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from c2 import fixed_xor
from collections import Counter
def frequency_table():
with open('alice.txt', 'r') as f:
chars = {}
while True:
l = f.readline()
if not l: break
for c in l:
chars[c] = chars.get(c, 0) + 1
result = {}
for c, n in chars.items():
result[c] = n / len(chars)
return result
def score(bs, freqs):
return sum(freqs.get(b, 0) for b in bs)
def decode_cipher(x):
freqs = frequency_table()
if not freqs:
raise Error("Cannot decode cipher without a populated frequency table")
x = bytearray.fromhex(x)
num_bytes = len(x)
mx, result, key = 0, None, None
for b in range(0, 1 << 8):
mask = bytearray(b.to_bytes(1, 'big') * num_bytes)
try:
y = fixed_xor(x, mask, decode_hex=False, encode_hex=False).decode('ascii')
except:
continue
test = score(y, freqs)
if test > mx:
result = y
mx = test
key = mask.decode('ascii')
return result
run_tests = False
if run_tests:
print(decode_cipher("1b37373331363f78151b7f2b783431333d78397828372d363c78373e783a393b3736"))
################################################################################
# Answer
################################################################################
"Cooking MC's like a pound of bacon"
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