def attack_single_byte_xor(ciphertext):
# a variable to keep track of the best candidate so far
best = None
for i in range(2**8): # for every possible key
# converting the key from a number to a byte
candidate_key = i.to_bytes(1, byteorder='big')
# the byte string we will XOR the message against is usually called the "keystream"
keystream = candidate_key * len(ciphertext)
candidate_message = xor(ciphertext, keystream)
ascii_text_chars = list(range(97, 122)) + [32]
nb_letters = sum([x in ascii_text_chars for x in candidate_message])
# if the obtained message has more letters than any other candidate before
if best == None or nb_letters > best['nb_letters']:
# store the current key and message as our best candidate so far
best = {
"message": candidate_message,
'nb_letters': nb_letters,
'key': candidate_key
}
# if the best message is too low quality
if best['nb_letters'] > 0.7 * len(ciphertext):
return best
else:
raise InvalidMessageException('best candidate message is: %s' %
best['message'])
def decrypt_aes_128_cbc(ctxt, iv, key):
result = b''
previous_ctxt_block = iv
blocks = split_bytes_in_blocks(ctxt, blocksize=16)
for block in blocks:
to_xor = decrypt_aes_128_block(block, key)
result += xor(to_xor, previous_ctxt_block)
assert len(result) != 0
# for the next iteration
previous_ctxt_block = block
return pkcs7_strip(result)
def encrypt_aes_128_cbc(msg, iv, key):
result = b''
previous_ctxt_block = iv
padded_ptxt = pkcs7_padding(msg, block_size=16)
blocks = split_bytes_in_blocks(padded_ptxt, blocksize=16)
for block in blocks:
to_encrypt = xor(block, previous_ctxt_block)
new_ctxt_block = encrypt_aes_128_block(to_encrypt, key)
result += new_ctxt_block
# for the next iteration
previous_ctxt_block = new_ctxt_block
return result
def hamming_distance(a, b):
#humming distance is between two strings of equal length is the number of positions at which the corresponding
#symbols are different. In other words, it measures the minimum number of substitutions required to change one
#string into the other, or the minimum number of errors that could have transformed one string into the other
return sum(bin(byte).count('1') for byte in xor(a, b))
def xor_with_key(message, key):
#form keystream, repeat as many times as needed
keystream = key * (len(message) // len(key) + 1)
#XOR
ciphertext = xor(message, keystream)
return hexlify(ciphertext)