Homework #0:   Many Time Pad

Due date: Jan. 14, 2019

Let us see what goes wrong when a stream cipher key is used more than once. Below are eleven hex-encoded ciphertexts that are the result of encrypting eleven plaintexts with a stream cipher, all with the same stream cipher key. Your goal is to decrypt the last ciphertext, and submit the secret message within it as solution.

Hint: XOR the ciphertexts together, and consider what happens when a space is XORed with a character in [a-zA-Z].

ciphertext #1:

ciphertext #2:

ciphertext #3:

ciphertext #4:

ciphertext #5:

ciphertext #6:

ciphertext #7:

ciphertext #8:

ciphertext #9:

ciphertext #10:

target ciphertext (decrypt this one):


For completeness, here is the python script used to generate the ciphertexts. (it doesn't matter if you can't read this)

import sys

MSGS = ( ---  11 secret messages  --- )

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 random(size=16):
    return open("/dev/urandom").read(size)

def encrypt(key, msg):
    c = strxor(key, msg)
    print
    print c.encode('hex')
    return c

def main():
    key = random(1024)
    ciphertexts = [encrypt(key, msg) for msg in MSGS]