DES implementation in Python

After coding RC4, I thought it was time to face a more difficult challenge: DES (Data Encryption Standard). Although it has been replaced by AES (Advanced Encryption Standard) more than a decade ago, it still has some appeal to me because it was the first symmetric algorithm I learned (and also because its creation, 40 years ago, was surrounded by mystery). Before fighting the monster, I faced a lighter opponent, S-DES (Simplified DES), to get used to the awkward bit manipulation DES take advantage of. Since S-DES is just a toy cryptographic algorithm, it isn’t worthwhile to spend much time and space writing about it. For this reason, without further ado, this is the DES Python code I wrote:

#!/usr/bin/python3
#
# Author: Joao H de A Franco (jhafranco@acm.org)
#
# Description: DES implementation in Python 3
#
# Note: only single DES in ECB mode (with PKCS5 padding)
#       is supported
#
# License: Attribution-NonCommercial-ShareAlike 3.0 Unported
#          (CC BY-NC-SA 3.0)
#===========================================================

subKeyList = 16*[[None]*8]

IPtable = (58, 50, 42, 34, 26, 18, 10, 2,
           60, 52, 44, 36, 28, 20, 12, 4,
           62, 54, 46, 38, 30, 22, 14, 6,
           64, 56, 48, 40, 32, 24, 16, 8,
           57, 49, 41, 33, 25, 17,  9, 1,
           59, 51, 43, 35, 27, 19, 11, 3,
           61, 53, 45, 37, 29, 21, 13, 5,
           63, 55, 47, 39, 31, 23, 15, 7)

EPtable = (32,  1,  2,  3,  4,  5,
            4,  5,  6,  7,  8,  9,
            8,  9, 10, 11, 12, 13,
           12, 13, 14, 15, 16, 17,
           16, 17, 18, 19, 20, 21,
           20, 21, 22, 23, 24, 25,
           24, 25, 26, 27, 28, 29,
           28, 29, 30, 31, 32,  1)

PFtable = (16,  7, 20, 21, 29, 12, 28, 17,
            1, 15, 23, 26,  5, 18, 31, 10,
            2,  8, 24, 14, 32, 27,  3,  9,
           19, 13, 30,  6, 22, 11,  4, 25)

FPtable = (40, 8, 48, 16, 56, 24, 64, 32,
           39, 7, 47, 15, 55, 23, 63, 31,
           38, 6, 46, 14, 54, 22, 62, 30,
           37, 5, 45, 13, 53, 21, 61, 29,
           36, 4, 44, 12, 52, 20, 60, 28,
           35, 3, 43, 11, 51, 19, 59, 27,
           34, 2, 42, 10, 50, 18, 58, 26,
           33, 1, 41,  9, 49, 17, 57, 25)

sBox = 8*[64*[0]]

sBox[0] = (14,  4, 13,  1,  2, 15, 11,  8,  3, 10,  6, 12,  5,  9,  0,  7,
            0, 15,  7,  4, 14,  2, 13,  1, 10,  6, 12, 11,  9,  5,  3,  8,
            4,  1, 14,  8, 13,  6,  2, 11, 15, 12,  9,  7,  3, 10,  5,  0,
           15, 12,  8,  2,  4,  9,  1,  7,  5, 11,  3, 14, 10,  0,  6, 13)

sBox[1] = (15,  1,  8, 14,  6, 11,  3,  4,  9,  7,  2, 13, 12,  0,  5, 10,
            3, 13,  4,  7, 15,  2,  8, 14, 12,  0,  1, 10,  6,  9, 11,  5,
            0, 14,  7, 11, 10,  4, 13,  1,  5,  8, 12,  6,  9,  3,  2, 15,
           13,  8, 10,  1,  3, 15,  4,  2, 11,  6,  7, 12,  0,  5, 14,  9)

sBox[2] = (10,  0,  9, 14,  6,  3, 15,  5,  1, 13, 12,  7, 11,  4,  2,  8,
           13,  7,  0,  9,  3,  4,  6, 10,  2,  8,  5, 14, 12, 11, 15,  1,
           13,  6,  4,  9,  8, 15,  3,  0, 11,  1,  2, 12,  5, 10, 14,  7,
            1, 10, 13,  0,  6,  9,  8,  7,  4, 15, 14,  3, 11,  5,  2, 12)

sBox[3] = ( 7, 13, 14,  3,  0,  6,  9, 10,  1,  2,  8,  5, 11, 12,  4, 15,
           13,  8, 11,  5,  6, 15,  0,  3,  4,  7,  2, 12,  1, 10, 14,  9,
           10,  6,  9,  0, 12, 11,  7, 13, 15,  1,  3, 14,  5,  2,  8,  4,
            3, 15,  0,  6, 10,  1, 13,  8,  9,  4,  5, 11, 12,  7,  2, 14)

