AES CBC mismatch after decryption.

[jeevanbmanoj]

Hi,
Please help with the following issue I'm facing. Looking forward to your inputs
https://stackoverflow.com/questions/49767968/pycryprodome-aes-cbc-mismatch-after-decryption-in-python

[SolarDon]

If you use MODE_ECB instead of MODE_CBC it works. I have several other examples here:
https://github.com/SolarDon/pycryptodome/tree/master/Examples

from Crypto.Cipher import AES
# Padding for the input string --not related to encryption itself.
BLOCK_SIZE = 16  # Bytes
pad = lambda s: s + (BLOCK_SIZE - len(s) % BLOCK_SIZE) * \
                chr(BLOCK_SIZE - len(s) % BLOCK_SIZE)
unpad = lambda s: s[:-ord(s[len(s) - 1:])]

key = b'Sixteen byte key'
data = 'Jeevan B Manoj'.encode("UTF-8")
data = pad(data)
#cipher = AES.new(key, AES.MODE_CBC)
cipher = AES.new(key, AES.MODE_ECB)
print("data before encryption")
print(data)
ciphertext = cipher.encrypt(data)
#cipher = AES.new(key, AES.MODE_CBC)
cipher = AES.new(key, AES.MODE_ECB)
plaintext = cipher.decrypt(ciphertext)
print(unpad(plaintext))

[frispete]

@jeevanbmanoj: you missed the random IV, as noted before.

Guys, why aren't you using AEAD modes here? You get:

• rid of padding
• tamper resistant encryption

On the downside, you need to handle nonce and mac in the process.

>>> from Crypto.Cipher import AES
>>> key = b'Sixteen byte key'
>>> data = 'Jeevan B Manoj'.encode('utf-8')
>>> cipher = AES.new(key, AES.MODE_GCM)
>>> ciphertext, mac = cipher.encrypt_and_digest(data)
>>> # transfer cipher.nonce, ciphertext and mac
>>> cipher = AES.new(key, AES.MODE_GCM, cipher.nonce)
>>> print(cipher.decrypt_and_verify(ciphertext, mac))
b'Jeevan B Manoj'

nonce and mac are always 16 byte values in these schemes, hence they can be combined with the ciphertext easily.

Please read and understand:
https://blog.cryptographyengineering.com/2012/05/19/how-to-choose-authenticated-encryption/
https://www.pycryptodome.org/en/latest/src/examples.html

BTW, using AEAD, the weakest part is the key, where a KDF like Argon2 could improve the situation significantly.

Here's an example, where these components are used together in action:
https://github.com/frispete/keyrings.cryptfile