Encrypt and decrypt using PyCrypto AES-256

You can get a passphrase out of an arbitrary password by using a cryptographic hash function (NOT Python’s builtin hash) like SHA-1 or SHA-256. Python includes support for both in its standard library:

import hashlib

hashlib.sha1("this is my awesome password").digest() # => a 20 byte string
hashlib.sha256("another awesome password").digest() # => a 32 byte string

You can truncate a cryptographic hash value just by using [:16] or [:24] and it will retain its security up to the length you specify.

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 you’re asking the length of key? You can use the MD5 hash 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 here’s my implementation:

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 = Random.new().read( AES.block_size )
        cipher = AES.new( 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.new(self.key, AES.MODE_CBC, iv )
        return unpad(cipher.decrypt( enc[16:] ))

For the benefit of others, here is my decryption implementation which I got to by combining the answers of @Cyril and @Marcus. This assumes that this coming in via HTTP Request with the encryptedText quoted and base64 encoded.

import base64
import urllib2
from Crypto.Cipher import AES


def decrypt(quotedEncodedEncrypted):
    key = 'SecretKey'

    encodedEncrypted = urllib2.unquote(quotedEncodedEncrypted)

    cipher = AES.new(key)
    decrypted = cipher.decrypt(base64.b64decode(encodedEncrypted))[:16]

    for i in range(1, len(base64.b64decode(encodedEncrypted))/16):
        cipher = AES.new(key, AES.MODE_CBC, base64.b64decode(encodedEncrypted)[(i-1)*16:i*16])
        decrypted += cipher.decrypt(base64.b64decode(encodedEncrypted)[i*16:])[:16]

    return decrypted.strip()

For someone who would like to use urlsafe_b64encode and urlsafe_b64decode, here are the version that’re working for me (after spending some time with the unicode issue)

BS = 16
key = hashlib.md5(settings.SECRET_KEY).hexdigest()[:BS]
pad = lambda s: s + (BS - len(s) % BS) * chr(BS - len(s) % BS)
unpad = lambda s : s[:-ord(s[len(s)-1:])]

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

    def encrypt(self, raw):
        raw = pad(raw)
        iv = Random.new().read(AES.block_size)
        cipher = AES.new(self.key, AES.MODE_CBC, iv)
        return base64.urlsafe_b64encode(iv + cipher.encrypt(raw)) 

    def decrypt(self, enc):
        enc = base64.urlsafe_b64decode(enc.encode('utf-8'))
        iv = enc[:BS]
        cipher = AES.new(self.key, AES.MODE_CBC, iv)
        return unpad(cipher.decrypt(enc[BS:]))

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

BLOCK_SIZE=16
def trans(key):
     return md5.new(key).digest()

def encrypt(message, passphrase):
    passphrase = trans(passphrase)
    IV = Random.new().read(BLOCK_SIZE)
    aes = AES.new(passphrase, AES.MODE_CFB, IV)
    return base64.b64encode(IV + aes.encrypt(message))

def decrypt(encrypted, passphrase):
    passphrase = trans(passphrase)
    encrypted = base64.b64decode(encrypted)
    IV = encrypted[:BLOCK_SIZE]
    aes = AES.new(passphrase, AES.MODE_CFB, IV)
    return aes.decrypt(encrypted[BLOCK_SIZE:])

You can use a scheme like PKCS#7 padding. You can use it instead the previous functions to pad (when doing encryption) and unpad (when doing decryption). I will provide the full source code below.

import base64
import hashlib
from Crypto import Random
from Crypto.Cipher import AES
import pkcs7

class Encryption:

    def __init__(self):
        pass

    def Encrypt(self, PlainText, SecurePassword):
        pw_encode = SecurePassword.encode('utf-8')
        text_encode = PlainText.encode('utf-8')

        key = hashlib.sha256(pw_encode).digest()
        iv = Random.new().read(AES.block_size)

        cipher = AES.new(key, AES.MODE_CBC, iv)
        pad_text = pkcs7.encode(text_encode)
        msg = iv + cipher.encrypt(pad_text)

