how to write IvParameterSpec to a file?

Question

I wrote my SecretKey to a file using the following code. Similarly, I have to write my ivParameterSpec to another file. How can I do this?

SecretKey key = KeyGenerator.getInstance("AES").generateKey();
ObjectOutputStream secretkeyOS = new ObjectOutputStream(new FileOutputStream("publicKeyFile"));
secretkeyOS.writeObject(key);
secretkeyOS.close();

AlgorithmParameterSpec paramSpec1 = new IvParameterSpec(iv);
session.setAttribute("secParam", paramSpec1);
ObjectOutputStream paramOS = new ObjectOutputStream(new FileOutputStream("paramFile"));
paramOS.writeObject(paramSpec1);
paramOS.close();

Answer 1

Don't try to store the IvParameterSpec object. It is not serializable, because it is not intended to be stored. The IV is the important part. Store this and create a new IvSpec from the IV. I have changed example code from here for AES encryption to store the IV and use the loaded IV to decrypt the ciphertext so you can see a possible workflow.

Please be aware that this is a minimal example. In a real usecase you would store and load the key as well and exception handling should also be reconsidered :-D

public class Test {
    public static void main(String[] args) throws Exception {
        String message = "This string contains a secret message.";

        // generate a key
        KeyGenerator keygen = KeyGenerator.getInstance("AES");
        keygen.init(128);
        byte[] key = keygen.generateKey().getEncoded();
        SecretKeySpec skeySpec = new SecretKeySpec(key, "AES");

        byte[] iv = { 0, 1, 0, 2, 0, 3, 0, 4, 0, 5, 0, 6, 0, 7, 0, 8 };
        IvParameterSpec ivspec = new IvParameterSpec(iv);

        // initialize the cipher for encrypt mode
        Cipher cipher = Cipher.getInstance("AES/CBC/PKCS5Padding");
        cipher.init(Cipher.ENCRYPT_MODE, skeySpec, ivspec);

        // encrypt the message
        byte[] encrypted = cipher.doFinal(message.getBytes());
        System.out.println("Ciphertext: " + hexEncode(encrypted) + "\n");

        // Write IV
        FileOutputStream fs = new FileOutputStream(new File("paramFile"));
        BufferedOutputStream bos = new BufferedOutputStream(fs);
        bos.write(iv);
        bos.close();

        // Read IV
        byte[] fileData = new byte[16];
        DataInputStream dis = null;

        dis = new DataInputStream(new FileInputStream(new File("paramFile")));
        dis.readFully(fileData);
        if (dis != null) {
            dis.close();
        }

        // reinitialize the cipher for decryption
        cipher.init(Cipher.DECRYPT_MODE, skeySpec, new IvParameterSpec(fileData));

        // decrypt the message
        byte[] decrypted = cipher.doFinal(encrypted);
        System.out.println("Plaintext: " + new String(decrypted) + "\n");
    }

    [...]
}