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#!/usr/bin/env python3 | |
from array import array | |
ctfile = './cd/debian-40r9-amd64-businesscard.iso' | |
ptfile = './debian-40r9-amd64-businesscard.iso' | |
flagfile = './flags/flag.txt' | |
with open(ctfile, 'rb') as f1: | |
ctbytes = array('B', f1.read()) | |
with open(ptfile, 'rb') as f2: | |
ptbytes = array('B', f2.read()) | |
with open(flagfile, 'rb') as f3: | |
flagbytes = array('B', f3.read()) | |
offset = 32 # IV is 3 for debian iso, 5 for flag.txt. this is two AES blocks of 16 bytes each for a total of 32 bytes | |
keystream = [ctbytes[i] ^ ptbytes[i] for i in range(len(flagbytes) + offset)] # KPA on the encrypted debian ISO to derive a big chunk of keystream | |
decrypted = [flagbytes[i] ^ keystream[i+offset] for i in range(len(flagbytes))] # using the keystream at the correct offset to decrypt the flag | |
[print(chr(d), end='') for d in decrypted] |
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