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challenge3.py
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challenge3.py
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import base64
from challenge2 import xor
# word is actually a byte array
def score(word):
# from http://www.data-compression.com/english.html
frequencies = {
' ': .1918,
'e': .1202,
't': .0910,
'a': .0812,
'o': .0768,
'i': .0731,
'n': .0695,
's': .0628,
'r': .0602,
'h': .0592,
'd': .0432,
'l': .0398,
'u': .0288,
'c': .0271,
'm': .0261,
'f': .0230,
'y': .0211,
'w': .0209,
'g': .0203,
'p': .0182,
'b': .0149,
'v': .0111,
'k': .0069,
'x': .0017,
'q': .0011,
'j': .0010,
'z': .0007,
}
score = 0
for letter, percent in frequencies.items():
score = score + percent * (word.count(ord(letter)) + word.count(ord(letter.upper())))
score = score - word.count(ord(' ')) * frequencies[' ']
return score
def decrypt(encrypted_bytes, key):
decrypted = ""
for byte in encrypted_bytes:
decrypted = decrypted + chr(byte ^ ord(key))
return decrypted
def guess_single_key(encrypted_bytes):
best_score = 0
best_key = None
for n in range(255):
this_score = score(xor(encrypted_bytes, [n]))
if this_score > best_score:
best_score = this_score
best_key = chr(n)
return best_key
def find_best_string(encrypted):
encrypted = [x for x in base64.b16decode(encrypted, True)]
key = guess_single_key(encrypted)
decrypted = decrypt(encrypted, key)
return (key, decrypted)
if __name__ == "__main__":
key, decrypted = find_best_string("1b37373331363f78151b7f2b783431333d78397828372d363c78373e783a393b3736")
assert(decrypted == "Cooking MC's like a pound of bacon")
print(key, decrypted)