Java Authenticated Encryption with AES and GCM

AesGcmTest.java

package at.favre.lib.bytes.otherPackage;

import org.junit.Test;

import javax.crypto.Cipher;
import javax.crypto.SecretKey;
import javax.crypto.spec.GCMParameterSpec;
import javax.crypto.spec.SecretKeySpec;
import java.nio.ByteBuffer;
import java.nio.charset.StandardCharsets;
import java.security.SecureRandom;
import java.security.spec.AlgorithmParameterSpec;

import static org.junit.Assert.assertEquals;

/**
 * A simple showcase for encryption and decryption with AES + GCM in Java
 */
public class AesGcmTest {
    private final SecureRandom secureRandom = new SecureRandom();
    private final static int GCM_IV_LENGTH = 12;

    @Test
    public void testEncryption() throws Exception {
        //create new random key
        byte[] key = new byte[16];
        secureRandom.nextBytes(key);
        SecretKey secretKey = new SecretKeySpec(key, "AES");
        byte[] associatedData = "ProtocolVersion1".getBytes(StandardCharsets.UTF_8); //meta data you want to verify with the secret message

        String message = "the secret message";

        byte[] cipherText = encrypt(message, secretKey, associatedData);
        String decrypted = decrypt(cipherText, secretKey, associatedData);

        assertEquals(message, decrypted);
    }

    /**
     * Encrypt a plaintext with given key.
     *
     * @param plaintext      to encrypt (utf-8 encoding will be used)
     * @param secretKey      to encrypt, must be AES type, see {@link SecretKeySpec}
     * @param associatedData optional, additional (public) data to verify on decryption with GCM auth tag
     * @return encrypted message
     * @throws Exception if anything goes wrong
     */
    private byte[] encrypt(String plaintext, SecretKey secretKey, byte[] associatedData) throws Exception {

        byte[] iv = new byte[GCM_IV_LENGTH]; //NEVER REUSE THIS IV WITH SAME KEY
        secureRandom.nextBytes(iv);
        final Cipher cipher = Cipher.getInstance("AES/GCM/NoPadding");
        GCMParameterSpec parameterSpec = new GCMParameterSpec(128, iv); //128 bit auth tag length
        cipher.init(Cipher.ENCRYPT_MODE, secretKey, parameterSpec);

        if (associatedData != null) {
            cipher.updateAAD(associatedData);
        }

        byte[] cipherText = cipher.doFinal(plaintext.getBytes(StandardCharsets.UTF_8));

        ByteBuffer byteBuffer = ByteBuffer.allocate(iv.length + cipherText.length);
        byteBuffer.put(iv);
        byteBuffer.put(cipherText);
        return byteBuffer.array();
    }

    /**
     * Decrypts encrypted message (see {@link #encrypt(String, SecretKey, byte[])}).
     *
     * @param cipherMessage  iv with ciphertext
     * @param secretKey      used to decrypt
     * @param associatedData optional, additional (public) data to verify on decryption with GCM auth tag
     * @return original plaintext
     * @throws Exception if anything goes wrong
     */
    private String decrypt(byte[] cipherMessage, SecretKey secretKey, byte[] associatedData) throws Exception {
        final Cipher cipher = Cipher.getInstance("AES/GCM/NoPadding");
        //use first 12 bytes for iv
        AlgorithmParameterSpec gcmIv = new GCMParameterSpec(128, cipherMessage, 0, GCM_IV_LENGTH);
        cipher.init(Cipher.DECRYPT_MODE, secretKey, gcmIv);

        if (associatedData != null) {
            cipher.updateAAD(associatedData);
        }
        //use everything from 12 bytes on as ciphertext
        byte[] plainText = cipher.doFinal(cipherMessage, GCM_IV_LENGTH, cipherMessage.length - GCM_IV_LENGTH);

        return new String(plainText, StandardCharsets.UTF_8);
    }
}