Homework #1:   Many Time Pad

Due date: Jan. 15, 2025, 11:59pm on Gradescope

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. Your submission should only contain the secret message.

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 python2 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]