AES-256 Cipher – Python Cryptography Examples

#cryptography #programming #python #security

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 pycryptodome, which will give us access to an implementation of AES-256:



pip3 install pycryptodomex

Padding – Handled by GCM

AES-256 typically requires that the data to be encrypted is supplied in 16-byte blocks, and you may have seen that on other sites or tutorials. AES-256 in GCM mode, however, doesn’t require an special padding to be done by us manually.

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 = get_random_bytes(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)

# create cipher config
cipher_config = AES.new(private_key, AES.MODE_GCM)

# return a dictionary with the encrypted text
cipher_text, tag = cipher_config.encrypt_and_digest(bytes(plain_text, 'utf-8'))
return {
    'cipher_text': b64encode(cipher_text).decode('utf-8'),
    'salt': b64encode(salt).decode('utf-8'),
    'nonce': b64encode(cipher_config.nonce).decode('utf-8'),
    'tag': b64encode(tag).decode('utf-8')
}

Notes on encrypt() function

Nonce: A random nonce (arbitrary value) must be a random and unique value for each time our encryption function is used with the same key. Think of it as a random salt for a cipher. The library supplies us with a secure nonce.
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
Tag: The tag is used to authenticate the data when using AES in GCM mode. This ensures no one can change our data without us knowing about it when we decrypt.

Decrypting



def decrypt(enc_dict, password):

    # decode the dictionary entries from base64

    salt = b64decode(enc_dict['salt'])

    cipher_text = b64decode(enc_dict['cipher_text'])

    nonce = b64decode(enc_dict['nonce'])

    tag = b64decode(enc_dict['tag'])

# 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_GCM, nonce=nonce)

# decrypt the cipher text
decrypted = cipher.decrypt_and_verify(cipher_text, tag)

return decrypted

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.tag.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 pycryptodome library


from base64 import b64encode, b64decode

import hashlib

from Cryptodome.Cipher import AES

import os

from Cryptodome.Random import get_random_bytes

  
  
  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 = get_random_bytes(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)

# create cipher config
cipher_config = AES.new(private_key, AES.MODE_GCM)

# return a dictionary with the encrypted text
cipher_text, tag = cipher_config.encrypt_and_digest(bytes(plain_text, 'utf-8'))
return {
    'cipher_text': b64encode(cipher_text).decode('utf-8'),
    'salt': b64encode(salt).decode('utf-8'),
    'nonce': b64encode(cipher_config.nonce).decode('utf-8'),
    'tag': b64encode(tag).decode('utf-8')
}



    

    




def decrypt(enc_dict, password):

    # decode the dictionary entries from base64

    salt = b64decode(enc_dict['salt'])

    cipher_text = b64decode(enc_dict['cipher_text'])

    nonce = b64decode(enc_dict['nonce'])

    tag = b64decode(enc_dict['tag'])

# 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_GCM, nonce=nonce)

# decrypt the cipher text
decrypted = cipher.decrypt_and_verify(cipher_text, tag)

return decrypted



    

    




def main():

    password = input("Password: ")

# First let us encrypt secret message
encrypted = encrypt("The secretest message here", password)
print(encrypted)

# Let us decrypt using our original password
decrypted = decrypt(encrypted, password)
print(bytes.decode(decrypted))



    

    




main()