Skip to main content

Insecure generation of random numbers - Static IV

Need​

Secure generation of random numbers and dynamic initialization vectors

Context​

  • Usage of Java for building cross-platform applications
  • Usage of javax.crypto.Cipher for encryption and decryption operations
  • Usage of IvParameterSpec for specifying an initialization vector (IV) in cryptographic operations
  • Usage of javax.crypto.spec.SecretKeySpec for generating a secret key specification

Description​

Non compliant code​

import javax.crypto.Cipher;
import javax.crypto.spec.IvParameterSpec;
import javax.crypto.spec.SecretKeySpec;

public class EncryptionUtil {

    private static final String ALGORITHM = "AES";
    private static final String MODE = "CBC";
    private static final String PADDING = "PKCS5Padding";
    private static final String IV = "0123456789abcdef"; // static and hardcoded initialization vector

    private static SecretKeySpec getKey(String password) {
        return new SecretKeySpec(password.getBytes(), ALGORITHM);
    }

    public static byte[] encrypt(String data, String password) throws Exception {
        Cipher cipher = Cipher.getInstance(ALGORITHM + "/" + MODE + "/" + PADDING);
        cipher.init(Cipher.ENCRYPT_MODE, getKey(password), new IvParameterSpec(IV.getBytes()));
        return cipher.doFinal(data.getBytes());
    }
}

The EncryptionUtil class in the above code snippet is used for encrypting data with the AES algorithm, CBC mode, and PKCS5Padding. It uses a static, hardcoded initialization vector (IV) for the encryption process.

The vulnerability lies in the usage of a static, hardcoded IV. The IV variable is set as a constant string "0123456789abcdef". This initialization vector is used in the encrypt method when initializing the Cipher object.

The IV in encryption algorithms is used to ensure that the same plaintext encrypted multiple times will result in different ciphertexts. It should be random and unique for each encryption process. By using a static, hardcoded IV, the encryption becomes predictable. If an attacker knows this IV, they can perform reverse engineering attacks to predict the random values that should remain secret.

This vulnerability can lead to serious security risks such as data breaches and unauthorized access to sensitive information. It violates the principle of least predictability which states that the security of a system should not depend on the secrecy of the implementation or its components.

Steps​

  • Avoid using static and hardcoded values for initialization vectors.
  • Generate a random initialization vector for each encryption operation.
  • Use a secure random number generator to generate the random initialization vector.
  • Ensure that the initialization vector is unique for each encryption operation.
  • Store the initialization vector securely along with the encrypted data.
  • Use a secure encryption algorithm and mode, such as AES in CBC mode.
  • Use a secure padding scheme, such as PKCS5Padding.
  • Use a strong and unique encryption key for each encryption operation.
  • Consider using a key derivation function to derive the encryption key from a password.
  • Keep the encryption key secure and protect it from unauthorized access.

Compliant code​

import javax.crypto.Cipher;
import javax.crypto.spec.IvParameterSpec;
import javax.crypto.spec.SecretKeySpec;
import java.security.SecureRandom;

public class EncryptionUtil {

    private static final String ALGORITHM = "AES";
    private static final String MODE = "CBC";
    private static final String PADDING = "PKCS5Padding";
    private static final int IV_LENGTH = 16; // 16 bytes for AES

    private static SecretKeySpec getKey(String password) {
        return new SecretKeySpec(password.getBytes(), ALGORITHM);
    }

    public static byte[] encrypt(String data, String password) throws Exception {
        Cipher cipher = Cipher.getInstance(ALGORITHM + "/" + MODE + "/" + PADDING);

        // Generate a random initialization vector
        SecureRandom secureRandom = new SecureRandom();
        byte[] iv = new byte[IV_LENGTH];
        secureRandom.nextBytes(iv);
        IvParameterSpec ivParameterSpec = new IvParameterSpec(iv);

        cipher.init(Cipher.ENCRYPT_MODE, getKey(password), ivParameterSpec);
        return cipher.doFinal(data.getBytes());
    }
}

The updated code now generates a random initialization vector (IV) for each encryption operation, instead of using a static, hardcoded IV. This is done using the SecureRandom class, which provides a cryptographically strong random number generator (RNG). The size of the IV is set to 16 bytes, which is the block size for AES.

The encrypt method has been updated to generate a new IV for each call. The SecureRandom.nextBytes method is used to fill a byte array with random bytes, which are then used to create an IvParameterSpec object. This object is passed to the Cipher.init method along with the encryption key.

This approach ensures that the IV is unique for each encryption operation, which is a requirement for secure use of CBC mode. The IV does not need to be secret, and can be stored or transmitted in the clear along with the ciphertext.

The encryption key is derived from a password using a simple SecretKeySpec for the sake of this example. In a real-world application, a key derivation function (KDF) should be used to derive the key from the password. The key should be kept secure and protected from unauthorized access.

References​

Last updated on