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:
- 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.
- R is the block size.
- 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()
Thanks For Reading
Lane on Twitter: @wagslane
Download Qvault: https://qvault.io
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