In acest laborator vom face niste exercitii ce folosesc algoritmul DES si variatii ale acestuia discutate la curs. Prezentarea PowerPoint pentru acest laborator poate fi găsită aici.

utils.py

import base64
 
# CONVERSION FUNCTIONS
def _chunks(string, chunk_size):
    for i in range(0, len(string), chunk_size):
        yield string[i:i+chunk_size]
 
def byte_2_bin(bval):
    """
      Transform a byte (8-bit) value into a bitstring
    """
    return bin(bval)[2:].zfill(8)
 
def _hex(x):
    return format(x, '02x')
 
def hex_2_bin(data):
    return ''.join(f'{int(x, 16):08b}' for x in _chunks(data, 2))
 
def str_2_bin(data):
    return ''.join(f'{ord(c):08b}' for c in data)
 
def bin_2_hex(data):
    return ''.join(f'{int(b, 2):02x}' for b in _chunks(data, 8))
 
def str_2_hex(data):
    return ''.join(f'{ord(c):02x}' for c in data)
 
def bin_2_str(data):
    return ''.join(chr(int(b, 2)) for b in _chunks(data, 8))
 
def hex_2_str(data):
    return ''.join(chr(int(x, 16)) for x in _chunks(data, 2))
 
# XOR FUNCTIONS
def strxor(a, b):  # xor two strings, trims the longer input
    return ''.join(chr(ord(x) ^ ord(y)) for (x, y) in zip(a, b))
 
def bitxor(a, b):  # xor two bit-strings, trims the longer input
    return ''.join(str(int(x) ^ int(y)) for (x, y) in zip(a, b))
 
def hexxor(a, b):  # xor two hex-strings, trims the longer input
    return ''.join(_hex(int(x, 16) ^ int(y, 16)) for (x, y) in zip(_chunks(a, 2), _chunks(b, 2)))
 
# BASE64 FUNCTIONS
def b64decode(data):
    return bytes_to_string(base64.b64decode(string_to_bytes(data)))
 
def b64encode(data):
    return bytes_to_string(base64.b64encode(string_to_bytes(data)))
 
# PYTHON3 'BYTES' FUNCTIONS
def bytes_to_string(bytes_data):
    return bytes_data.decode()  # default utf-8
 
def string_to_bytes(string_data):
    return string_data.encode()  # default utf-8

DESX este definit sub forma DESX( (k1,k2,k3), m) = k1 ⊕ DES(k2, m ⊕ k3). Construiti un atac impotriva DESX care sa ruleze intr-un timp proportional cu 2¹²⁰.

De ce urmatoarele scheme nu aduc nicio imbunatatire fata de DES? Justificati!

• a) c = k1 ⊕ DES(k2, m)
• b) c = DES(k2, m ⊕ k1)

The goal of this exercise is to implement the meet-in-the-middle attack on double DES. For this, you are given a starter code (see below), implemented using the pycrypto library.

Perform the following tasks:

See https://pypi.org/project/pycryptodome/

Starting from the starter code (see below), write methods to encrypt and decrypt using double-DES (2DES), defined as follows: 2DES( (k1,k2), m) = DES(k1, DES(k2, m))

You are given the ciphertexts

c1 = 'cda98e4b247612e5b088a803b4277710f106beccf3d020ffcc577ddd889e2f32'

c2 = '54826ea0937a2c34d47f4595f3844445520c0995331e5d492f55abcf9d8dfadf'

Decrypt them using the following keys:

k1 = 'Smerenie'

k2 = 'Dragoste'

The plaintext corresponding to c1 is m1='Fericiti cei saraci cu duhul, ca'. Find the plaintext m2 corresponding to c2.

Decrypt the entire ciphertext (c1 || c2) with k1 and k2 using 2DES and check it matches the messages m1||m2 above.

In this last but most important task, you are given the following ciphertext/plaintext pairs for 2DES with unknown keys:

m1 = 'Pocainta' (in byte string, i.e. can be used directly with pycrypto DES)

c1 = '9f98dbd6fe5f785d' (in hex string, you need to hex-decode)

m2 = 'Iertarea'

c2 = '6e266642ef3069c2'

You also know the last 6-bytes of each key (note we now have a different key than for the previous tasks): k1 (last 6 bytes) = 'oIkvH5' k2 (last 6 bytes) = 'GK4EoU'

Your task is now to find the full keys k1 and k2 by applying the meet-in-the-middle attack over 2DES.

To build a table, we recommend using a list of tuples, where you add new (key,enc) pairs as follows:

tb = []
tb.append(('keyval', 'encval'))

To sort the table, you can do this:

tbs = sorted(tb, key=itemgetter(1))

To search with binary search, first select just the second column (to search the encryptions):

tenc = [value for _,value in tbs]

then use the bisect library (e.g. bisect.bisect_left) https://docs.python.org/2/library/bisect.html

The starter code is this:

desmitm.py

from utils import *
from operator import itemgetter
import bisect
from Crypto.Cipher import DES
 
def get_index(a, x):
    """Locate the leftmost value exactly equal to x in list a"""
    i = bisect.bisect_left(a, x)
    if i != len(a) and a[i] == x:
        return i
    else:
        return -1
 
def des_enc(k, m):
    """
    Encrypt a message m with a key k using DES as follows:
    c = DES(k, m)
 
    Args:
        m should be a bytestring (i.e. a sequence of characters such as 'Hello' or '\x02\x04')
        k should be a bytestring of length exactly 8 bytes.
 
    Note that for DES the key is given as 8 bytes, where the last bit of
    each byte is just a parity bit, giving the actual key of 56 bits, as expected for DES.
    The parity bits are ignored.
 
    Return:
        The bytestring ciphertext c
    """
    d = DES.new(k, DES.MODE_ECB)
    c = d.encrypt(m)
    return c
 
def des_dec(k, c):
    """
    Decrypt a message c with a key k using DES as follows:
    m = DES(k, c)
 
    Args:
        c should be a bytestring (i.e. a sequence of characters such as 'Hello' or '\x02\x04')
        k should be a bytestring of length exactly 8 bytes.
 
    Note that for DES the key is given as 8 bytes, where the last bit of
    each byte is just a parity bit, giving the actual key of 56 bits, as expected for DES.
    The parity bits are ignored.
 
    Return:
        The bytestring plaintext m
    """
    d = DES.new(k, DES.MODE_ECB)
    m = d.decrypt(c)
    return m
 
def main():
    # Exercitiu pentru test des2_enc
    key1 = 'Smerenie'
    key2 = 'Dragoste'
    m1_given = 'Fericiti cei saraci cu duhul, ca'
    c1 = 'cda98e4b247612e5b088a803b4277710f106beccf3d020ffcc577ddd889e2f32'
    # TODO: implement des2_enc and des2_dec
    m1 = des2_dec(key1, key2, hex_2_str(c1))
 
    print('ciphertext: ' + c1)
    print('plaintext: ' + m1)
    print('plaintext in hexa: ' + str_2_hex(m1))
 
    # TODO: run meet-in-the-middle attack for the following plaintext/ciphertext
    m1 = 'Pocainta'
    c1 = '9f98dbd6fe5f785d' # in hex string
    m2 = 'Iertarea'
    c2 = '6e266642ef3069c2'
 
    # Note: you only need to search for the first 2 bytes of the each key:
    k1 = '??oIkvH5'
    k2 = '??GK4EoU'
 
if __name__ == "__main__":
    main()