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Insecure encryption algorithm - Blowfish

Need​

Secure encryption algorithm for data protection

Context​

  • Usage of Python 3 for developing Python applications
  • Usage of pycryptodome for cryptographic operations in Python

Description​

Non compliant code​

from Crypto.Cipher import Blowfish
import os

def encrypt_blowfish(data, key):
    cipher = Blowfish.new(key, Blowfish.MODE_ECB)
    return cipher.encrypt(data)

def decrypt_blowfish(ciphertext, key):
    cipher = Blowfish.new(key, Blowfish.MODE_ECB)
    return cipher.decrypt(ciphertext)

# sample usage
key = os.urandom(16) # generate a random key
data = 'hello world'.encode()

encrypted_data = encrypt_blowfish(data, key)
print(f'Encrypted data: {encrypted_data}')

decrypted_data = decrypt_blowfish(encrypted_data, key)
print(f'Decrypted data: {decrypted_data.decode()}')

In the above code, the encrypt_blowfish and decrypt_blowfish functions are used to encrypt and decrypt data using the Blowfish encryption algorithm.

The encrypt_blowfish function takes in the data and a key as parameters. It creates a new Blowfish cipher object with the given key and the ECB (Electronic Codebook) mode. Then, it encrypts the data using the created cipher and returns the encrypted data.

The decrypt_blowfish function also takes in the encrypted data (ciphertext) and a key as parameters. It creates a new Blowfish cipher object with the given key and the ECB mode. Then, it decrypts the ciphertext using the created cipher and returns the decrypted data.

In the sample usage, a random key is generated using os.urandom(16). Then, a string 'hello world' is encrypted and decrypted using the generated key.

The vulnerability in this code lies in the use of the Blowfish encryption algorithm, which is considered insecure due to its small block size and susceptibility to attacks. Furthermore, the use of ECB mode can lead to patterns in the encrypted data, which can be exploited by attackers.

Steps​

  • Replace the Blowfish encryption algorithm with a more secure one such as AES (Advanced Encryption Standard).
  • Ensure that the new encryption algorithm is used in Cipher Block Chaining (CBC) mode or another secure mode, instead of ECB (Electronic Codebook) mode which is less secure.
  • Use a secure method to generate the encryption key. The os.urandom function is a good choice for this.
  • Update the encrypt and decrypt functions to use the new encryption algorithm and mode.
  • Test the updated code to ensure that it still works as expected and that the encryption is secure.

Compliant code​

from Crypto.Cipher import AES
from Crypto.Util.Padding import pad, unpad
from Crypto.Random import get_random_bytes
import os

BLOCK_SIZE = 16

def encrypt_aes(data, key):
    cipher = AES.new(key, AES.MODE_CBC)
    iv = cipher.iv
    return iv + cipher.encrypt(pad(data, BLOCK_SIZE))

def decrypt_aes(ciphertext, key):
    iv = ciphertext[:BLOCK_SIZE]
    cipher = AES.new(key, AES.MODE_CBC, iv=iv)
    return unpad(cipher.decrypt(ciphertext[BLOCK_SIZE:]), BLOCK_SIZE)

# sample usage
key = os.urandom(16) # generate a random key
data = 'hello world'.encode()

encrypted_data = encrypt_aes(data, key)
print(f'Encrypted data: {encrypted_data}')

decrypted_data = decrypt_aes(encrypted_data, key)
print(f'Decrypted data: {decrypted_data.decode()}')

The updated code replaces the insecure Blowfish encryption algorithm with the more secure AES (Advanced Encryption Standard) algorithm.

The AES algorithm is used in Cipher Block Chaining (CBC) mode, which is more secure than the previously used ECB (Electronic Codebook) mode. In CBC mode, each block of plaintext is XORed with the previous ciphertext block before being encrypted. This way, each ciphertext block depends on all plaintext blocks processed up to that point, which provides more security.

The encryption key is generated using the os.urandom function, which is a secure method for generating random numbers.

The encrypt_aes and decrypt_aes functions have been updated to use the AES algorithm and CBC mode. The data is padded before encryption using the pad function from the Crypto.Util.Padding module, and unpadded after decryption using the unpad function. This is necessary because AES is a block cipher that requires the input to be a multiple of the block size.

The encrypted and decrypted data is printed to the console for testing purposes. The decrypted data is decoded from bytes to a string before being printed.

References​

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