Insecure encryption algorithm - ECB - Python

Insecure encryption algorithm - ECB - Python

Need

Secure encryption algorithm implementation

Context

  • Usage of Python 3 for writing and executing Python code
  • Usage of Crypto for cryptographic operations
  • Usage of base64 for encoding and decoding data in base64 format

Description

Non compliant code

        from Crypto.Cipher import AES
import base64

def insecure_encryption(data, key):
    cipher = AES.new(key, AES.MODE_ECB)
    encoded = base64.b64encode(cipher.encrypt(data))
    return encoded

The above Python code is an example of a common encryption vulnerability. The function insecure_encryption takes a string data and a key as input and returns the encrypted data. The encryption is handled by the Python cryptography library pycrypto.

The vulnerability lies in the use of Electronic Codebook (ECB) mode for the AES encryption. ECB mode is considered insecure for most uses because it does not use an initialization vector (IV). This means that the same plaintext will always produce the same ciphertext, making it vulnerable to pattern analysis.

For example, if the data string contains repetitive information, the output encoded string will also contain repetitive patterns. An attacker can use these patterns to infer information about the original data.

Moreover, ECB mode does not provide serious message confidentiality, and it does not hide data patterns well. In some senses, it doesn't provide serious message confidentiality at all; an attacker who can modify the ciphertext in storage can cause predictable changes to the decrypted plaintext.

In summary, the use of ECB mode in this code makes it vulnerable to various types of cryptographic attacks.

Steps

  • Switch from ECB mode to a more secure mode such as CBC, CFB, OFB, or GCM. These modes use an initialization vector (IV) or nonce to ensure that encrypting the same data with the same key produces different results.
  • Generate a random initialization vector (IV) or nonce for each encryption operation. This should be done using a secure random number generator.
  • Include the IV or nonce with the ciphertext when storing or transmitting it. It does not need to be kept secret and can be stored or transmitted in plaintext alongside the ciphertext.
  • Ensure that the key used for encryption is kept secret and is not hard-coded into the application. It should be generated using a secure method and stored securely.
  • Consider using authenticated encryption modes like GCM that provide both confidentiality and integrity protection. This can help prevent attacks that modify the encrypted data.

Compliant code

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

def secure_encryption(data, key):
    iv = get_random_bytes(16)
    cipher = AES.new(key, AES.MODE_CBC, iv)
    encoded = base64.b64encode(iv + cipher.encrypt(data))
    return encoded

The above code fixes the vulnerability by switching from ECB mode to CBC mode, which is a more secure encryption mode.

In the secure_encryption function, we first generate a random initialization vector (IV) using the get_random_bytes function from the Crypto.Random module. This IV is 16 bytes long, as required for the AES algorithm.

Next, we create a new AES cipher object using the provided key, but this time we specify AES.MODE_CBC instead of AES.MODE_ECB. We also pass the IV to the AES.new function.

When we encrypt the data, we prepend the IV to the ciphertext. This is because the IV needs to be known for decryption, but it doesn't need to be kept secret. By prepending it to the ciphertext, we ensure that it's always available with the ciphertext.

Finally, we encode the IV and ciphertext together using base64 encoding and return the result.

This code assumes that the key argument is a secret key that has been securely generated and stored. It should not be hard-coded into the application. If you need to generate a new key, you can use a function like Crypto.Protocol.KDF.PBKDF2 to derive a key from a password, or get_random_bytes to generate a completely random key.

This code does not provide integrity protection, which would prevent an attacker from modifying the encrypted data without detection. If you need integrity protection, consider using an authenticated encryption mode like GCM.

References