Flag: can_you_believe_xor_provides_perfect_security?
The challenge flavortext says "Intercepted some messages encrypted using this high-tech algorithm" and provides these messages:
msg1: This message is not important, so I send it twice encrypted and plain.
msg1_enc: XOvjumbdDj+Ny3Dz/dicO2bs/ukv3RsjjN52+Kmdz2hno8PpNdUFKN7DY7apxoZ4baPvpyXCEjyKz3O2vN+LO3jv66Aong==
msg2_enc: Ruz96TLYDmyXx2f5r8WOdXyj56w1wworm4pg/6nZz29g5qqvKtEMdt6Icfq81pR4ae3VsCnFNC6bxn7zq9SwY2fx1bk03x0lms9kya3UnX1t4P6WNdUIOYzDY+/izM01
msg3_enc: QOb4rGbZGGyKwn7kuZGCfnvw664jnEs4kYp0+bPXmmhto+W8NJAEPJHEcvipnc95ffeqoSOQCC2QjWO2r9SOfyj34qA1kAYpjdl28biRjnVx9OuwaA==
The linked script shows that the encryption and decryption mechanisms are just an XOR with a repeating 16-byte key:
import base64
import os
def xor(data, key):
return "".join(map(lambda (x,y): chr(ord(x) ^ ord(y)), zip(data, key*(len(data)/len(key)+1))))
keylen = 16
def generate_key():
return os.urandom(keylen)
def encrypt(data, key):
return base64.b64encode(xor(data, key))
def decrypt(data, key):
return xor(base64.b64decode(data), key)
We have both a message msg1 and the resulting ciphertext msg1_enc.
We know that msg1_enc = msg1 ^ key. Thus:
msg1_enc ^ msg1 = msg1 ^ key ^ msg1 = key
If we XOR msg1 and msg1_enc1, we can recover the key to decrypt the
remaining messages:
msg1 = "This message is not important, so I send it twice encrypted and plain."
msg1_enc = "XOvjumbdDj+Ny3Dz/dicO2bs/ukv3RsjjN52+Kmdz2hno8PpNdUFKN7DY7apxoZ4baPvpyXCEjyKz3O2vN+LO3jv66Aong=="
key = xor(base64.b64decode(msg1_enc), msg1)
key = key[:16]
msg2_enc = "Ruz96TLYDmyXx2f5r8WOdXyj56w1wworm4pg/6nZz29g5qqvKtEMdt6Icfq81pR4ae3VsCnFNC6bxn7zq9SwY2fx1bk03x0lms9kya3UnX1t4P6WNdUIOYzDY+/izM01"
msg3_enc = "QOb4rGbZGGyKwn7kuZGCfnvw664jnEs4kYp0+bPXmmhto+W8NJAEPJHEcvipnc95ffeqoSOQCC2QjWO2r9SOfyj34qA1kAYpjdl28biRjnVx9OuwaA=="
print decrypt(msg2_enc, key)
print decrypt(msg3_enc, key)
Running the revised txt_securer.py, we get:
$ python txt_securer.py
Now the important message with the flag: "flag{can_you_believe_xor_provides_perfect_security?}".
Here is third message, to confuse our oponent, but he can't read this message anyway.
The flag is thus can_you_believe_xor_provides_perfect_security?.