python – Encrypt & Decrypt using PyCrypto AES 256

python – Encrypt & Decrypt using PyCrypto AES 256

Here is my implementation and works for me with some fixes and enhances the alignment of the key and secret phrase with 32 bytes and iv to 16 bytes:

import base64
import hashlib
from Crypto import Random
from Crypto.Cipher import AES

class AESCipher(object):

    def __init__(self, key): = AES.block_size
        self.key = hashlib.sha256(key.encode()).digest()

    def encrypt(self, raw):
        raw = self._pad(raw)
        iv =
        cipher =, AES.MODE_CBC, iv)
        return base64.b64encode(iv + cipher.encrypt(raw.encode()))

    def decrypt(self, enc):
        enc = base64.b64decode(enc)
        iv = enc[:AES.block_size]
        cipher =, AES.MODE_CBC, iv)
        return self._unpad(cipher.decrypt(enc[AES.block_size:])).decode(utf-8)

    def _pad(self, s):
        return s + ( - len(s) % * chr( - len(s) %

    def _unpad(s):
        return s[:-ord(s[len(s)-1:])]

You may need the following two functions: pad– to pad(when doing encryption) and unpad– to unpad (when doing decryption) when the length of input is not a multiple of BLOCK_SIZE.

BS = 16
pad = lambda s: s + (BS - len(s) % BS) * chr(BS - len(s) % BS) 
unpad = lambda s : s[:-ord(s[len(s)-1:])]

So youre asking the length of key? You can use the md5sum of the key rather than use it directly.

More, according to my little experience of using PyCrypto, the IV is used to mix up the output of a encryption when input is same, so the IV is chosen as a random string, and use it as part of the encryption output, and then use it to decrypt the message.

And heres my implementation, hope it will be useful for you:

import base64
from Crypto.Cipher import AES
from Crypto import Random

class AESCipher:
    def __init__( self, key ):
        self.key = key

    def encrypt( self, raw ):
        raw = pad(raw)
        iv = AES.block_size )
        cipher = self.key, AES.MODE_CBC, iv )
        return base64.b64encode( iv + cipher.encrypt( raw ) ) 

    def decrypt( self, enc ):
        enc = base64.b64decode(enc)
        iv = enc[:16]
        cipher =, AES.MODE_CBC, iv )
        return unpad(cipher.decrypt( enc[16:] ))

python – Encrypt & Decrypt using PyCrypto AES 256

Let me address your question about modes. AES256 is a kind of block cipher. It takes as input a 32-byte key and a 16-byte string, called the block and outputs a block. We use AES in a mode of operation in order to encrypt. The solutions above suggest using CBC, which is one example. Another is called CTR, and its somewhat easier to use:

from Crypto.Cipher import AES
from Crypto.Util import Counter
from Crypto import Random

# AES supports multiple key sizes: 16 (AES128), 24 (AES192), or 32 (AES256).
key_bytes = 32

# Takes as input a 32-byte key and an arbitrary-length plaintext and returns a
# pair (iv, ciphtertext). iv stands for initialization vector.
def encrypt(key, plaintext):
    assert len(key) == key_bytes

    # Choose a random, 16-byte IV.
    iv =

    # Convert the IV to a Python integer.
    iv_int = int(binascii.hexlify(iv), 16) 

    # Create a new Counter object with IV = iv_int.
    ctr = * 8, initial_value=iv_int)

    # Create AES-CTR cipher.
    aes =, AES.MODE_CTR, counter=ctr)

    # Encrypt and return IV and ciphertext.
    ciphertext = aes.encrypt(plaintext)
    return (iv, ciphertext)

# Takes as input a 32-byte key, a 16-byte IV, and a ciphertext, and outputs the
# corresponding plaintext.
def decrypt(key, iv, ciphertext):
    assert len(key) == key_bytes

    # Initialize counter for decryption. iv should be the same as the output of
    # encrypt().
    iv_int = int(iv.encode(hex), 16) 
    ctr = * 8, initial_value=iv_int)

    # Create AES-CTR cipher.
    aes =, AES.MODE_CTR, counter=ctr)

    # Decrypt and return the plaintext.
    plaintext = aes.decrypt(ciphertext)
    return plaintext

(iv, ciphertext) = encrypt(key, hella)
print decrypt(key, iv, ciphertext)

This is often referred to as AES-CTR. I would advise caution in using AES-CBC with PyCrypto. The reason is that it requires you to specify the padding scheme, as exemplified by the other solutions given. In general, if youre not very careful about the padding, there are attacks that completely break encryption!

Now, its important to note that the key must be a random, 32-byte string; a password does not suffice. Normally, the key is generated like so:

# Nominal way to generate a fresh key. This calls the systems random number
# generator (RNG).
key1 =

A key may be derived from a password, too:

# Its also possible to derive a key from a password, but its important that
# the password have high entropy, meaning difficult to predict.
password = This is a rather weak password.

# For added # security, we add a salt, which increases the entropy.
# In this example, we use the same RNG to produce the salt that we used to
# produce key1.
salt_bytes = 8 
salt =

# Stands for Password-based key derivation function 2
key2 = PBKDF2(password, salt, key_bytes)

Some solutions above suggest using SHA256 for deriving the key, but this is generally considered bad cryptographic practice.
Check out wikipedia for more on modes of operation.

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