def break_ctr(ciphertext_bytearr, key):
attack_text = bytearray([97] * len(ciphertext_bytearr))
original_ciphertext_bytearr = copy.copy(ciphertext_bytearr)
attack_ciphertext = seek(ciphertext_bytearr, key, 0, attack_text)
keystream = xor(attack_ciphertext, attack_text)
plaintext_bytearr = xor(original_ciphertext_bytearr, keystream)
return plaintext_bytearr
def attack(ciphertexts, reference_letter_frequency):
guessed_keystream = guess_keystream(ciphertexts, reference_letter_frequency)
plaintexts = []
for ciphertext in ciphertexts:
plaintexts.append(xor(ciphertext, guessed_keystream))
return plaintexts
def attack_ciphertext(ciphertext_bytearr, blocksize, key):
ciphertext_bytearr = modify_ciphertext(ciphertext_bytearr, blocksize)
is_successful_decrypt, plaintext_bytearr = decrypt(ciphertext_bytearr, key)
if is_successful_decrypt:
raise Exception("No ASCII error occurred - impossible to find key!")
else:
key = xor(plaintext_bytearr[:blocksize], plaintext_bytearr[2*blocksize:3*blocksize])
return key
def ctr(target, key):
cipher = AES.new(key, AES.MODE_ECB)
result = bytearray()
for i in range(math.ceil(len(target) / len(key))):
ctr_block = i.to_bytes(16, "little")
encrypted_ctr_block = cipher.encrypt(ctr_block)
start = i * len(key)
stop = min(start + len(key), len(target))
result = result + xor(target[start:stop], encrypted_ctr_block)
return result
def seek(ciphertext_bytearr, key, offset, new_text):
cipher = AES.new(key, AES.MODE_ECB)
for i in range(math.ceil(offset + len(new_text) / len(key))):
ctr_block = i.to_bytes(16, "little")
encrypted_ctr_block = cipher.encrypt(ctr_block)
start = i * len(key)
stop = min(start + len(key), offset + len(new_text))
if start >= offset:
ciphertext_bytearr[start:stop] = xor(
new_text[start - offset:stop - offset], encrypted_ctr_block)
return ciphertext_bytearr