decrypt.py

# ******************************************************************************
#
#                            dec_decrypt_aes_part.py
#
# Deccrypt a string using a method based on AES
# 
# STUDENTS YOU MUST ADD CODE TO MAKE THIS WORK.

import sys

# needed for sys.argv

# ------------------------------------------------------------------------------
#
# The text to decode and key to use

text = \
    [132, 201, 141, 74, 140, 94, 141, 140, 141, 15, 31, 164, 90, 229, 201, 141, 78,
     114, 241, 217, 141, 217, 140, 180, 141, 164, 51, 141, 188, 221, 31, 164, 241,
     177, 141, 140, 51, 217, 141, 201, 229, 152, 141, 78, 241, 114, 78, 102, 94,
     141, 74, 152, 31, 152, 141, 94, 201, 31, 164, 102, 164, 51, 90, 141, 201, 229,
     164, 31, 201, 152, 152, 51, 115]

key = 84


# ------------------------------------------------------------------------------
#
# Decrypt the list of integers passed as argument using the integer key passed
# as second argument and return the decrypted form. The first arg is a list of
# integers, one for each byte, e.g. [ 132, 201 ... ]. The second arg is an
# integer <=255 (i.e. it corresponds to a byte) The output returned is a string.

def dec_decrypt(int_list, int_key):
    crypt_bytes = bytearray(int_list)
    crypt_bytes_as_nibbles = dec_convert_bytes_to_nibbles(crypt_bytes)
    # List of bytearrays, each holding four bytes, one for each nibble 

    key_as_nibbles = dec_byte_to_nibbles(int_key)
    # one bytearray, holding four bytes, one for each nibble of the key

    decrypt_bytes_as_nibbles = []
    for nibbles in crypt_bytes_as_nibbles:
        decrypt_bytes_as_nibbles += \
            [dec_decrypt_byte_as_nibbles(nibbles, key_as_nibbles)]

    decrypt_bytes = \
        dec_convert_nibbles_to_bytes(decrypt_bytes_as_nibbles)
    # Integers in fact

    decrypt_string = dec_convert_bytes_to_string(decrypt_bytes)
    return decrypt_string


# ------------------------------------------------------------------------------
#
# Input is a bytearray. Output is a list of quadruples. Each quadruple is a
# bytearray of four elements, one for each nibble.

def dec_convert_bytes_to_nibbles(a_bytearray):
    quadruple_list = []
    for a_byte in a_bytearray:
        quadruple = dec_byte_to_nibbles(a_byte)
        quadruple_list += [quadruple]

    return quadruple_list


# ------------------------------------------------------------------------------
#
# Input is a byte. Output is a bytearray of four bytes, each containing one
# nibble.

def dec_byte_to_nibbles(a_byte):
    nibble4 = a_byte & 0b00000011
    nibble3 = (a_byte >> 2) & 0b00000011
    nibble2 = (a_byte >> 4) & 0b00000011
    nibble1 = (a_byte >> 6) & 0b00000011

    return (bytearray([nibble1, nibble2, nibble3, nibble4]))


# ------------------------------------------------------------------------------
#
# Input is a list of bytearrays, each with four nibbles. Output is a list of
# bytes.

def dec_convert_nibbles_to_bytes(bytes_as_nibbles):
    byte_list = []
    for nibble_quad in bytes_as_nibbles:
        byte_list += [dec_convert_nibbles_to_byte(nibble_quad)]

    return byte_list


# ------------------------------------------------------------------------------
#
# Input is a bytearray of length four containing the four nibbles. Output is
# these nibbles combined into one byte.

def dec_convert_nibbles_to_byte(quadruple_bytearray):
    a_byte = \
        (quadruple_bytearray[0] << 6) | \
        (quadruple_bytearray[1] << 4) | \
        (quadruple_bytearray[2] << 2) | \
        (quadruple_bytearray[3])

    return a_byte


# ------------------------------------------------------------------------------
#
# First argument nibble_quadruple is a bytearray containing four nibbles. This
# corresponds to a byte in the original encrypted text. Second arg
# key_quadruple is the key, also converted to nibbles in a bytearray. This
# function carries out the decryption process and returns the answer as a
# quadruple bytearray. In other words, the bytearray returned is the decrypted
# form of the input nibble_quadruple, done using the key key_quadruple.
# 
# To write this, you need to follow the instructions in the assignment
# specification. You will need to start off by writing dec_xor,
# dec_swap_last_two, dec_s_box, dec_subtract_last_two_first_two. Using the
# command-line interface you can conveniently test these other functions. Then
# you must use them all to define this function.

def dec_decrypt_byte_as_nibbles(nibble_quadruple, key_quadruple):
    # STUDENTS YOU MUST WRITE THIS FUNCTION

    block = dec_xor(nibble_quadruple, key_quadruple)
    block = dec_swap_last_two(block)
    block = dec_s_box(block)
    block = dec_xor(block, key_quadruple)
    block = dec_subtract_last_two_first_two(block)
    block = dec_swap_last_two(block)
    block = dec_s_box(block)
    block = dec_xor(block, key_quadruple)

    return block

    # return (bytearray([1, 0, 0, 1]))
    # THIS LINE IS JUST TO MAKE PROGRAM RUN BEFORE IT IS COMPLETED
    # It corresponds to 0b01000001 - check what ASCII code that is.