        EncodeMsg = base64.b64encode(msg)
        return EncodeMsg

    def Decrypt(self, Encrypted, SecurePassword):
        decodbase64 = base64.b64decode(Encrypted.decode("utf-8"))
        pw_encode = SecurePassword.decode('utf-8')

        iv = decodbase64[:AES.block_size]
        key = hashlib.sha256(pw_encode).digest()

        cipher = AES.new(key, AES.MODE_CBC, iv)
        msg = cipher.decrypt(decodbase64[AES.block_size:])
        pad_text = pkcs7.decode(msg)

        decryptedString = pad_text.decode('utf-8')
        return decryptedString

import StringIO
import binascii


def decode(text, k=16):
    nl = len(text)
    val = int(binascii.hexlify(text[-1]), 16)
    if val > k:
        raise ValueError('Input is not padded or padding is corrupt')

    l = nl - val
    return text[:l]


def encode(text, k=16):
    l = len(text)
    output = StringIO.StringIO()
    val = k - (l % k)
    for _ in xrange(val):
        output.write('%02x' % val)
    return text + binascii.unhexlify(output.getvalue())

Another take on this (heavily derived from solutions above) but

• uses null for padding
• does not use lambda (never been a fan)

• tested with python 2.7 and 3.6.5

  #!/usr/bin/python2.7
  # you'll have to adjust for your setup, e.g., #!/usr/bin/python3
  
  
  import base64, re
  from Crypto.Cipher import AES
  from Crypto import Random
  from django.conf import settings
  
  class AESCipher:
      """
        Usage:
        aes = AESCipher( settings.SECRET_KEY[:16], 32)
        encryp_msg = aes.encrypt( 'ppppppppppppppppppppppppppppppppppppppppppppppppppppppp' )
        msg = aes.decrypt( encryp_msg )
        print("'{}'".format(msg))
      """
      def __init__(self, key, blk_sz):
          self.key = key
          self.blk_sz = blk_sz
  
      def encrypt( self, raw ):
          if raw is None or len(raw) == 0:
              raise NameError("No value given to encrypt")
          raw = raw + '' * (self.blk_sz - len(raw) % self.blk_sz)
          raw = raw.encode('utf-8')
          iv = Random.new().read( AES.block_size )
          cipher = AES.new( self.key.encode('utf-8'), AES.MODE_CBC, iv )
          return base64.b64encode( iv + cipher.encrypt( raw ) ).decode('utf-8')
  
      def decrypt( self, enc ):
          if enc is None or len(enc) == 0:
              raise NameError("No value given to decrypt")
          enc = base64.b64decode(enc)
          iv = enc[:16]
          cipher = AES.new(self.key.encode('utf-8'), AES.MODE_CBC, iv )
          return re.sub(b'x00*$', b'', cipher.decrypt( enc[16:])).decode('utf-8')
  

Let me address your question about "modes." AES-256 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 it’s 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 = Random.new().read(AES.block_size)

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

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

    # Create AES-CTR cipher.
    aes = AES.new(key, 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 = Counter.new(AES.block_size * 8, initial_value=iv_int)

    # Create AES-CTR cipher.
    aes = AES.new(key, 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 you’re not very careful about the padding, there are attacks that completely break encryption!

Now, it’s 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 system's random number
# generator (RNG).
key1 = Random.new().read(key_bytes)

A key may be derived from a password, too:

# It's also possible to derive a key from a password, but it's 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 = Random.new().read(salt_bytes)

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

Some solutions above suggest using SHA-256 for deriving the key, but this is generally considered bad cryptographic practice.
Check out Wikipedia for more on modes of operation.

I am grateful for the other answers which inspired me, but it didn’t work for me.

