0
      while j < 8:
        # Work out the offsets
        m = (B[j][0] << 1) + B[j][5]
        n = (B[j][1] << 3) + (B[j][2] << 2) + (B[j][3] << 1) + B[j][4]
        # Find the permutation value
        v = des.__sbox[j][(m << 4) + n]
        # Turn value into bits, add it to result: Bn
        Bn[pos] = (v & 8) >> 3
        Bn[pos + 1] = (v & 4) >> 2
        Bn[pos + 2] = (v & 2) >> 1
        Bn[pos + 3] = v & 1
        pos += 4
        j += 1
      # Permutate the concatination of B[1] to B[8] (Bn)
      self.R = self.__permutate(des.__p, Bn)
      # Xor with L[i - 1]
      self.R = list(map(lambda x, y: x ^ y, self.R, self.L))
      # Optimization: This now replaces the below commented code
      #j = 0
      #while j < len(self.R):
      #  self.R[j] = self.R[j] ^ self.L[j]
      #  j += 1
      # L[i] becomes R[i - 1]
      self.L = tempR
      i += 1
      iteration += iteration_adjustment
    # Final permutation of R[16]L[16]
    self.final = self.__permutate(des.__fp, self.R + self.L)
    return self.final

  # Data to be encrypted/decrypted
  def crypt(self, data, crypt_type):
    """Crypt the data in blocks, running it through des_crypt()"""
    # Error check the data
    if not data:
      return ''
    if len(data) % self.block_size != 0:
      if crypt_type == des.DECRYPT: # Decryption must work on 8 byte blocks
        raise ValueError("Invalid data length, data must be a multiple of " + str(self.block_size) + " bytesn.")
      if not self.getPadding():
        raise ValueError("Invalid data length, data must be a multiple of " + str(self.block_size) + " bytesn. Try setting the optional padding character")
      else:
        data += (self.block_size - (len(data) % self.block_size)) * self.getPadding()
      # print "Len of data: %f" % (len(data) / self.block_size)
    if self.getMode() == CBC:
      if self.getIV():
        iv = self.__String_to_BitList(self.getIV())
      else:
        raise ValueError("For CBC mode, you must supply the Initial Value (IV) for ciphering")
    # Split the data into blocks, crypting each one seperately
    i = 0
    dict = {}
    result = []
    #cached = 0
    #lines = 0
    while i < len(data):
      # Test code for caching encryption results
      #lines += 1
      #if dict.has_key(data[i:i+8]):
        #print "Cached result for: %s" % data[i:i+8]
      #  cached += 1
      #  result.append(dict[data[i:i+8]])
      #  i += 8
      #  continue
      block = self.__String_to_BitList(data[i:i+8])
      # Xor with IV if using CBC mode
      if self.getMode() == CBC:
        if crypt_type == des.ENCRYPT:
          block = list(map(lambda x, y: x ^ y, block, iv))
          #j = 0
          #while j < len(block):
          #  block[j] = block[j] ^ iv[j]
          #  j += 1
        processed_block = self.__des_crypt(block, crypt_type)
        if crypt_type == des.DECRYPT:
          processed_block = list(map(lambda x, y: x ^ y, processed_block, iv))
          #j = 0
          #while j < len(processed_block):
          #  processed_block[j] = processed_block[j] ^ iv[j]
          #  j += 1
          iv = block
        else:
          iv = processed_block
      else:
        processed_block = self.__des_crypt(block, crypt_type)

      # Add the resulting crypted block to our list
      #d = self.__BitList_to_String(processed_block)
      #result.append(d)
      result.append(self.__BitList_to_String(processed_block))
      #dict[data[i:i+8]] = d
      i += 8
    # print "Lines: %d, cached: %d" % (lines, cached)
    # Return the full crypted string
    if _pythonMajorVersion < 3:
      return ''.join(result)
    else:
      return bytes.fromhex('').join(result)
  def encrypt(self, data, pad=None, padmode=None):
    """encrypt(data, [pad], [padmode]) -> bytes
    data : Bytes to be encrypted
    pad : Optional argument for encryption padding. Must only be one byte
    padmode : Optional argument for overriding the padding mode.
