AES-128 CBC decryption in Python

Question

I'm trying to implement this code in python (I'm new to python) and it gives me the following error:

AttributeError: 'str' object has no attribute 'decode'

If we remove .decode ('hex') only to avoid such error:

from itertools import product
from Crypto.Cipher import AES
import Crypto.Cipher.AES

key = ('2b7e151628aed2a6abf7158809cf4f3c').decode('hex')
IV = ('000102030405060708090a0b0c0d0e0f').decode('hex')
plaintext1 = ('6bc1bee22e409f96e93d7e117393172a').decode('hex')
plaintext2 = ('ae2d8a571e03ac9c9eb76fac45af8e51').decode('hex')
plaintext3 = ('30c81c46a35ce411e5fbc1191a0a52ef').decode('hex')
cipher = AES.new(key, AES.MODE_CBC, IV)
ciphertext = cipher.encrypt(plaintext1 + plaintext2 + plaintext3)
(ciphertext).encode('hex')
decipher = AES.new(key, AES.MODE_CBC, IV)
plaintext = decipher.decrypt(ciphertext)
(plaintext).encode('hex')

but it gives me the following error:

ValueError: IV must be 16 bytes long

since the algorithm would need the .decode ('hex') that I had to remove

from itertools import product
from Crypto.Cipher import AES
import Crypto.Cipher.AES

key = ('2b7e151628aed2a6abf7158809cf4f3c')
IV = ('000102030405060708090a0b0c0d0e0f')
plaintext1 = ('6bc1bee22e409f96e93d7e117393172a')
plaintext2 = ('ae2d8a571e03ac9c9eb76fac45af8e51')
plaintext3 = ('30c81c46a35ce411e5fbc1191a0a52ef')
cipher = AES.new(key,AES.MODE_CBC,IV)
ciphertext = cipher.encrypt(plaintext1 + plaintext2 + plaintext3)
(ciphertext).encode('hex')
decipher = AES.new(key,AES.MODE_CBC,IV)
plaintext = decipher.decrypt(ciphertext)
(plaintext).encode('hex')

Does anyone have any idea how I could do to make this code work?

Solution

You are using Python 3, not Python 2. You can't use decode() on strings in Python 3, they are already text, so bytes-to-bytes codecs such as 'hex' can't be applied that way.

Use the binascii module instead:

from binascii import hexlify, unhexlify

key = unhexlify('2b7e151628aed2a6abf7158809cf4f3c')
IV = unhexlify('000102030405060708090a0b0c0d0e0f')
plaintext1 = unhexlify('6bc1bee22e409f96e93d7e117393172a')
plaintext2 = unhexlify('ae2d8a571e03ac9c9eb76fac45af8e51')
plaintext3 = unhexlify('30c81c46a35ce411e5fbc1191a0a52ef')

and

ciphertext_hex = hexlify(ciphertext)
# ...
plaintext_hex = hexlify(plaintext)

So to decode from a hex string to bytes, use binascii.unhexlify(), and to encode back to hex, use binascii.hexlify(). Note that you you can't convert data in-place, you do have to store the result back in a variable (or print out the value, etc.).

Demo:

>>> from Crypto.Cipher import AES
>>> import Crypto.Cipher.AES
>>> from binascii import hexlify, unhexlify
>>> key = unhexlify('2b7e151628aed2a6abf7158809cf4f3c')
>>> IV = unhexlify('000102030405060708090a0b0c0d0e0f')
>>> plaintext1 = unhexlify('6bc1bee22e409f96e93d7e117393172a')
>>> plaintext2 = unhexlify('ae2d8a571e03ac9c9eb76fac45af8e51')
>>> plaintext3 = unhexlify('30c81c46a35ce411e5fbc1191a0a52ef')
>>> cipher = AES.new(key,AES.MODE_CBC,IV)
>>> ciphertext = cipher.encrypt(plaintext1 + plaintext2 + plaintext3)
>>> hexlify(ciphertext)
b'7649abac8119b246cee98e9b12e9197d5086cb9b507219ee95db113a917678b273bed6b8e3c1743b7116e69e22229516'
>>> decipher = AES.new(key,AES.MODE_CBC,IV)
>>> plaintext = decipher.decrypt(ciphertext)
>>> plaintext == plaintext1 + plaintext2 + plaintext3  # test if decryption was successful
True
>>> hexlify(plaintext)
b'6bc1bee22e409f96e93d7e117393172aae2d8a571e03ac9c9eb76fac45af8e5130c81c46a35ce411e5fbc1191a0a52ef'

Comments

[Martijn Pieters]

I'd love to hear what is not helpful or wrong about my answer, to deserve a downvote. That way I can improve my answer!

[rene]

Requesting comments on down votes only gets you more down votes ... have another one ... wait ... no ... scrap that ....

[Martijn Pieters]

@rene I clean these comments up periodically; sometimes, just sometimes, I do actually get helpful feedback when I ask for it after a dv. I am known to make mistakes from time to time, after all!

[rene]

Fair enough, I'm allowed one joke a day, right? (I admit it is my poor Dutch humor)

[Martijn Pieters]

@rene: Dutch humour is underrated, in my personal opinion.

[Jon Ander Díez]

Martijn Pieters ♦, I voted positively, you have helped me a lot, without you I would have been looking for the answer for a long time, I am very grateful. Sorry for the misunderstanding