CODEWITHSUNDEEP
python-3.xcryptographypycrypto
Max
Max

Reputation: 219

CBC mode AES encryption problem with Pycrypto

I have the following code running like a charm

from Crypto.Cipher import AES
from Crypto.Util.Padding import pad, unpad

key = b'Sixteen byte key'
data = 'some text to encrypt'.encode("UTF-8")
data = pad(data, AES.block_size)
encryptor = AES.new(key, AES.MODE_CBC)
iv = encryptor.IV
decryptor = AES.new(key, AES.MODE_CBC, IV=iv)

ciphertext = encryptor.encrypt(data)
print(ciphertext)
plaintext = decryptor.decrypt(ciphertext)
print(unpad(plaintext, 16))

But when I try to convert to a function I got an padding error. My adapted code is

def cbc(msg, op):
    key = b'Sixteen byte key'
    encryptor = AES.new(key, AES.MODE_CBC)
    iv = encryptor.IV
    decryptor = AES.new(key, AES.MODE_CBC, IV=iv)
    if op == 1:
        data = msg.encode("UTF-8")
        data = pad(data, AES.block_size)
        ciphertext = encryptor.encrypt(data)
        print(ciphertext)
    else:
        plaintext = decryptor.decrypt(msg)
        print(unpad(plaintext, 16))

My log is 

Traceback (most recent call last):
  File "D:/Google Drive/max/AES.py", line 48, in <module>
    cbc(b'*\xd3\xc1Y\xc2f;\xf0\xc0@\xd9E\xc5x\x11\xb4', 2)
  File "D:/Google Drive/max/AES.py", line 19, in cbc
    print(unpad(plaintext, 16))
  File "C:\Users\Evilmaax\AppData\Local\Programs\Python\Python36\lib\site-packages\Crypto\Util\Padding.py", line 90, in unpad
    raise ValueError("Padding is incorrect.")
ValueError: Padding is incorrect.

Error occurs in else statement when I try to decrypt a byte message like *\xd3\xc1Y\xc2f;\xf0\xc0@\xd9E\xc5x\x11\xb4. Important: This message was generated by the if statement of same function.

Does anyone know why this happens? __________________________________________________________

EDIT: Using Rob Napier's tip (thank you so much) I solved the problem. If you experiencing the same issue, this is a functional version:

def cbc(key, data, op):
if op == 1:
    cipher = AES.new(key, AES.MODE_CBC, iv)
    data = key.encrypt(data)
    print(f"Coded text: {data}")
else:
    decipher = AES.new(key, AES.MODE_CBC, IV=iv)
    print(f'Plaintext: {unpad(decipher.decrypt(data), BLOCK_SIZE).decode("utf-8")}')

Upvotes: 2

Views: 1961

Answers (1)

Rob Napier
Rob Napier

Reputation: 299565

You're generating a random IV, which is good, but you then throw it away, which means you can't decrypt the data.

This line creates an encryptor with a random IV:

encryptor = AES.new(key, AES.MODE_CBC)

This line creates a decryptor with the same IV:

decryptor = AES.new(key, AES.MODE_CBC, IV=iv)

That's fine in your initial code, but you don't save the IV anywhere, so when you put it in a function which you call twice, you're trying to decrypt with a different IV than you encrypted with.

You need to save the IV as part of the cipher text, and use it during decryption. If you want an example of such a format, see the RNCryptor spec, and if you want to see an example in Python, see RNCryptor-python.

Upvotes: 2

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