Monday, October 15, 2012

hackyou CTF: Crypto 200

This challenge was a bit more of a traditional crypto challenge (Download Here) with a big clue coming in the form of the name of the challenge, XOROWbIu WbI(|)P. The big thing I got from this was the reference to XOR, enter xortool.py (xortool). At xortool's site they say that the most common character in ASCII is 0x20 so that is the first thing I tested.
xortool.py cry200.txt.enc -c 20
This printed out a key of '\x96\xa4*\xc3\xc4:' which, when applied to the file gave me something close to an answer.
Cong (tula& ons!r hiler=he q' ck b &wn f=1 jum": ove ithe > ........
I noticed that the only every 5th and 6th byte of this message was unintelligible, so I changed the 5th and 6th chars of the key manually in a python script until the message was correct. The script is below.
key = '\x96\xa4*\xc3\x96\x73'
counter = 0
answer = ''
for i in open('cry200.txt', 'rb').read():
    answer += chr(ord(i)^ord(key[counter%6]))
    counter += 1

print answer
Answer: Congratulations! While the quick brown fox jumps over the lazy dog, the plain xor cipher is still very unsecure when the key is much shorter than the message. Your flag: Foxie Dogzie Crypto Pwnd
And that's all there is to it!
-- suntzu_II

1 comment:

  1. I've solved this task using hellman's xortool too :)

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