week2.py

import sys
from operator import methodcaller
import re
import numpy
from Crypto.Cipher import AES
from Crypto.Util import Counter
from Crypto import Random

def main():
	blockSize = 16 
	q1 = cbcDecrypt("140b41b22a29beb4061bda66b6747e14", "4ca00ff4c898d61e1edbf1800618fb2828a226d160dad07883d04e008a7897ee2e4b7465d5290d0c0e6c6822236e1daafb94ffe0c5da05d9476be028ad7c1d81", blockSize)	
	q2 = cbcDecrypt("140b41b22a29beb4061bda66b6747e14", "5b68629feb8606f9a6667670b75b38a5b4832d0f26e1ab7da33249de7d4afc48e713ac646ace36e872ad5fb8a512428a6e21364b0c374df45503473c5242a253", blockSize)	
	q3 = ctrDecrypt("36f18357be4dbd77f050515c73fcf9f2", "69dda8455c7dd4254bf353b773304eec0ec7702330098ce7f7520d1cbbb20fc388d1b0adb5054dbd7370849dbf0b88d393f252e764f1f5f7ad97ef79d59ce29f5f51eeca32eabedd9afa9329", blockSize)	
	q4 = ctrDecrypt("36f18357be4dbd77f050515c73fcf9f2", "770b80259ec33beb2561358a9f2dc617e46218c0a53cbeca695ae45faa8952aa0e311bde9d4e01726d3184c34451", blockSize)	
	
	print "\n\nAnswers:"
	print "Q1. ",q1
	print "Q2. ",q2
	print "Q3. ",q3
	print "Q4. ",q4

	 	
def ctrDecrypt(key, cypherText, blockSize):
	print "\nCTR decryption of key/cypher",key," / ",cypherText
	res1 = ctrDecrypt1(key, cypherText, blockSize)
	res2 = ctrDecrypt2(key, cypherText, blockSize)
	print "1: ", res1 
	print "2: ", res2
	return res1

 
def ctrDecrypt1(key, cypherText, blockSize):
	k = key.decode('hex')
	ct = cypherText.decode('hex')
	iv = ct[:blockSize]
	ct1 = ct[blockSize:]
	ctr = Counter.new(blockSize*8,initial_value=long(iv.encode('hex'),16))
	obj = AES.new(k,AES.MODE_CTR,counter=ctr)
	paddedStr = obj.decrypt(ct1)
	return paddedStr


def ctrDecrypt2(key, cypherText, blockSize):
	cypherTextBlocks =  [cypherText[i:i+(blockSize*2)] for i in range(0, len(cypherText), (blockSize*2))]	
	iv =  long(cypherTextBlocks.pop(0), 16) 

	cypherTextBlocksDecoded = map(methodcaller("decode", "hex"), cypherTextBlocks)
	
	k = key.decode('hex')

	pt = ""

	i = 0
	for c in cypherTextBlocksDecoded:
	 	ctr = hex(iv+i << 64)[2:(2*blockSize)+2]
	 	encIV = AES.new(k, AES.MODE_ECB).encrypt(ctr)	
	 	plaintext =  strxor(encIV, c)
	  	i = i + 1
	  	pt = plaintext + pt
	

	return "?"#pt
  

def cbcDecrypt(key, cypherText, blockSize):
	print "\nCBC decryption of key/cypher",key," / ",cypherText
	res1 = cbcDecrypt1(key, cypherText, blockSize)
	res2 = cbcDecrypt2(key, cypherText, blockSize)
	print "1: ", res1 
	print "2: ", res2
	return res2

def cbcDecrypt1(key, cypherText, blockSize):
	k = key.decode('hex')
	ct = cypherText.decode('hex')
	iv = ct[:blockSize]
	ct1 = ct[blockSize:]
	obj = AES.new(k,AES.MODE_CBC,iv)
	paddedStr = obj.decrypt(ct1)
	paddingAmount = ord(paddedStr[len(paddedStr)-1:])
	return paddedStr[:-paddingAmount]



def cbcDecrypt2(key, cypherText, blockSize):
	cypherTextBlocks =  [cypherText[i:i+(blockSize*2)] for i in range(0, len(cypherText), (blockSize*2))]
	cypherTextBlocksDecoded = map(methodcaller("decode", "hex"), cypherTextBlocks)
	k = key.decode('hex')

	pt = ""

	iter = len(cypherTextBlocksDecoded)
	for c in reversed(cypherTextBlocksDecoded):
		iter = iter - 1
		if(iter > 0):
			cipher = AES.new(k, AES.MODE_ECB).decrypt(c)			
			plaintext = strxor(cipher, cypherTextBlocksDecoded[iter - 1])

			pt = plaintext + pt

	paddingAmount = ord(pt[len(pt)-1:])
			
	return pt[:-paddingAmount]



def strxor(a, b):     
    if len(a) > len(b):
        return "".join([chr(ord(x) ^ ord(y)) for (x, y) in zip(a[:len(b)], b)])
    else:
        return "".join([chr(ord(x) ^ ord(y)) for (x, y) in zip(a, b[:len(a)])])

main()