Week 2 - Programming Assignment

Author: urlan

README.md

@@ -1,4 +1,7 @@
 Course: Cryptography - www.coursera.org
 Professor: Dan Boneh
 
-Week 1: Many Time Pad
+Programmin Assignments exercises implemented on the course.
+
+- Week 1: Many Time Pad
+- Week 2: CBC mode and CTR mode

Week_02/Week 2 - Programming Assignment.pdf

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+import sys
+from Crypto.Cipher import AES
+import os
+import ctypes
+
+BLOCK_SIZE = 32
+
+# Xor two strings of different lengths
+
+def strxor(a, b):
+   if len(a) > len(b):
+      xor_string = "".join([chr(ord(x) ^ ord(y)) for (x, y) in zip(a[:len(b)], b)])
+   else:
+      xor_string = "".join([chr(ord(x) ^ ord(y)) for (x, y) in zip(a, b[:len(a)])]) 
+
+   return xor_string
+
+# Decrypts a cipher text
+
+def decrypt(key, cipher_text):
+
+   num_blocks = len(cipher_text) / len(key)
+
+   iv = cipher_text[0:BLOCK_SIZE].decode('hex')
+
+   aes = AES.new(key.decode('hex'))
+ 
+   index_min = BLOCK_SIZE
+   index_max = index_min + BLOCK_SIZE
+
+   current_xor = iv
+
+   pt = ''
+
+   for i in range(1, num_blocks):
+
+      block_ciphered = cipher_text[index_min:index_max].decode('hex')
+
+      block_decrypted = aes.decrypt(block_ciphered)
+
+      message_block = strxor(block_decrypted, current_xor)
+
+      pt = pt + message_block
+
+      current_xor = block_ciphered
+
+      index_min = index_max
+      index_max = index_max + BLOCK_SIZE
+
+   return pt
+
+# Main function
+
+def main(argv):
+
+   key = argv[1]
+   cipher_text = argv[2]
+
+   pt = decrypt(key, cipher_text)
+
+   print '\nplain text'
+   print pt
+   print '\n'
+
+   return 0
+
+if __name__ == '__main__':
+   sys.exit(main(sys.argv))

Week_02/cbc.py

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+import sys
+from Crypto.Cipher import AES
+import os
+import ctypes
+
+BLOCK_SIZE = 32
+IV_LENGTH = 32
+
+# Convert String to List
+
+def convertStringToList(string):
+   list = []
+
+   for i in range(0, len(string)):
+      list.append(string[i])
+
+   return list
+
+# Convert List to String
+
+def convertListToString(list):
+   string = ''
+
+   for i in range(0, len(list)):
+      string += list[i]
+
+   return string
+
+# Increment IV List by 1
+
+def incrIV(s, i):
+   if i < 0:
+      return ""
+
+   if s[i] == 'f':
+      s[i] = '0'
+      s = incrIV(s, i-1)
+   else:
+      hex_value = int(s[i], 16)
+      hex_value += 1
+      s[i] = hex(hex_value)[2:];
+
+   return s;
+
+def strxor(a, b):
+   if len(a) > len(b):
+      xor_string = "".join([chr(ord(x) ^ ord(y)) for (x, y) in zip(a[:len(b)], b)])
+   else:
+      xor_string = "".join([chr(ord(x) ^ ord(y)) for (x, y) in zip(a, b[:len(a)])]) 
+
+   return xor_string
+
+# Decrypt a cipher text
+
+def decrypt(key, cipher_text):
+
+   num_blocks = len(cipher_text) / len(key)
+
+   iv = cipher_text[0:IV_LENGTH].decode('hex')
+
+   aes = AES.new(key.decode('hex'))
+ 
+   index_min = IV_LENGTH
+   index_max = index_min + BLOCK_SIZE
+
+   current_xor = iv
+
+   pt = ''
+
+   for i in range(1, num_blocks+1):
+
+      block_ciphered = cipher_text[index_min:index_max].decode('hex')
+
+      block_decrypted = aes.encrypt(current_xor)
+
+      message_block = strxor(block_decrypted, block_ciphered)
+
+      print 'message_block'
+      print message_block
+
+      pt = pt + message_block
+
+      list = incrIV(convertStringToList(current_xor.encode('hex')), IV_LENGTH-1)
+
+      current_xor = convertListToString(list).decode('hex')
+
+      print 'current_xor'
+      print current_xor.encode('hex')
+      print ''
+
+      index_min = index_max
+      index_max = index_max + BLOCK_SIZE
+
+   return pt
+
+# Main function
+
+def main(argv):
+
+   key = argv[1]
+   cipher_text = argv[2]
+
+   pt = decrypt(key, cipher_text)
+
+   print '\nplain text'
+   print pt
+   print '\n'
+
+   return 0
+
+if __name__ == '__main__':
+   sys.exit(main(sys.argv))