    The data must be a multiple of 8 bytes and will be encrypted
    with the already specified key. Data does not have to be a
    multiple of 8 bytes if the padding character is supplied, or
    the padmode is set to PAD_PKCS5, as bytes will then added to
    ensure the be padded data is a multiple of 8 bytes.
    """
    data = self._guardAgainstUnicode(data)
    if pad is not None:
      pad = self._guardAgainstUnicode(pad)
    data = self._padData(data, pad, padmode)
    return self.crypt(data, des.ENCRYPT)
  def decrypt(self, data, pad=None, padmode=None):
    """decrypt(data, [pad], [padmode]) -> bytes
    data : Bytes to be encrypted
    pad : Optional argument for decryption padding. Must only be one byte
    padmode : Optional argument for overriding the padding mode.
    The data must be a multiple of 8 bytes and will be decrypted
    with the already specified key. In PAD_NORMAL mode, if the
    optional padding character is supplied, then the un-encrypted
    data will have the padding characters removed from the end of
    the bytes. This pad removal only occurs on the last 8 bytes of
    the data (last data block). In PAD_PKCS5 mode, the special
    padding end markers will be removed from the data after decrypting.
    """
    data = self._guardAgainstUnicode(data)
    if pad is not None:
      pad = self._guardAgainstUnicode(pad)
    data = self.crypt(data, des.DECRYPT)
    return self._unpadData(data, pad, padmode)

#############################################################################
#         Triple DES          #
#############################################################################
class triple_des(_baseDes):
  """Triple DES encryption/decrytpion class
  This algorithm uses the DES-EDE3 (when a 24 byte key is supplied) or
  the DES-EDE2 (when a 16 byte key is supplied) encryption methods.
  Supports ECB (Electronic Code Book) and CBC (Cypher Block Chaining) modes.
  pyDes.des(key, [mode], [IV])
  key -> Bytes containing the encryption key, must be either 16 or
 bytes long
  mode -> Optional argument for encryption type, can be either pyDes.ECB
    (Electronic Code Book), pyDes.CBC (Cypher Block Chaining)
  IV  -> Optional Initial Value bytes, must be supplied if using CBC mode.
    Must be 8 bytes in length.
  pad -> Optional argument, set the pad character (PAD_NORMAL) to use
    during all encrypt/decrpt operations done with this instance.
  padmode -> Optional argument, set the padding mode (PAD_NORMAL or
    PAD_PKCS5) to use during all encrypt/decrpt operations done
    with this instance.
  """
  def __init__(self, key, mode=ECB, IV=None, pad=None, padmode=PAD_NORMAL):
    _baseDes.__init__(self, mode, IV, pad, padmode)
    self.setKey(key)
  def setKey(self, key):
    """Will set the crypting key for this object. Either 16 or 24 bytes long."""
    self.key_size = 24 # Use DES-EDE3 mode
    if len(key) != self.key_size:
      if len(key) == 16: # Use DES-EDE2 mode
        self.key_size = 16
      else:
        raise ValueError("Invalid triple DES key size. Key must be either 16 or 24 bytes long")
    if self.getMode() == CBC:
      if not self.getIV():
        # Use the first 8 bytes of the key
        self._iv = key[:self.block_size]
      if len(self.getIV()) != self.block_size:
        raise ValueError("Invalid IV, must be 8 bytes in length")
    self.__key1 = des(key[:8], self._mode, self._iv,
         self._padding, self._padmode)
    self.__key2 = des(key[8:16], self._mode, self._iv,
         self._padding, self._padmode)
    if self.key_size == 16:
      self.__key3 = self.__key1
    else:
      self.__key3 = des(key[16:], self._mode, self._iv,
           self._padding, self._padmode)
    _baseDes.setKey(self, key)
  # Override setter methods to work on all 3 keys.
  def setMode(self, mode):
    """Sets the type of crypting mode, pyDes.ECB or pyDes.CBC"""
    _baseDes.setMode(self, mode)
    for key in (self.__key1, self.__key2, self.__key3):
      key.setMode(mode)
  def setPadding(self, pad):
    """setPadding() -> bytes of length 1. Padding character."""