# ------------------------------------------------------------------------------
#
# Both arguments are bytearrays of four elements. XORs them and returns the
# result in a new bytearray called ans.

def dec_xor(nibbles, key):
    # STUDENTS YOU MUST WRITE THIS FUNCTION
    ans = bytearray([0, 0, 0, 0])
    for i in range(4):
        ans[i] = nibbles[i] ^ key[i]
    return ans


# ------------------------------------------------------------------------------
#
# Both arguments are bytearrays of four elements. Swaps last two nibbles and
# returns the result in the new bytearray called ans.

def dec_swap_last_two(nibbles):
    # STUDENTS YOU MUST WRITE THIS FUNCTION

    ans = bytearray([0, 0, 0, 0])
    ans = nibbles
    temp = ans[-2]
    ans[-2] = ans[-1]
    ans[-1] = temp

    return ans


# ------------------------------------------------------------------------------
#
# Implements the s box transformation. The input is a bytearray of four
# elements, one for each nibble. We return the answer as a new bytearray
# called ans. 

def dec_s_box(nibbles):
    # STUDENTS YOU MUST WRITE THIS FUNCTION

    ans = bytearray([0, 0, 0, 0])
    ans = nibbles
    for i in range(4):
        if nibbles[i] == 0:
            ans[i] = 2
        elif nibbles[i] == 1:
            ans[i] = 0
        elif nibbles[i] == 2:
            ans[i] = 3
        elif nibbles[i] == 3:
            ans[i] = 1

    return ans


# ------------------------------------------------------------------------------
#
# Subtracts the last two from the first two. Leaves the third and fourth
# (i.e. the last two) the same. Returns the answer in a new bytearray called
# ans. Uses modulo 4 (% 4). In the event of a negative result, % makes it
# positive again. e.g. (3-2)%4=1 (2-3)%4 = +3 . % is a standard python operator.
#
# Explanation of modulo operations on negative numbers:
# http://stackoverflow.com/questions/3883004/negative-numbers-modulo-in-python

def dec_subtract_last_two_first_two(nibbles):
    # STUDENTS YOU MUST WRITE THIS FUNCTION

    ans = bytearray([0, 0, 0, 0])
    ans = nibbles
    ans[0] = (nibbles[0] - nibbles[2]) % 4
    ans[1] = (nibbles[1] - nibbles[3]) % 4

    return ans


# ------------------------------------------------------------------------------
#
# Input is a list of bytes (integers in fact). Converts them to a string and
# returns.

def dec_convert_bytes_to_string(byte_list):
    ans = ''
    for a_byte in byte_list:
        ans += chr(a_byte)

    return ans


# ------------------------------------------------------------------------------
#
# Allow the program to be run from the command line:
#
# python dec_decrypt_aes

if __name__ == '__main__':

    if len(sys.argv) < 2:

        print("Must have another command-line argument")

    elif sys.argv[1] == 'a':

        print("dec_decrypt:")
        print(dec_decrypt(text, key))

    elif sys.argv[1] == 'b':

        ans = \
            dec_xor(dec_byte_to_nibbles(0b10101010),
                    dec_byte_to_nibbles(0b10001000))
        ans2 = dec_convert_nibbles_to_byte(ans)
        print("dec_xor \n10101010 with \n10001000:\n")
        print(format(ans2, '08b'))

    elif sys.argv[1] == 'c':

        ans = \
            dec_swap_last_two(dec_byte_to_nibbles(0b10001000))
        ans2 = dec_convert_nibbles_to_byte(ans)
        print("dec_swap_last_two of \n10001000:\n")
        print(format(ans2, '08b'))

    elif sys.argv[1] == 'd':

        ans = \
            dec_s_box(dec_byte_to_nibbles(0b00011011))
        ans2 = dec_convert_nibbles_to_byte(ans)
        print("dec_s_box of \n00011011:\n")
        print(format(ans2, '08b'))

    elif sys.argv[1] == 'e':

        ans = \
            dec_subtract_last_two_first_two(dec_byte_to_nibbles(0b00011011))
        ans2 = dec_convert_nibbles_to_byte(ans)
        print("dec_subtract_last_two_first_two of \n00011011:\n")
        print(format(ans2, '08b'))

    else:
        print('Bad argument')