92 lines
2.6 KiB
Python
92 lines
2.6 KiB
Python
'''
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Solution for https://cryptohack.org/courses/symmetric/bean_counter/
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'''
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import os
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import requests
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from math import ceil
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from Crypto.Cipher import AES
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from imp.math.util import xor_bytes
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class StepUpCounter(object):
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def __init__(self, step_up=False):
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self.value = os.urandom(16).hex()
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self.step = 1
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self.stup = step_up
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def increment(self):
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if self.stup:
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self.newIV = hex(int(self.value, 16) + self.step)
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else:
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self.newIV = hex(int(self.value, 16) - self.stup)
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self.value = self.newIV[2:len(self.newIV)]
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return bytes.fromhex(self.value.zfill(32))
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def __repr__(self):
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self.increment()
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return self.value
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'''
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NOTE: Since step_up is used ONLY as false, we can simplify:
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class StepUpCounter(object):
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def __init__(self):
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self.value = os.urandom(16).hex()
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# SAME AS DOING NOTHING
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def increment(self):
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return self.value()
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def __repr__(self):
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return self.value
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'''
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URL = 'https://aes.cryptohack.org/bean_counter'
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def get_encrypted() -> bytes:
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resp = requests.get(f'{URL}/encrypt/')
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encrypted = bytes.fromhex(resp.json()['encrypted'])
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return encrypted
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PNG_HEADER = [
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'89', '50', '4e', '47',
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'0d', '0a', '1a', '0a',
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'00', '00', '00', '0d',
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'49', '48', '44', '52'
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]
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def main() -> None:
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# NOTE: the counter is constant!
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ctr = StepUpCounter()
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# init = ctr.increment()
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# init_val = ctr.value
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# for i in range(2560000):
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# if ctr.increment() != init:
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# print('Counter Changed')
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# print(i)
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# break
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# elif ctr.value != init_val:
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# print('valued changed')
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# print(i)
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# break
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# PNGs *should* have a constant header, we will use the first 16 bytes
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# to determine what the counter value must be set to
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encrypted = get_encrypted()
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png_header = bytes.fromhex(''.join(PNG_HEADER))
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ctr_value = xor_bytes(encrypted[:16], png_header)
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# apply the counter value to the entire encrypted image (in blocks of 16 bytes)
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BLOCK_SIZE = 16
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with open('bean_flag.png', 'wb') as f:
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for i in range(ceil(len(encrypted) / BLOCK_SIZE)):
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block = encrypted[i*BLOCK_SIZE : (i+1)*BLOCK_SIZE]
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# NOTE: the block we get is not guaranteed to be block size (ie if at end)
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plaintext_block = xor_bytes(block, ctr_value[:len(block)])
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f.write(plaintext_block)
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if __name__ == '__main__':
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try:
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main()
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except (KeyboardInterrupt, EOFError):
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print('\n[!] Interrupt')
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