JDK8 AES-GCM code example

AESGCMUpdateAAD2.java

import javax.crypto.*;
import javax.crypto.spec.GCMParameterSpec;
import java.nio.ByteBuffer;
import java.security.SecureRandom;
import java.util.Arrays;

public class AESGCMUpdateAAD2 {

    // AES-GCM parameters
    public static final int AES_KEY_SIZE = 128; // in bits
    public static final int GCM_NONCE_LENGTH = 12; // in bytes
    public static final int GCM_TAG_LENGTH = 16; // in bytes

    public static void main(String[] args) throws Exception {
        int testNum = 0; // pass

        if (args.length > 0) {
            testNum = Integer.parseInt(args[0]);
            if (testNum <0 || testNum > 3) {
                System.out.println("Usage: java AESGCMUpdateAAD2 [X]");
                System.out.println("X can be 0, 1, 2, 3");
                System.exit(1);
            }
        }
        byte[] input = "Hello AES-GCM World!".getBytes();

        // Initialise random and generate key
        SecureRandom random = SecureRandom.getInstanceStrong();
        KeyGenerator keyGen = KeyGenerator.getInstance("AES");
        keyGen.init(AES_KEY_SIZE, random);
        SecretKey key = keyGen.generateKey();

        // Encrypt
        Cipher cipher = Cipher.getInstance("AES/GCM/NoPadding", "SunJCE");
        final byte[] nonce = new byte[GCM_NONCE_LENGTH];
        random.nextBytes(nonce);
        GCMParameterSpec spec = new GCMParameterSpec(GCM_TAG_LENGTH * 8, nonce);
        cipher.init(Cipher.ENCRYPT_MODE, key, spec);

        byte[] aad = "Whatever I like".getBytes();;
        cipher.updateAAD(aad);

        byte[] cipherText = cipher.doFinal(input);

        // Decrypt; nonce is shared implicitly
        cipher.init(Cipher.DECRYPT_MODE, key, spec);

        // EXPECTED: Uncommenting this will cause an AEADBadTagException when decrypting
        // because AAD value is altered
        if (testNum == 1) aad[1]++;

        cipher.updateAAD(aad);

        // EXPECTED: Uncommenting this will cause an AEADBadTagException when decrypting
        // because the encrypted data has been altered
        if (testNum == 2) cipherText[10]++;

        // EXPECTED: Uncommenting this will cause an AEADBadTagException when decrypting
        // because the tag has been altered
        if (testNum == 3) cipherText[cipherText.length - 2]++;

        try {
            byte[] plainText = cipher.doFinal(cipherText);
            if (testNum != 0) {
                System.out.println("Test Failed: expected AEADBadTagException not thrown");
            } else {
                // check if the decryption result matches
                if (Arrays.equals(input, plainText)) {
                    System.out.println("Test Passed: match!");
                } else {
                    System.out.println("Test Failed: result mismatch!");
                    System.out.println(new String(plainText));
                }
            }
        } catch(AEADBadTagException ex) {
            if (testNum == 0) {
                System.out.println("Test Failed: unexpected ex " + ex);
                ex.printStackTrace();
            } else {
                System.out.println("Test Passed: expected ex " + ex);
            }
        }
    }
}

With the default JDK8 implementation is it necessary to provide the tag length? I think the default itself is 128 bytes and the same is being provided here as well.

For completeness you should also add the IV to the tag, e.g. cipher.updateAAD(nonce);

How to alter key size from 128 to 256? cannot only alter param AES_KEY_SIZE to 256
Thanks!

I know the reason now: https://stackoverflow.com/questions/6481627/java-security-illegal-key-size-or-default-parameters

This code throws a error
java.security.InvalidAlgorithmParameterException: unknown parameter type.
while running the same on JDK 12

For completeness you should also add the IV to the tag, e.g. cipher.updateAAD(nonce);

The nonce is already included in the calculation of the authentication tag. Not really any reason why you would need to include it in AAD as you can't verify it without already having the IV in the first place.