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--- ic:laboratoare:05 [2017/11/10 18:00]
marios.choudary
+++ ic:laboratoare:05 [2020/11/04 14:00] (current)
philip.dumitru [Exercise 3 (5p)]
@@ Line 1: / Line 1: @@
 ===== Laboratorul 05 - DES =====
+In this lab we'll do some exercises with DES and some of its variants, as we discussed in the last lecture [[https://drive.google.com/file/d/1Fjybv6k5QudRB1bkAi5shVUGlUyTO_0U/view|here]].
-In this lab we'll do some exercises with DES and some of its variants, as we discussed in the last lecture:
+<file python utils.py>
-http://cs.curs.pub.ro/2014/pluginfile.php/13095/mod_resource/content/2/sasc_curs4_5.pdf
+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):
+    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
+# THIS ONE IS NEW
+def hex_2_bytes(hex_data):
+    return bytes.fromhex(hex_data)  # default utf-8
+</file>
 ==== Exercise 1 (2p) ====
@@ Line 21: / Line 80: @@
 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.
+For this, you are given a starter code (see below), implemented using the pycrypto library.
 Perform the following tasks:
@@ Line 27: / Line 86: @@
 === A. Install the pycrypto library ===
-See https://pypi.python.org/pypi/pycrypto.
+<hidden>this is outdated: https://pypi.python.org/pypi/pycrypto.</hidden>
+See https://pypi.org/project/pycryptodome/
 === B. Implement 2DES ===
-Starting from the starter code (see below), write methods to encrypt and decrypt using double-DES (2DES), defined as follows:
+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))
-<quote> 2DES( (k1,k2), m) = DES(k1, DES(k2, m))</quote>
 === C. Test 2DES ===
@@ Line 41: / Line 101: @@
 c2 = '54826ea0937a2c34d47f4595f3844445520c0995331e5d492f55abcf9d8dfadf'
-as hex strings (i.e. you need to decode them to get the actual byte strings to use with DES, e.g. c1.decode('hex'))
 Decrypt them using the following keys:
@@ Line 65: / Line 123: @@
 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)
+m1 = 'Pocainta' (in byte string, i.e. can be used directly with pycrypto DES)
 c1 = '9f98dbd6fe5f785d' (in hex string, you need to hex-decode)
@@ Line 79: / Line 137: @@
 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, I recommend using a list of tuples, where you add new (key,enc) pairs as follows:
+To build a table, we recommend using a list of tuples, where you add new (key,enc) pairs as follows:
 <code>
 tb = []
-tb.append(('keyval', 'encva'))
+tb.append(('keyval', 'encval'))
 </code>
@@ Line 100: / Line 158: @@
 <code python desmitm.py>
-import sys
+from utils import *
-import random
-import string
 from operator import itemgetter
-import time
 import bisect
 from Crypto.Cipher import DES
-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 hexxor(a, b): # xor two hex strings (trims the longer input)
-  ha = a.decode('hex')
-  hb = b.decode('hex')
-  return "".join([chr(ord(x) ^ ord(y)).encode('hex') for (x, y) in zip(ha, hb)])
-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 str2bin(ss):
-  """
-    Transform a string (e.g. 'Hello') into a string of bits
-  """
-  bs = ''
-  for c in ss:
-    bs = bs + bin(ord(c))[2:].zfill(8)
-  return bs
-def str2int(ss):
-  """
-    Transform a string (e.g. 'Hello') into a (long) integer by converting
-    first to a bistream
-  """
-  bs = str2bin(ss)
-  li = int(bs, 2)
-  return li
-def hex2bin(hs):
-  """
-    Transform a hex string (e.g. 'a2') into a string of bits (e.g.10100010)
-  """
-  bs = ''
-  for c in hs:
-    bs = bs + bin(int(c,16))[2:].zfill(4)
-  return bs
-def bin2hex(bs):
-  """
-    Transform a bit string into a hex string
-  """
-  bv = int(bs,2)
-  return int2hexstring(bv)
-def byte2bin(bval):
-  """
-    Transform a byte (8-bit) value into a bitstring
-  """
-  return bin(bval)[2:].zfill(8)
-def int2hexstring(bval):
-  """
-    Transform an int value into a hexstring (even number of characters)
-  """
-  hs = hex(bval)[2:]
-  lh = len(hs)
-  return hs.zfill(lh + lh%2)
 def get_index(a, x):
-  'Locate the leftmost value exactly equal to x in list a'
+    """Locate the leftmost value exactly equal to x in list a"""
-  i = bisect.bisect_left(a, x)
+    i = bisect.bisect_left(a, x)
-  if i != len(a) and a[i] == x:
+    if i != len(a) and a[i] == x:
-    return i
+        return i
-  else:
+    else:
-    return -1
+        return -1
 def des_enc(k, m):
-  """
+    """
-  Encrypt a message m with a key k using DES as follows:
+    Encrypt a message m with a key k using DES as follows:
-  c = DES(k, m)
+    c = DES(k, m)
-  Args:
+    Args:
-    m should be a bytestring (i.e. a sequence of characters such as 'Hello' or '\x02\x04')
+        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.