sBox[4] = ( 2, 12,  4,  1,  7, 10, 11,  6,  8,  5,  3, 15, 13,  0, 14,  9,
           14, 11,  2, 12,  4,  7, 13,  1,  5,  0, 15, 10,  3,  9,  8,  6,
            4,  2,  1, 11, 10, 13,  7,  8, 15,  9, 12,  5,  6,  3,  0, 14,
           11,  8, 12,  7,  1, 14,  2, 13,  6, 15,  0,  9, 10,  4,  5,  3)

sBox[5] = (12,  1, 10, 15,  9,  2,  6,  8,  0, 13,  3,  4, 14,  7,  5, 11,
           10, 15,  4,  2,  7, 12,  9,  5,  6,  1, 13, 14,  0, 11,  3,  8,
            9, 14, 15,  5,  2,  8, 12,  3,  7,  0,  4, 10,  1, 13, 11,  6,
            4,  3,  2, 12,  9,  5, 15, 10, 11, 14,  1,  7,  6,  0,  8, 13)

sBox[6] = ( 4, 11,  2, 14, 15,  0,  8, 13,  3, 12,  9,  7,  5, 10,  6,  1,
           13,  0, 11,  7,  4,  9,  1, 10, 14,  3,  5, 12,  2, 15,  8,  6,
            1,  4, 11, 13, 12,  3,  7, 14, 10, 15,  6,  8,  0,  5,  9,  2,
            6, 11, 13,  8,  1,  4, 10,  7,  9,  5,  0, 15, 14,  2,  3, 12)

sBox[7] = (13,  2,  8,  4,  6, 15, 11,  1, 10,  9,  3, 14,  5,  0, 12,  7,
            1, 15, 13,  8, 10,  3,  7,  4, 12,  5,  6, 11,  0, 14,  9,  2,
            7, 11,  4,  1,  9, 12, 14,  2,  0,  6, 10, 13, 15,  3,  5,  8,
            2,  1, 14,  7,  4, 10,  8, 13, 15, 12,  9,  0,  3,  5,  6, 11)

def bit2Byte(bitList):
    """Convert bit list into a byte list"""
    return [int("".join(map(str,bitList[i*8:i*8+8])),2) for i in range(len(bitList)//8)]

def byte2Bit(byteList):
    """Convert byte list into a bit list"""
    return [(byteList[i//8]>>(7-(i%8)))&0x01 for i in range(8*len(byteList))]

def permBitList(inputBitList,permTable):
    """Permute input bit list according to input permutation table"""
    return [inputBitList[e - 1] for e in permTable]

def permByteList(inByteList,permTable):
    """Permute input byte list according to input permutation table"""
    outByteList = (len(permTable)>>3)*[0]
    for index,elem in enumerate(permTable):
        i = index%8
        e = (elem-1)%8
        if i>=e:
            outByteList[index>>3] |= \
                (inByteList[(elem-1)>>3]&(128>>e))>>(i-e)
        else:
            outByteList[index>>3] |= \
                (inByteList[(elem-1)>>3]&(128>>e))<<(e-i)
    return outByteList

def getIndex(inBitList):
    """Permute bits to properly index the S-boxes"""
    return (inBitList[0]<<5)+(inBitList[1]<<3)+ \
           (inBitList[2]<<2)+(inBitList[3]<<1)+ \
           (inBitList[4]<<0)+(inBitList[5]<<4)

def padData(string):
    """Add PKCS5 padding to plaintext"""
    padLength = 8-(len(string)%8)
    return [ord(s) for s in string]+padLength*[padLength]

def unpadData(byteList):
    """Remove PKCS5 padding from plaintext"""
    return "".join(chr(e) for e in byteList[:-byteList[-1]])

def setKey(keyByteList):
    """Generate all sixteen round subkeys"""
    PC1table = (57, 49, 41, 33, 25, 17,  9,
                 1, 58, 50, 42, 34, 26, 18,
                10,  2, 59, 51, 43, 35, 27,
                19, 11,  3, 60, 52, 44, 36,
                63, 55, 47, 39, 31, 23, 15,
                 7, 62, 54, 46, 38, 30, 22,
                14,  6, 61, 53, 45, 37, 29,
                21, 13,  5, 28, 20, 12,  4)

    PC2table= (14, 17, 11, 24,  1,  5,  3, 28,
               15,  6, 21, 10, 23, 19, 12,  4,
               26,  8, 16,  7, 27, 20, 13,  2,
               41, 52, 31, 37, 47, 55, 30, 40,
               51, 45, 33, 48, 44, 49, 39, 56,
               34, 53, 46, 42, 50, 36, 29, 32)

    def leftShift(inKeyBitList,round):
        """Perform one (or two) circular left shift(s) on key"""
        LStable = (1, 1, 2, 2, 2, 2, 2, 2, 1, 2, 2, 2, 2, 2, 2, 1)

        outKeyBitList = 56*[0]
        if LStable[round] == 2:
            outKeyBitList[:26] = inKeyBitList[2:28]
            outKeyBitList[26] = inKeyBitList[0]
            outKeyBitList[27] = inKeyBitList[1]
            outKeyBitList[28:54] = inKeyBitList[30:]
            outKeyBitList[54] = inKeyBitList[28]
            outKeyBitList[55] = inKeyBitList[29]
        else:
            outKeyBitList[:27] = inKeyBitList[1:28]
            outKeyBitList[27] = inKeyBitList[0]
            outKeyBitList[28:55] = inKeyBitList[29:]
            outKeyBitList[55] = inKeyBitList[28]
        return outKeyBitList