After spending hours trying to figure out how it works, I came up with the implementation below with the newest PyCryptodomex library (it is another story how I managed to set it up behind proxy, on Windows, in a virtualenv… phew)

It is working on your implementation. Remember to write down padding, encoding, and encrypting steps (and vice versa). You have to pack and unpack, keeping in mind the order.

import base64
import hashlib
from Cryptodome.Cipher import AES
from Cryptodome.Random import get_random_bytes

__key__ = hashlib.sha256(b'16-character key').digest()

def encrypt(raw):
    BS = AES.block_size
    pad = lambda s: s + (BS - len(s) % BS) * chr(BS - len(s) % BS)

    raw = base64.b64encode(pad(raw).encode('utf8'))
    iv = get_random_bytes(AES.block_size)
    cipher = AES.new(key= __key__, mode= AES.MODE_CFB,iv= iv)
    return base64.b64encode(iv + cipher.encrypt(raw))

def decrypt(enc):
    unpad = lambda s: s[:-ord(s[-1:])]

    enc = base64.b64decode(enc)
    iv = enc[:AES.block_size]
    cipher = AES.new(__key__, AES.MODE_CFB, iv)
    return unpad(base64.b64decode(cipher.decrypt(enc[AES.block_size:])).decode('utf8'))

See mnothic’s answer.

Compatible UTF-8 encoding:

def _pad(self, s):
    s = s.encode()
    res = s + (self.bs - len(s) % self.bs) * chr(self.bs - len(s) % self.bs).encode()
    return res

I have used both Crypto and PyCryptodomex library and it is blazing fast…

import base64
import hashlib
from Cryptodome.Cipher import AES as domeAES
from Cryptodome.Random import get_random_bytes
from Crypto import Random
from Crypto.Cipher import AES as cryptoAES

BLOCK_SIZE = AES.block_size

key = "my_secret_key".encode()
__key__ = hashlib.sha256(key).digest()
print(__key__)

def encrypt(raw):
    BS = cryptoAES.block_size
    pad = lambda s: s + (BS - len(s) % BS) * chr(BS - len(s) % BS)
    raw = base64.b64encode(pad(raw).encode('utf8'))
    iv = get_random_bytes(cryptoAES.block_size)
    cipher = cryptoAES.new(key= __key__, mode= cryptoAES.MODE_CFB,iv= iv)
    a= base64.b64encode(iv + cipher.encrypt(raw))
    IV = Random.new().read(BLOCK_SIZE)
    aes = domeAES.new(__key__, domeAES.MODE_CFB, IV)
    b = base64.b64encode(IV + aes.encrypt(a))
    return b

def decrypt(enc):
    passphrase = __key__
    encrypted = base64.b64decode(enc)
    IV = encrypted[:BLOCK_SIZE]
    aes = domeAES.new(passphrase, domeAES.MODE_CFB, IV)
    enc = aes.decrypt(encrypted[BLOCK_SIZE:])
    unpad = lambda s: s[:-ord(s[-1:])]
    enc = base64.b64decode(enc)
    iv = enc[:cryptoAES.block_size]
    cipher = cryptoAES.new(__key__, cryptoAES.MODE_CFB, iv)
    b=  unpad(base64.b64decode(cipher.decrypt(enc[cryptoAES.block_size:])).decode('utf8'))
    return b

encrypted_data =encrypt("Hi Steven!!!!!")
print(encrypted_data)
print("=======")
decrypted_data = decrypt(encrypted_data)
print(decrypted_data)

Encryption and decryption of Latin and special characters (Chinese) using AES-256 with utf8mb4:

For those who need to encrypt and decrypt Latin and special values, such as Chinese, here is a modification of the @MIkee code to do this task.