+        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
+    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.
+    each byte is just a parity bit, giving the actual key of 56 bits, as expected for DES.
-  The parity bits are ignored.
+    The parity bits are ignored.
-  Return:
+    Return:
-    The bytestring ciphertext c
+        The bytestring ciphertext c
-  """
+    """
-  d = DES.new(k)
+    d = DES.new(k, DES.MODE_ECB)
-  c = d.encrypt(m)
+    c = d.encrypt(m)
+    return c
-  return c
 def des_dec(k, c):
-  """
+    """
-  Decrypt a message c with a key k using DES as follows:
+    Decrypt a message c with a key k using DES as follows:
-  m = DES(k, c)
+    m = DES(k, c)
-  Args:
+    Args:
-    c should be a bytestring (i.e. a sequence of characters such as 'Hello' or '\x02\x04')
+        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.
+        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
+    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.
+    each byte is just a parity bit, giving the actual key of 56 bits, as expected for DES.
-  The parity bits are ignored.
+    The parity bits are ignored.
-  Return:
+    Return:
-    The bytestring plaintext m
+        The bytestring plaintext m
-  """
+    """
-  d = DES.new(k)
+    d = DES.new(k, DES.MODE_ECB)
-  m = d.decrypt(c)
+    m = d.decrypt(c)
+    return m
-  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))
-  # Exercitiu pentru test des2_enc
+    print('ciphertext: ' + c1)
-  key1 = 'Smerenie'
+    print('plaintext: ' + m1)
-  key2 = 'Dragoste'
+    print('plaintext in hexa: ' + str_2_hex(m1))
-  m1_given = 'Fericiti cei saraci cu duhul, ca'
-  c1 = 'cda98e4b247612e5b088a803b4277710f106beccf3d020ffcc577ddd889e2f32'
+    # TODO: run meet-in-the-middle attack for the following plaintext/ciphertext
-  # TODO: implement des2_enc and des2_dec
+    m1 = 'Pocainta'
-  m1 = des2_dec(key1, key2, c1.decode('hex'))
+    c1 = '9f98dbd6fe5f785d' # in hex string
+    m2 = 'Iertarea'
-  print 'ciphertext: ' + c1
+    c2 = '6e266642ef3069c2'
-  print 'plaintext: ' + m1
-  print 'plaintext in hexa: ' + m1.encode('hex')
-  # 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:
+    # Note: you only need to search for the first 2 bytes of the each key:
-  k1 = '??oIkvH5'
+    k1 = '??oIkvH5'
-  k2 = '??GK4EoU'
+    k2 = '??GK4EoU'
 if __name__ == "__main__":
-  main()
+    main()
 </code>
-<hidden>The solution is {{:ic:laboratoare:lab5_sol.zip|here}}.</hidden>

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ic/laboratoare/05.1510329601.txt.gz · Last modified: 2017/11/10 18:00 by marios.choudary

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