CSAW QUALS 2016 - Neo [Crypto]

Competition: CSAW QUALS CTF 2016
Challenge Name: Neo
Type: Crypto
Points: 200 pts
URL: http://crypto.chal.csaw.io:8001/

tl;dr: presenting a multithreaded fork of python-paddingoracle to speed up decryption

Neo was a classic padding oracle challenge where we had to decrypt some ciphertext to get the flag. Since padding oracle challenges are recurrent in CTFs, I thought it would be great to add multithreading to the awesome python-paddingoracle project to decrypt each block in parallel.

The Neo challenge presented us with a website that simply consisted of a single form with the POST parameter matrix-id storing some base64-encoded binary blob (80 bytes in total).

Let's byte-flip that blob to see if we can detect any discrepancies in the server responses:

from cryptopal import byteflip # https://github.com/lanjelot/cryptopal  
from hashlib import md5  
import requests

blob = 'JaaEqilpk1W1ZDErYGkUeZQmxwVGQYHHYnfv+G/S+p5FNCyETblM2hWeq5I5r+7QpUv/iuXVlqEwjKvefmMi9ZuCxWEHsFB5kgKwof4y1ZE='.decode('base64')

def flipit():  
    def submit(s):
        return requests.post('http://crypto.chal.csaw.io:8001/', proxies={'http': ''}, data={'matrix-id': s.encode('base64').strip()})

    for i, r in byteflip(blob, submit):
        print '%03d %d %d %s' % (i, r.status_code, len(r.text), md5(r.text).hexdigest())

We observe a Caught exception during AES decryption... error message in the html response from the 48th byte to the last 80th byte, so we indeed have a CBC padding oracle with blocksize=16:

000 200 3833 25fef633e4bcd7c2ada6f7210d132e1d  
001 200 3833 25fef633e4bcd7c2ada6f7210d132e1d  
048 200 3833 25fef633e4bcd7c2ada6f7210d132e1d  
049 200 4061 56a6fbca3d2ac12a1bb0487f48044d9e  
050 200 4061 3b160cfcc9eaa63449de5a093a7c8865  
079 200 4061 0b39a68b6c43fc825147df37492963f5  

Let's now recover the plaintext and use one thread per block to speed up the decryption process:

from cryptopal import PaddingOracle, PaddingException # https://github.com/lanjelot/cryptopal

class PadBuster(PaddingOracle):  
    def __init__(self):
        self.session = requests.Session()

    def oracle(self, data):
        r = self.session.post('http://crypto.chal.csaw.io:8001/', data={'matrix-id': data.encode('base64').strip()})

        print '%s %d %d %d' % (data.encode('hex'), r.status_code, len(str(r.headers)), len(r.text))

        if 'Caught exception during AES decryption' in r.text:
            raise PaddingException

def decrypt():  
    padbuster = PadBuster()
    print 'Decrypted: %r' % padbuster.decrypt(blob, block_size=16)

Yay we get the flag:

Decrypted: 'flag{what_if_i_told_you_you_solved_the_challenge}\x0f\x0f\x0f\x0f\x0f\x0f\x0f\x0f\x0f\x0f\x0f\x0f\x0f\x0f\x0f'  

Thanks CSAW for delivering a great CTF (as usual). This time TheGoonies ranked 28th! :)



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