def chal6():
"""
The text in input/6.txt has been base64 encoded after being
encrypted with repeating-key XOR. Decrypt it.
"""
f = open('input/6.txt')
ciphertext = utils.base64_to_bytes(f.read())
best_english_score = 0
result = ''
keysizes = utils.find_possible_keysizes(ciphertext, 3, 2, 40)
for keysize in keysizes:
key = [0] * keysize
# Split the ciphertext into blocks of length keysize
blocks = [ciphertext[i:i+keysize] for i in range(0,
len(ciphertext), keysize)]
# Transpose the blocks
transposed = [bytes(t) for t in zip_longest(*blocks,
fillvalue=0)]
# Treat each block as if it's been encrypted with a
# repeating-key XOR
for i in range(0, len(transposed)-1):
key[i] = utils.get_popular_byte(transposed[i])
plaintext = _crypto.xor_repeating_key(ciphertext, bytes(key))
english_score = utils.english_score(plaintext.decode('utf-8'))
if english_score > best_english_score:
result = plaintext.decode('utf-8')
def chal5():
"""Encrypt the input under the key 'ICE' using repeating-key XOR."""
IN = "Burning 'em, if you ain't quick and nimble\nI go crazy" \
" when I hear a cymbal"
OUT = '0b3637272a2b2e63622c2e69692a23693a2a3c6324202d623d63343c2a2622' \
'6324272765272a282b2f20430a652e2c652a3124333a653e2b2027630c69' \
'2b20283165286326302e27282f'
KEY = 'ICE'
ciphertext = _crypto.xor_repeating_key(bytes(IN, 'utf-8'),
bytes(KEY, 'utf-8'))
result = utils.bytes_to_hex(ciphertext)
expect(result, OUT)
