7aaddb3d31
Create a frequency table of alphabetic characters by reading each character in "Alice in Wonderland"; use this frequency table to score cleartext when decoding ciphers. Change-Id: Id322af64d792c15231a1a02794f396c46196c207 Reviewed-on: https://cl.tvl.fyi/c/depot/+/4788 Tested-by: BuildkiteCI Reviewed-by: wpcarro <wpcarro@gmail.com> Autosubmit: wpcarro <wpcarro@gmail.com>
50 lines
1.4 KiB
Python
50 lines
1.4 KiB
Python
from c2 import fixed_xor
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from collections import Counter
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def frequency_table():
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with open('alice.txt', 'r') as f:
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chars = {}
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while True:
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l = f.readline()
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if not l: break
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for c in l:
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chars[c] = chars.get(c, 0) + 1
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result = {}
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for c, n in chars.items():
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result[c] = n / len(chars)
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return result
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def score(bs, freqs):
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return sum(freqs.get(b, 0) for b in bs)
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def decode_cipher(x):
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freqs = frequency_table()
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if not freqs:
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raise Error("Cannot decode cipher without a populated frequency table")
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x = bytearray.fromhex(x)
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num_bytes = len(x)
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mx, result, key = 0, None, None
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for b in range(0, 1 << 8):
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mask = bytearray(b.to_bytes(1, 'big') * num_bytes)
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try:
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y = fixed_xor(x, mask, decode_hex=False, encode_hex=False).decode('ascii')
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except:
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continue
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test = score(y, freqs)
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if test > mx:
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result = y
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mx = test
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key = mask.decode('ascii')
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return result
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run_tests = False
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if run_tests:
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print(decode_cipher("1b37373331363f78151b7f2b783431333d78397828372d363c78373e783a393b3736"))
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################################################################################
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# Answer
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################################################################################
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"Cooking MC's like a pound of bacon"
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