Java AES GCM Tag mismatch

Question

I have a problem with Java AES GCM encryption. I have followed the instructions on this page to generate the following code:

package aes;

import java.nio.ByteBuffer;
import java.security.SecureRandom;
import javax.crypto.Cipher;
import javax.crypto.spec.GCMParameterSpec;
import javax.crypto.spec.SecretKeySpec;

public class AES {
    private Cipher encryptCipher;
    private Cipher decryptCipher;
    private byte[] key;
    private int keyLength;
    private SecretKeySpec keySpec;
    private SecureRandom random;

    public AES(String key) {
        try {
            this.key = key.getBytes();
            this.keyLength = this.key.length*8; // key length in bits
            this.keySpec = new SecretKeySpec(this.key, "AES");
            this.encryptCipher = Cipher.getInstance("AES/GCM/NoPadding");
            this.decryptCipher = Cipher.getInstance("AES/GCM/NoPadding");
            this.random = SecureRandom.getInstance("SHA1PRNG");
        }
        catch (Exception e) {
            System.err.println(e.getMessage());
        }
    }
    public byte[] encrypt(byte[] plaintext) {
        try {
            byte[] iv = new byte[12]; // create new IV
            random.nextBytes(iv); 
            encryptCipher.init(Cipher.ENCRYPT_MODE, keySpec, new GCMParameterSpec(keyLength, iv));
            byte[] encrypted = encryptCipher.doFinal(plaintext);
            ByteBuffer byteBuffer = ByteBuffer.allocate(iv.length + encrypted.length);
            byteBuffer.put(iv);
            byteBuffer.put(encrypted);
            return byteBuffer.array(); // IV(0)...IV(11) + ENCRYPTED(0)...ENCRYPTED(N)
        }
        catch (Exception e) {
            System.err.println(e.getMessage());
            e.printStackTrace();
            return null;
        }
    }
    public byte[] decrypt(byte[] ciphertext) {
        try {
            ByteBuffer byteBuffer = ByteBuffer.wrap(ciphertext);
            byte[] iv = new byte[12];
            byteBuffer.get(iv);
            byte[] encrypted = new byte[byteBuffer.remaining()];
            byteBuffer.get(encrypted);
            decryptCipher.init(Cipher.DECRYPT_MODE, keySpec, new GCMParameterSpec(keyLength, iv));
            return decryptCipher.doFinal(ciphertext);
        }
        catch (Exception e) {
            System.err.println(e.getMessage());
            e.printStackTrace();
            return null;
        }
    } 
}

In the main, I call the whole thing as follows:

package aes;

public class Main {
    static byte[] plaintext = new byte[] {0x41, 0x42, 0x43, 0x44, 0x45, 0x46, 0x47, 0x48, 0x49, 0x4a, 0x4b, 0x4c, 0x4d, 0x4e, 0x4f, 0x50, 0x51, 0x52, 0x53};
    public static void main(String[] args) {
        AES aes = new AES("Random09Random09");

        byte[] encrypted = aes.encrypt(plaintext);
        byte[] decrypted = aes.decrypt(encrypted);  
    }
}

Now I always get a tag mismatch error like this:

Tag mismatch!
javax.crypto.AEADBadTagException: Tag mismatch!
    at com.sun.crypto.provider.GaloisCounterMode.decryptFinal(GaloisCounterMode.java:578)
    at com.sun.crypto.provider.CipherCore.finalNoPadding(CipherCore.java:1049)
    at com.sun.crypto.provider.CipherCore.doFinal(CipherCore.java:985)
    at com.sun.crypto.provider.CipherCore.doFinal(CipherCore.java:847)
    at com.sun.crypto.provider.AESCipher.engineDoFinal(AESCipher.java:446)
    at javax.crypto.Cipher.doFinal(Cipher.java:2164)
    at aes.AES.decrypt(AES.java:55)
    at aes.Main.main(Main.java:9)

for which I can not determine the cause. The StackTrace is not helpful to me. I would be grateful if someone could help me with that. As you can see in the StackTrace, the error is in decryption in step

doFinal (...)

Answer 1

I finally found the mistake by myself. The mistake was that in the decrypt method I tried to do doFinal (...) on the whole received message and not just on the extracted ciphertext. I left the mistake in my question and post here the relevant (now correct) part of the program. @kelalaka Thank you for your efforts.

    public byte[] decrypt(byte[] ciphertext) {
        try {
            ByteBuffer byteBuffer = ByteBuffer.wrap(ciphertext);
            byte[] iv = new byte[12];
            byteBuffer.get(iv);
            byte[] encrypted = new byte[byteBuffer.remaining()];
            byteBuffer.get(encrypted);
            decryptCipher.init(Cipher.DECRYPT_MODE, keySpec, new GCMParameterSpec(keyLength, iv));
            return decryptCipher.doFinal(encrypted); // here was the mistake
        }
        catch (Exception e) {
            System.err.println(e.getMessage());
            e.printStackTrace();
            return null;
        }
    } 

And the main:

package aes;

public class Main {
    static byte[] plaintext = new byte[] {0x41, 0x42, 0x43, 0x44, 0x45, 0x46, 0x47, 0x48, 0x49, 0x4a, 0x4b, 0x4c, 0x4d, 0x4e, 0x4f, 0x50, 0x51, 0x52, 0x53};
    public static void main(String[] args) {
        AES aes = new AES("Random09Random09");

        byte[] encrypted = aes.encrypt(plaintext);
        byte[] decrypted = aes.decrypt(encrypted);  
        System.out.println("Original:\n" + new String(plaintext) + "\nEncrypted:\n" + new String(encrypted) + "\nDecrypted:\n" + new String(decrypted));
    }
}

The (now correct) output of the program:

Original:
ABCDEFGHIJKLMNOPQRS
Encrypted:
�~q꽕kl�9���&�ZB=�WPU�"�'�H���]:?Bo
Decrypted:
ABCDEFGHIJKLMNOPQRS