def test_challenge20(self):
plaintexts = [
base64_to_bytes(line)
for line in utils.readlines('20.txt')
]
key = encryption_key()
nonce = '\0' * 8
ciphertexts = [
ctr_encrypt(m, key, nonce)
for m in plaintexts
]
# Because of the fixed-nonce, the encrypted keystream bytes are
# repeated for every plaintext message.
#
# ciphertext[i] ^ keystream[i] = plaintext[i]
#
# We can create a transposed ciphertext message by concatenating
# ciphertext[i] from every encrypted message and then xor'ing that
# against a guessed keystream byte. Then we can test whether the
# resulting plaintext looks like english based on character
# distributions. If so, then we've figured out the keystream byte.
keystream = ''
for index in itertools.count():
transposed = ''.join(m[index:index+1] for m in ciphertexts)
if not transposed:
break
allowed_chars = None
if index == 0:
allowed_chars = string.ascii_uppercase + '"\''
score, _, key = crack.find_best_single_byte_key(
transposed,
allowed_chars=allowed_chars
)
# print 'Best score for index {}: {}'.format(index, score)
keystream += key[0]
recovered_plaintexts = [
bitops.xor(m, keystream) for m in ciphertexts
]
# for m in recovered_plaintexts:
# print m
self.assertIn(
'\'Cause my girl is definitely mad / \'Cause it took us too long to do this album',
recovered_plaintexts
)
def test_challenge18(self):
ciphertext = base64_to_bytes('L77na/nrFsKvynd6HzOoG7GHTLXsTVu9qvY/2syLXzhPweyyMTJULu/6/kXX0KSvoOLSFQ==')
nonce = '\x00' * 8
plaintext = ctr_decrypt(ciphertext, 'YELLOW SUBMARINE', nonce)
self.assertEqual(
'Yo, VIP Let\'s kick it Ice, Ice, baby Ice, Ice, baby ',
plaintext
)
plaintext = 'the quick brown fox jumped over the lazy dog.'
key = encryption_key()
ciphertext = ctr_encrypt(plaintext, key, nonce)
self.assertEqual(plaintext, ctr_decrypt(ciphertext, key, nonce))
def test_challenge25(self):
ciphertext = convert.base64_to_bytes(utils.read('25.txt'))
plaintext = crypto.ecb_decrypt(ciphertext, 'YELLOW SUBMARINE')
key = crypto.encryption_key()
nonce = '\x00' * 8
ciphertext = crypto.ctr_encrypt(plaintext, key, nonce)
# The edit function lets us recover the original keystream because
# in CTR mode: plaintext ^ keystream = ciphertext and then through
# the magic of xor:
#
# plaintext ^ ciphertext = keystream
#
# Through the edit function, we know the plaintext and ciphertext
# for every index in the byte stream.
keystream = ''
for offset in range(len(ciphertext)):
new_ciphertext = edit(ciphertext, key, nonce, offset, 'A')
keystream += bitops.xor(new_ciphertext[offset], 'A')
recovered_plaintext = bitops.xor(ciphertext, keystream)
self.assertEquals(plaintext, recovered_plaintext)
'U28gZGFyaW5nIGFuZCBzd2VldCBoaXMgdGhvdWdodC4=',
'VGhpcyBvdGhlciBtYW4gSSBoYWQgZHJlYW1lZA==',
'QSBkcnVua2VuLCB2YWluLWdsb3Jpb3VzIGxvdXQu',
'SGUgaGFkIGRvbmUgbW9zdCBiaXR0ZXIgd3Jvbmc=',
'VG8gc29tZSB3aG8gYXJlIG5lYXIgbXkgaGVhcnQs',
'WWV0IEkgbnVtYmVyIGhpbSBpbiB0aGUgc29uZzs=',
'SGUsIHRvbywgaGFzIHJlc2lnbmVkIGhpcyBwYXJ0',
'SW4gdGhlIGNhc3VhbCBjb21lZHk7',
'SGUsIHRvbywgaGFzIGJlZW4gY2hhbmdlZCBpbiBoaXMgdHVybiw=',
'VHJhbnNmb3JtZWQgdXR0ZXJseTo=',
'QSB0ZXJyaWJsZSBiZWF1dHkgaXMgYm9ybi4=',
]
]
key = encryption_key()
nonce = '\x00' * 8
ciphertexts = [
ctr_encrypt(m, key, nonce)
for m in plaintexts
]
translations = Translations(ciphertexts)
view = TranslationsView(translations)
curses.wrapper(view.event_loop)
# Answer: Easter 1916 by Yeats
# Set the line third from the bottom to:
#
# He, too, has been changed in his turn,
#
# To generate the complete keystream
def encrypt_kvps(content, key, nonce):
full_content = ('comment1=cooking%20MCs;userdata=' +
urllib.quote(content) +
';comment2=%20like%20a%20pound%20of%20bacon')
return crypto.ctr_encrypt(full_content, key, nonce)
def edit(ciphertext, key, nonce, offset, plaintext):
orig_plaintext = crypto.ctr_decrypt(ciphertext, key, nonce)
new_plaintext = (orig_plaintext[:offset] + plaintext +
orig_plaintext[offset + len(plaintext):])
return crypto.ctr_encrypt(new_plaintext, key, nonce)