Does AES/CBC really requires IV parameter?

Question

I am writing a simple app to encrypt my message using AES / CBC (mode). As my understanding CBC mode requires IV parameter but I don't know why my code work without IV parameter used. Anyone can explain why? Thanks.

The encrypted message printed: T9KdWxVZ5xStaisXn6llfg== without exception.

public class TestAES {

public static void main(String[] args) {

    try {
        byte[] salt = new byte[8];
        new SecureRandom().nextBytes(salt);

        SecretKeyFactory keyFactory = SecretKeyFactory.getInstance("PBKDF2WithHmacSHA1");
        KeySpec keySpec = new PBEKeySpec("myPassword".toCharArray(), salt, 100, 128);

        SecretKey tmp = keyFactory.generateSecret(keySpec);
        SecretKeySpec key = new SecretKeySpec(tmp.getEncoded(), "AES");

        Cipher enCipher = Cipher.getInstance("AES/CBC/PKCS5Padding");
        enCipher.init(Cipher.ENCRYPT_MODE, key);

        // enCipher.init(Cipher.ENCRYPT_MODE, key, new IvParameterSpec(iv));

        byte[] cipherBytes = enCipher.doFinal("myMessage".getBytes());
        String cipherMsg = BaseEncoding.base64().encode(cipherBytes);

        System.out.println("Encrypted message: " + cipherMsg);

    } catch (Exception ex) {
        ex.printStackTrace();
    }

}
}

Answer 1

When it is used without an IV, for certain types of ciphers including AES, it implicitly uses 0 IV. See Cipher class documentation.

The disadvantage of a null IV (or a deterministic IV) is that it is vulnerable to dictionary attacks. The requirement for IV is to prevent the same plain text block producing the same cipher text every time.