# C 105: Two-Time Pad (20 pts + 65 extra)

## 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 3.

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("ascii").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 = bytes.fromhex("130117004512080e100945410f1345000a1b004e") i = 0 plaintext = "" for c in ciphertext: k = key[i] ik = ord(k) ip = c ^ ik p = chr(ip) print(c, k, ik, ip, p) plaintext += p 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: Crib Drag (15 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. This is called the

crib drag attack. Submit either of the plaintext messages as the flag.

# C 105.3: Crib Drag (15 pts)

Both these ciphertexts were encrypted with the same key:
```13070b4954491b53114816450d1b101c100000090952480105044e4b000f4b4e5453 070d0a1e4c0c4e4d1d0f0d54431c10175311480a464604140a5b4e64373722233153 ```
One of the plaintext messages contains the flag.

# C 105.4: Crib Drag (50 pts)

Both these ciphertexts were encrypted with the same key:
```7e5e491142414350595441145c49435719454a1559425e5d4b114b49505c574e555d0016475e5757584f4651524054194149494c0c55595e144d4454195b494b1e19 734558115956551247545947510c5841195851500c535e544b544a58114844435b54495b190c665b540c545c514b1150460c7e76606a7962777c02 ```
One of the plaintext messages contains the flag.

Posted 1-15-19
Format fixes 2-24-19
Ported to new scoring engine 7-11-19
Updated to Python 3 7-10-2020
Points changed for challenge 2 9-13-2020
Extra points specified 9-17-2020
Points for .2 changed to 15 9-24-2020