# C 105: Two-Time Pad (15 pts)

## Background

One-time pads are very secure, but not
if you use the pad more than once.
Russia made this error during World War II
and it led to complete compromise of their
encryption system, as detailed in this book.
## What You Need

Any computer with Python 2.7.
## Using a One-Time Pad

Let's use this keystring:
Unbreakable awesome secret secure system

To encrypt this message:
Four score and seven

This Python code does the encryption:
key = "Unbreakable awesome secret secure system"
plaintext = "Four score and seven"
i = 0
ciphertext = ""
for p in plaintext:
ip = ord(p)
k = key[i]
ik = ord(k)
inew = ip ^ ik
ciphertext += chr(inew)
print p, hex(ip), k, hex(ik), hex(inew)
i += 1
print ciphertext.encode("hex")

This program encrypts the string,
as shown below.
Here's a shorter program that does the same
thing, with less output:

key = "Unbreakable awesome secret secure system"
plaintext = "Four score and seven"
i = 0
ciphertext = ""
for p in plaintext:
ciphertext += chr( ord(p) ^ ord(key[i]) )
i += 1
print ciphertext.encode("hex")

This program encrypts the string,
as shown below.
Decrypting uses the same operation,
since XOR reverses itself:

key = "Unbreakable awesome secret secure system"
ciphertext = "130117004512080e100945410f1345000a1b004e".decode("hex")
i = 0
plaintext = ""
for p in ciphertext:
plaintext += chr( ord(p) ^ ord(key[i]) )
i += 1
print plaintext

This string, is decrypted,
as shown below.

## C 105.1: Using a Known Key (5 pts)

Use this key:
Wow, what a great key!

Decrypt this ciphertext:
0e20220c67383c413d7440

The plaintext is the flag.

## C 105.2: Partial Key (10 pts)

The key starts with:
The

Both these ciphertexts were encrypted with the same key:
1a0d134503550e0a1d4542071f56
0d0710071d19490d0e5607004f00074e47

To break this, start with a guess at the key,
like this:
The................................

This shows the first three letters of the
two plaintext messages,
as shown below.
From the partial plaintexts, you can guess another letter
or two, and use those letters to extend the key.
Continue adding one or two letters at a time until you
get the whole plaintext.

The plaintext starting with "N" is the flag.

Posted 1-15-19

Format fixes 2-24-19

Ported to new scoring engine 7-11-19