    _baseDes.setPadding(self, pad)
    for key in (self.__key1, self.__key2, self.__key3):
      key.setPadding(pad)
  def setPadMode(self, mode):
    """Sets the type of padding mode, pyDes.PAD_NORMAL or pyDes.PAD_PKCS5"""
    _baseDes.setPadMode(self, mode)
    for key in (self.__key1, self.__key2, self.__key3):
      key.setPadMode(mode)
  def setIV(self, IV):
    """Will set the Initial Value, used in conjunction with CBC mode"""
    _baseDes.setIV(self, IV)
    for key in (self.__key1, self.__key2, self.__key3):
      key.setIV(IV)
  def encrypt(self, data, pad=None, padmode=None):
    """encrypt(data, [pad], [padmode]) -> bytes
    data : bytes to be encrypted
    pad : Optional argument for encryption padding. Must only be one byte
    padmode : Optional argument for overriding the padding mode.
    The data must be a multiple of 8 bytes and will be encrypted
    with the already specified key. Data does not have to be a
    multiple of 8 bytes if the padding character is supplied, or
    the padmode is set to PAD_PKCS5, as bytes will then added to
    ensure the be padded data is a multiple of 8 bytes.
    """
    ENCRYPT = des.ENCRYPT
    DECRYPT = des.DECRYPT
    data = self._guardAgainstUnicode(data)
    if pad is not None:
      pad = self._guardAgainstUnicode(pad)
    # Pad the data accordingly.
    data = self._padData(data, pad, padmode)
    if self.getMode() == CBC:
      self.__key1.setIV(self.getIV())
      self.__key2.setIV(self.getIV())
      self.__key3.setIV(self.getIV())
      i = 0
      result = []
      while i < len(data):
        block = self.__key1.crypt(data[i:i+8], ENCRYPT)
        block = self.__key2.crypt(block, DECRYPT)
        block = self.__key3.crypt(block, ENCRYPT)
        self.__key1.setIV(block)
        self.__key2.setIV(block)
        self.__key3.setIV(block)
        result.append(block)
        i += 8
      if _pythonMajorVersion < 3:
        return ''.join(result)
      else:
        return bytes.fromhex('').join(result)
    else:
      data = self.__key1.crypt(data, ENCRYPT)
      data = self.__key2.crypt(data, DECRYPT)
      return self.__key3.crypt(data, ENCRYPT)
  def decrypt(self, data, pad=None, padmode=None):
    """decrypt(data, [pad], [padmode]) -> bytes
    data : bytes to be encrypted
    pad : Optional argument for decryption padding. Must only be one byte
    padmode : Optional argument for overriding the padding mode.
    The data must be a multiple of 8 bytes and will be decrypted
    with the already specified key. In PAD_NORMAL mode, if the
    optional padding character is supplied, then the un-encrypted
    data will have the padding characters removed from the end of
    the bytes. This pad removal only occurs on the last 8 bytes of
    the data (last data block). In PAD_PKCS5 mode, the special
    padding end markers will be removed from the data after
    decrypting, no pad character is required for PAD_PKCS5.
    """
    ENCRYPT = des.ENCRYPT
    DECRYPT = des.DECRYPT
    data = self._guardAgainstUnicode(data)
    if pad is not None:
      pad = self._guardAgainstUnicode(pad)
    if self.getMode() == CBC:
      self.__key1.setIV(self.getIV())
      self.__key2.setIV(self.getIV())
      self.__key3.setIV(self.getIV())
      i = 0
      result = []
      while i < len(data):
        iv = data[i:i+8]
        block = self.__key3.crypt(iv,  DECRYPT)
        block = self.__key2.crypt(block, ENCRYPT)
        block = self.__key1.crypt(block, DECRYPT)
        self.__key1.setIV(iv)
        self.__key2.setIV(iv)
        self.__key3.setIV(iv)
        result.append(block)
        i += 8
      if _pythonMajorVersion < 3:
        data = ''.join(result)
      else:
        data = bytes.fromhex('').join(result)
    else:
      data = self.__key3.crypt(data, DECRYPT)
      data = self.__key2.crypt(data, ENCRYPT)
      data = self.__key1.crypt(data, DECRYPT)
    return self._unpadData(data, pad, padmode)