AES-256 Cipher – Python Cryptography Examples

By Lane Wagner – @wagslane on Twitter

Need to encrypt some text with a password or private key in Python? You came to the right place. AES-256 is a solid symmetric cipher that is commonly used to encrypt data for oneself. In other words, the same person who is encrypting the data is typically decrypting it as well (think password manager)

Dependencies

For this tutorial, we will be using Python 3. Make sure you install pycrypto, which will give us access to an implementation of AES-256:

pip install pycrypto

Padding

AES-256 requires that the data to be encrypted is supplied in 16-byte blocks. We will naively add spaces to the end of our ciphertext to satisfy that requirement:

# pad with spaces at the end of the text
# beacuse AES needs 16 byte blocks
def pad(s):
    block_size = 16
    remainder = len(s) % block_size
    padding_needed = block_size - remainder
    return s + padding_needed * ' '

We will also create an unpad() function that strips the extra spaces off after decryption:

# remove the extra spaces at the end
def unpad(s): 
    return s.rstrip()

Encrypting

Now we create a simple encrypt(plain_text, password) function. This function uses the password to encrypt the plain text. Anyone with access to the encrypted text and the password will be able to decrypt it.

def encrypt(plain_text, password):
    # generate a random salt
    salt = os.urandom(AES.block_size)

    # generate a random iv
    iv = Random.new().read(AES.block_size)

    # use the Scrypt KDF to get a private key from the password
    private_key = hashlib.scrypt(password.encode(), salt=salt, n=2**14, r=8, p=1, dklen=32)

    # pad text with spaces to be valid for AES CBC mode
    padded_text = pad(plain_text)
    
    # create cipher config
    cipher_config = AES.new(private_key, AES.MODE_CBC, iv)

    # return a dictionary with the encrypted text
    return {
        'cipher_text': base64.b64encode(cipher_config.encrypt(padded_text)),
        'salt': base64.b64encode(salt),
        'iv': base64.b64encode(iv)
    }

Notes on encrypt() function

IV: Initialization vector. The initialization vector must be random and new for each time our encryption function is used. Think of it as a random salt for a cipher.
Scrypt: Scrypt is used to generate a secure private key from the password. This will make it harder for an attacker to brute-force our encryption.
Salt: A new random salt is used for each run of our encryption. This makes it impossible for an attacker to use precomputed hashes in an attempt to crack the cipher. (see rainbow table)
Scrypt parameters:
1. N is the cost factor. It must be a power of two, and the higher it is the more secure the key, but the more resources it requires to run.
2. R is the block size.
3. P is the parallelization factor, useful for running on multiple cores.
Base64: We encode all of our bytes-type data into base64 for convenient string representation

Decrypting

def decrypt(enc_dict, password):
    # decode the dictionary entries from base64
    salt = base64.b64decode(enc_dict['salt'])
    enc = base64.b64decode(enc_dict['cipher_text'])
    iv = base64.b64decode(enc_dict['iv'])

    # generate the private key from the password and salt
    private_key = hashlib.scrypt(password.encode(), salt=salt, n=2**14, r=8, p=1, dklen=32)

    # create the cipher config
    cipher = AES.new(private_key, AES.MODE_CBC, iv)

    # decrypt the cipher text
    decrypted = cipher.decrypt(enc)

    # unpad the text to remove the added spaces
    original = unpad(decrypted)

    return original

Notes on decrypt() function

The decrypt() function needs the same salt and iv that were used for encryption. We used a dictionary for convenience in parsing, but if we instead wanted one string of ciphertext we could have used a scheme like salt.iv.cipher_text
The configuration parameters on the Scrypt and AES functions need to be the same as the encrypt function.

Give Me The Full Code!

You probably want to see it all work in an example script. Look no further!


# AES 256 encryption/decryption using pycrypto library
 
import base64
import hashlib
from Crypto.Cipher import AES
from Crypto import Random
import os


# pad with spaces at the end of the text
# beacuse AES needs 16 byte blocks
def pad(s):
    block_size = 16
    remainder = len(s) % block_size
    padding_needed = block_size - remainder
    return s + padding_needed * ' '

# remove the extra spaces at the end
def unpad(s): 
    return s.rstrip()
 
def encrypt(plain_text, password):
    # generate a random salt
    salt = os.urandom(AES.block_size)

    # generate a random iv
    iv = Random.new().read(AES.block_size)

    # use the Scrypt KDF to get a private key from the password
    private_key = hashlib.scrypt(password.encode(), salt=salt, n=2**14, r=8, p=1, dklen=32)

    # pad text with spaces to be valid for AES CBC mode
    padded_text = pad(plain_text)
    
    # create cipher config
    cipher_config = AES.new(private_key, AES.MODE_CBC, iv)

    # return a dictionary with the encrypted text
    return {
        'cipher_text': base64.b64encode(cipher_config.encrypt(padded_text)),
        'salt': base64.b64encode(salt),
        'iv': base64.b64encode(iv)
    }
 
 
def decrypt(enc_dict, password):
    # decode the dictionary entries from base64
    salt = base64.b64decode(enc_dict['salt'])
    enc = base64.b64decode(enc_dict['cipher_text'])
    iv = base64.b64decode(enc_dict['iv'])

    # generate the private key from the password and salt
    private_key = hashlib.scrypt(password.encode(), salt=salt, n=2**14, r=8, p=1, dklen=32)

    # create the cipher config
    cipher = AES.new(private_key, AES.MODE_CBC, iv)

    # decrypt the cipher text
    decrypted = cipher.decrypt(enc)

    # unpad the text to remove the added spaces
    original = unpad(decrypted)

    return original

def main():
    password = input("Password: ")
    
    # First let us encrypt secret message
    encrypted = encrypt("The secretest message", password)
    print(encrypted)
    
    # Let us decrypt using our original password
    decrypted = decrypt(encrypted, password)
    print(bytes.decode(decrypted))

Thanks For Reading

Lane on Twitter: @wagslane

Lane on Dev.to: wagslane

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