Implementation of DES encryption and decryption

DES.py

#!/usr/bin/env python3

#import sys

PLAINTEXT = "plaintxt" #sys.argv[0]
KEY = "seeecret" #sys.argv[1]

INITIAL_PERMUTATION = [
    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,
]

INVERSE_INITIAL_PERMUTATION = [
    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,
]

PERMUTED_CHOICE_1 = [
    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,
]

PERMUTED_CHOICE_2 = [
    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,
]

EXPANSION_PERMUTATION = [
    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,
]

SUBSTITUTION_BOXES = [
    [
        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
    ],
    [
        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
    ],
    [
        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
    ],
    [
         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
    ],
    [
         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
    ],
    [
        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
    ],
    [
         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
    ],
    [
        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
    ],
]

P_BOX = [
    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,
]

def permutate(input, permutation_table):
    result = [None] * len(permutation_table)

    for i in range(len(permutation_table)):
        result[i] = input[permutation_table[i] - 1]

    return ''.join(result)

def xor(a, b):
    if len(a) != len(b):
        raise Exception("xor: len(a) != len(b)")

    result = ""

    for i in range(len(a)):
        result += '0' if (a[i] == b[i]) else '1'

    return result

def sbox_substitute(input):
    output = ""

    for i in range(8):
        input_slice = input[i * 6:(i + 1) * 6]
        row_bits = input_slice[0] + input_slice[5]
        column_bits = input_slice[1:5]
        row = int(row_bits, 2)
        column = int(column_bits, 2)
        substituted_value = SUBSTITUTION_BOXES[i][row * 16 + column]
        substituted_bits = format(substituted_value, '04b')
        output += substituted_bits

    return output

def swap_32bit(input):
    input_left = input[:32]
    input_right = input[32:]
    output_left = input_right
    output_right = input_left
    output = output_left + output_right

    return output

def des_round(input, round_key):
    input_left = input[:32]
    input_right = input[32:]

    expanded = permutate(input_right, EXPANSION_PERMUTATION)
    xored = xor(expanded, round_key)
    substituted = sbox_substitute(xored)
    transpositioned = permutate(substituted, P_BOX)

    output_left = input_right
    output_right = xor(input_left, transpositioned)
    output = output_left + output_right

    return output

def generate_round_keys(key_56bit):
    round_keys = [None] * 16

    for i in range(16):
        old_key_56bit = key_56bit

        if (i + 1) in [1, 2, 9, 16]:
            shift = 1
        else:
            shift = 2

        # Circular left shift
        old_key_56bit_left = old_key_56bit[:28]
        old_key_56bit_right = old_key_56bit[28:]
        key_56bit_left = old_key_56bit_left[shift:] + old_key_56bit_left[:shift]
        key_56bit_right = old_key_56bit_right[shift:] + old_key_56bit_right[:shift]
        key_56bit = key_56bit_left + key_56bit_right

        round_keys[i] = permutate(key_56bit, PERMUTED_CHOICE_2)

    return round_keys

def encrypt(plaintext, key):
    plaintext_bits = ''.join(format(ord(i), '08b') for i in plaintext)
    key_bits = ''.join(format(ord(i), '08b') for i in key)

    key_permuted = permutate(key_bits, PERMUTED_CHOICE_1)
    round_keys = generate_round_keys(key_permuted)

    round_input = permutate(plaintext_bits, INITIAL_PERMUTATION)

    for i in range(16):
        round_input = des_round(round_input, round_keys[i])

    swapped = swap_32bit(round_input)

    ciphertext_bits = permutate(swapped, INVERSE_INITIAL_PERMUTATION)
    ciphertext = ''.join(chr(int(''.join(x), 2)) for x in zip(*[iter(ciphertext_bits)]*8))
    return ciphertext

def decrypt(ciphertext, key):
    ciphertext_bits = ''.join(format(ord(i), '08b') for i in ciphertext)
    key_bits = ''.join(format(ord(i), '08b') for i in key)

    key_permuted = permutate(key_bits, PERMUTED_CHOICE_1)
    round_keys = generate_round_keys(key_permuted)

    round_input = permutate(ciphertext_bits, INITIAL_PERMUTATION)

    for i in range(16):
        round_input = des_round(round_input, round_keys[15 - i])

    swapped = swap_32bit(round_input)

    plaintext_bits = permutate(swapped, INVERSE_INITIAL_PERMUTATION)
    plaintext = ''.join(chr(int(''.join(x), 2)) for x in zip(*[iter(plaintext_bits)]*8))
    return plaintext

if __name__ == "__main__":
    print("PLAINTEXT:\t" + repr(PLAINTEXT) + "\t(" + ":".join("{:02x}".format(ord(c)) for c in PLAINTEXT) + ")")
    print("KEY:\t\t\t" + repr(KEY) + "\t(" + ":".join("{:02x}".format(ord(c)) for c in KEY) + ")")
    ciphertext = encrypt(PLAINTEXT, KEY)
    print("ENCRYPTED:\t" + repr(ciphertext) + "\t(" + ":".join("{:02x}".format(ord(c)) for c in ciphertext) + ")")
    decrypted = decrypt(ciphertext, KEY)
    print("DECRYPTED:\t" + repr(decrypted) + "\t(" + ":".join("{:02x}".format(ord(c)) for c in decrypted) + ")")