    permKeyBitList = permBitList(byte2Bit(keyByteList),PC1table)
    for round in range(16):
        auxBitList = leftShift(permKeyBitList,round)
        subKeyList[round] = bit2Byte(permBitList(auxBitList,PC2table))
        permKeyBitList = auxBitList

def encryptBlock(inputBlock):
    """Encrypt an 8-byte block with already defined key"""
    inputData = permByteList(inputBlock,IPtable)
    leftPart,rightPart = inputData[:4],inputData[4:]
    for round in range(16):
        expRightPart = permByteList(rightPart,EPtable)
        key = subKeyList[round]
        indexList = byte2Bit([i^j for i,j in zip(key,expRightPart)])
        sBoxOutput = 4*[0]
        for nBox in range(4):
            nBox12 = 12*nBox
            leftIndex = getIndex(indexList[nBox12:nBox12+6])
            rightIndex = getIndex(indexList[nBox12+6:nBox12+12])
            sBoxOutput[nBox] = (sBox[nBox<<1][leftIndex]<<4)+ \
                                sBox[(nBox<<1)+1][rightIndex]
        aux = permByteList(sBoxOutput,PFtable)
        newRightPart = [i^j for i,j in zip(aux,leftPart)]
        leftPart = rightPart
        rightPart = newRightPart
    return permByteList(rightPart+leftPart,FPtable)

def decryptBlock(inputBlock):
    """Decrypt an 8-byte block with already defined key"""
    inputData = permByteList(inputBlock,IPtable)
    leftPart,rightPart = inputData[:4],inputData[4:]
    for round in range(16):
        expRightPart = permByteList(rightPart,EPtable)
        key = subKeyList[15-round]
        indexList = byte2Bit([i^j for i,j in zip(key,expRightPart)])
        sBoxOutput = 4*[0]
        for nBox in range(4):
            nBox12 = 12*nBox
            leftIndex = getIndex(indexList[nBox12:nBox12+6])
            rightIndex = getIndex(indexList[nBox12+6:nBox12+12])
            sBoxOutput[nBox] = (sBox[nBox*2][leftIndex]<<4)+ \
                                sBox[nBox*2+1][rightIndex]
        aux = permByteList(sBoxOutput,PFtable)
        newRightPart = [i^j for i,j in zip(aux,leftPart)]
        leftPart = rightPart
        rightPart = newRightPart
    return permByteList(rightPart+leftPart,FPtable)

def encrypt(key, inString):
    """Encrypt plaintext with given key"""
    setKey(key)
    inByteList,outByteList = padData(inString),[]
    for i in range(0,len(inByteList),8):
        outByteList += encryptBlock(inByteList[i:i+8])
    return outByteList

def decrypt(key, inByteList):
    """Decrypt ciphertext with given key"""
    setKey(key)
    outByteList = []
    for i in range(0,len(inByteList),8):
        outByteList += decryptBlock(inByteList[i:i+8])
    return unpadData(outByteList)

To verify this DES implementation, I also wrote a separate Python module (shown below) containing an interesting algorithm proposed by Ron Rivest a long time ago (1985) in the paper Testing Implementation of DES. I also added a simple multi-block test.

import sys
from pyDES import setKey, encryptBlock, decryptBlock, encrypt, decrypt

def sanityCheck1():
    """Tests single-block DES encryption & decryption
       using algorithm proposed by Ronald Rivest
       (http://people.csail.mit.edu/rivest/Destest.txt)"""
    x0 =  [0x94, 0x74, 0xb8, 0xe8, 0xc7, 0x3b, 0xca, 0x7d]
    x16 = [0x1b, 0x1a, 0x2d, 0xdb, 0x4c, 0x64, 0x24, 0x38]
    x = x0
    for i in range(16):
        setKey(x)
        if i % 2 == 0:
            x = encryptBlock(x) # if i is even, x[i+1] = E(x[i], x[i)
        else:
            x = decryptBlock(x) # if i is odd, x[i+1] = D(x[i], x[i)
    try:
        assert x == x16
    except AssertionError:
        return False
    return True

def sanityCheck2():
    """Tests multi-block DES encryption and decryption"""
    try:
        key = [0x0f, 0x15, 0x71, 0xc9, 0x47, 0xd9, 0xe8, 0x59]
        plaintext = "The quick brown fox jumps over the lazy dog"
        ciphertext = encrypt(key, plaintext)
        assert decrypt(key, ciphertext) == plaintext
    except AssertionError:
        return False
    return True

def main():
    if sanityCheck1() and sanityCheck2():
        print("All DES tests ok!")
    else:
        sys.exit(1)
    sys.exit()

if __name__ == '__main__':
    main()

One thought on “DES implementation in Python

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