Remembering that UTF-8 alone does not handle this type of encoding.

import base64, re
from Crypto.Cipher import AES
from Crypto import Random
from django.conf import settings

import codecs

# Make utf8mb4 recognizable.
codecs.register(lambda name: codecs.lookup('utf8') if name == 'utf8mb4' else None)


class AESCipher:

    def __init__(self, key, blk_sz):
        self.key = key
        self.blk_sz = blk_sz

    def encrypt( self, raw ):
        # raw is the main value
        if raw is None or len(raw) == 0:
            raise NameError("No value given to encrypt")
        raw = raw + '' * (self.blk_sz - len(raw) % self.blk_sz)
        raw = raw.encode('utf8mb4')
        # Initialization vector to avoid same encrypt for same strings.
        iv = Random.new().read( AES.block_size )
        cipher = AES.new( self.key.encode('utf8mb4'), AES.MODE_CFB, iv )
        return base64.b64encode( iv + cipher.encrypt( raw ) ).decode('utf8mb4')

    def decrypt( self, enc ):
        # enc is the encrypted value
        if enc is None or len(enc) == 0:
            raise NameError("No value given to decrypt")
        enc = base64.b64decode(enc)
        iv = enc[:16]
        # AES.MODE_CFB that allows bigger length or Latin values
        cipher = AES.new(self.key.encode('utf8mb4'), AES.MODE_CFB, iv )
        return re.sub(b'x00*$', b'', cipher.decrypt( enc[16:])).decode('utf8mb4')

Usage:

>>> from django.conf import settings
>>> from aesencryption import AESCipher
>>>
>>> aes = AESCipher(settings.SECRET_KEY[:16], 32)
>>>
>>> value = aes.encrypt('漢字')
>>>
>>> value
'hnuRwBjwAHDp5X0DmMF3lWzbjR0r81WlW9MRrWukgQwTL0ZI88oQaWvMfBM+W87w9JtSTw=='
>>> dec_value = aes.decrypt(value)
>>> dec_value
'漢字'
>>>

The same for Latin letters, such as ã, á, à, â, ã, ç, etc.

Attention point

Bear in mind that if you will store Latin values to your database, you need to set it to allow such type of data. Therefore, if your database is set as utf-8 it will not accept such type of data. You will need to change there as well.

You can use the new django-mirage-field package.

PyCrypto is old and busted.

The cryptography has better support these days.

Here’s another implementation. Note that this returns bytes, you’d need to use base64 to convert them to a string for transmission.

import os
import hashlib
from cryptography.hazmat.primitives.ciphers import Cipher, algorithms, modes
from cryptography.hazmat.backends import default_backend

_BLOCK_SIZE = 16

class AesStringCipher:
    def __init__(self, key):
        self._key = hashlib.sha256(key.encode()).digest()

    def encrypt_str(self, raw:str) -> bytes:
        iv = os.urandom(_BLOCK_SIZE)
        cipher = Cipher(algorithms.AES(self._key), modes.CBC(iv), default_backend())
        encryptor = cipher.encryptor()
        raw = _pad(raw)
        return iv + encryptor.update(raw.encode('utf-8')) + encryptor.finalize()

    def decrypt_str(self, enc:bytes) -> str:
        iv = enc[:_BLOCK_SIZE]
        enc = enc[_BLOCK_SIZE:]
        cipher = Cipher(algorithms.AES(self._key), modes.CBC(iv), default_backend())
        decryptor = cipher.decryptor()
        raw = decryptor.update(enc) + decryptor.finalize()
        raw = raw.decode('utf-8')
        return _unpad(raw)

def _pad(s:str) -> str:
    padding = (_BLOCK_SIZE - (len(s) % _BLOCK_SIZE))
    return s + padding * chr(padding)

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


if __name__ == '__main__':
    cipher = AesStringCipher('my secret password')

    secret_msg = 'this is a super secret msg ...'
    enc_msg = cipher.encrypt_str(secret_msg)
    dec_msg = cipher.decrypt_str(enc_msg)

    assert secret_msg == dec_msg