AES Encryption with AMPScript: Resolving Discrepancies

Question:

We want to encrypt URL parameters on our pages using the EncryptSymmetric() function in AMPScript with the AES algorithm. However, the result of this function does not match the output of standard online AES encryption tools. For example:

AMPScript Code:

%%[ 
SET @encData = EncryptSymmetric("test", "AES", @null, "1111", @null, "0000000000000000", @null, "00000000000000000000000000000000") 
]%%
AES: %%=v(@encData)=%%

The result of this code is AT1r0irnzM+Ax1j8zN+zAw==. However, when we encrypt the same value ("test") with an online AES tool (128-bit), we get l4Yo3sFofFD1EMmtoh397w==. This discrepancy suggests differences between the AES implementation in Marketing Cloud and standard tools.

What are the technical details behind the EncryptSymmetric() function? Is it possible to achieve consistent AES encryption across AMPScript and external tools? How can this be implemented?

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Answer:

The EncryptSymmetric() function in AMPScript uses AES encryption with customizable parameters for key size, initialization vector (IV), and padding. However, its implementation may differ from standard online AES tools. Here’s an in-depth explanation of the problem and how to resolve it.

Issue: The discrepancy arises because encryption standards such as AES rely on configurations like block size, key size, and padding schemes. If these parameters differ, the output will not match.

Solution: To achieve compatibility, ensure that the AES parameters match exactly. The solution involves these key aspects:

1. Key Derivation: Use PBKDF2 (Password-Based Key Derivation Function 2) with a specified number of iterations.
2. Block Size: Set the block size to 16 bytes (128 bits).
3. Key Size: Use a 32-byte (256-bit) key.
4. Padding: Apply PKCS7 padding.

Here’s an example implementation in Python that matches the AMPScript function’s behavior:

Python Code for AES Encryption Compatible with AMPScript:

import base64
from Cryptodome.Cipher import AES
from Cryptodome.Protocol.KDF import PBKDF2
from Cryptodome.Util.Padding import pad

data = b'Example'  # The plaintext to encrypt
password = b'password'  # The password used for encryption
salt = bytes.fromhex('0000000000000000')  # Fixed salt value
iv = bytes.fromhex('00000000000000000000000000000000')  # Fixed IV

if __name__ == '__main__':
    block_size = 16  # Block size for AES
    key_size = 32  # Key size for AES (256 bits)
    key = PBKDF2(password, salt, key_size)  # Derive key with PBKDF2

    # Initialize AES cipher in CBC mode
    cipher = AES.new(key, AES.MODE_CBC, iv)
    ct_bytes = cipher.encrypt(pad(data, block_size))  # Apply PKCS7 padding

    # Encode ciphertext as Base64
    ct_b64 = base64.b64encode(ct_bytes).decode('utf-8')

    result = {
        'data': data.decode('utf-8'),
        'password': password.decode('utf-8'),
        'salt': salt.hex(),
        'iv': iv.hex(),
        'ciphertext': ct_b64,
    }
    print('Expected ciphertext: L2pjkUH92JSH4KyVY0jGxw==')
    print(result)

Explanation:

1. The PBKDF2 function derives a secure key from the password and salt. By default, it uses 1,000 iterations.
2. The AES.new() function creates a new AES cipher object in CBC mode with the derived key and IV.
3. The pad() function ensures that the data conforms to the block size using PKCS7 padding.
4. Finally, the output ciphertext is encoded in Base64 for compatibility with other systems.

Using this implementation, you can ensure the encrypted output matches that generated in AMPScript. Similarly, you can use this approach to decrypt the data by ensuring the same parameters and padding are applied.

Conclusion

Achieving consistent AES encryption with AMPScript and external tools requires meticulous alignment of parameters such as block size, key size, padding, and key derivation. By implementing the correct configuration, as demonstrated in the Python example, you can ensure compatibility and reliability in encrypting and decrypting sensitive data like URL parameters. This alignment not only resolves discrepancies but also strengthens the overall security and integrity of your data encryption process.

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