Sender and Receiver Message bytes content size mismatch and javax.crypto.AEADBadTagException: Tag mismatch in AES-GCM mode

Question

I used sample AES-GCM sample java code from online.UDP client have to encrypt data using AES-GCM mode and send to sever.UDP server have to receive and decrypt it. I have two problem when i do that

1.I sent "hi" message(plaintext was 2 bytes and after encrypted it was 18 bytes) to server. After ,server received message we do following

DatagramPacket receivePacket = new DatagramPacket(receiveData, receiveData.length);
serverSocket.receive(receivePacket);
byte[] rec=receivePacket.getData();
String receivedData = new String(rec,0,receivePacket.getLength());
System.out.println(receivePacket.getLength());//18 bytes
System.out.println(receivedData.length);// 30 bytes(how???)

why both are not same size???

2.then,when try to decrypt 30 byte data(?), Got exception in following line

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

sample client:

class GCMClient
{
    // 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{        
        DatagramSocket clientSocket = new DatagramSocket();
        InetAddress IPAddress = InetAddress.getByName("192.168.1.8");
        byte[] sendData = new byte[1024];
        byte[] receiveData = new byte[1024];        
        byte[] input = "hi".getBytes(); //2 bytes 

        byte[] keyBytes ="qwertyuiopasdfgh".getBytes();
        SecretKey key = new SecretKeySpec(keyBytes, 0, keyBytes.length, "AES");       
        Cipher cipher = Cipher.getInstance("AES/GCM/NoPadding", "SunJCE");
        byte[] nonce = new byte[GCM_NONCE_LENGTH];
        nonce = "poiuytrewqlk".getBytes();;;        
        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); 
        System.out.println(cipherText.length+ "data sent!!!!!!! "); //18 bytes after encryption
        DatagramPacket sendPacket = new DatagramPacket(cipherText, cipherText.length, IPAddress, 9999);
        clientSocket.send(sendPacket);  
        clientSocket.close();     
   }
}

sample server:

class GCMServer
{
     // 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{ 
        try{
            DatagramSocket serverSocket = new DatagramSocket(9999,InetAddress.getByName("192.168.1.8"));
            byte[] receiveData = new byte[1024];
            byte[] sendData = new byte[1024];
            while(true){           
                DatagramPacket receivePacket = new DatagramPacket(receiveData, receiveData.length);
                receivePacket.setData(new byte[4096]);  
                serverSocket.receive(receivePacket);
                byte[] rec=receivePacket.getData();
                String receivedData = new String(rec,0,receivePacket.getLength());          
                byte[] cipherText = receivedData.getBytes();

                System.out.println("received packet size before convert to bytes "+receivePacket.getLength());//it displays 18
                System.out.println("received packet size after convert to bytes "+cipherText.length);//it display 30 how???? it must be 18          


                byte[] keyBytes ="qwertyuiopasdfgh".getBytes();
                SecretKey key = new SecretKeySpec(keyBytes, 0, keyBytes.length, "AES");       
                Cipher cipher = Cipher.getInstance("AES/GCM/NoPadding", "SunJCE");
                byte[] nonce = new byte[GCM_NONCE_LENGTH];
                nonce = "poiuytrewqlk".getBytes();;;        
                GCMParameterSpec spec = new GCMParameterSpec(GCM_TAG_LENGTH * 8, nonce);            
                byte[] aad = "Whatever I like".getBytes();;         
                cipher.init(Cipher.DECRYPT_MODE, key, spec);     
                cipher.updateAAD(aad);      
                byte[] plainText = cipher.doFinal(cipherText);      
                System.out.println("After decryption "+new String(plainText));
            }
        }catch(Exception e){
            System.out.println("Exception caught "+e);//got Exception caught javax.crypto.AEADBadTagException: Tag mismatch!
        }
     }
}

Answer 1

I dont know that why byte content size were different.But,it solved when i use following code

 DatagramPacket receivePacket = new DatagramPacket(receiveData, receiveData.length);
    serverSocket.receive(receivePacket);
    byte[] rec=receivePacket.getData();
    byte[] cipherText=new byte[receivePacket.getLength()];
    System.arraycopy(rec, 0, cipherText, 0, receivePacket.getLength());

instead

DatagramPacket receivePacket = new DatagramPacket(receiveData, receiveData.length);
serverSocket.receive(receivePacket);
byte[] rec=receivePacket.getData();
String receivedData = new String(rec,0,receivePacket.getLength());

and solved javax.crypto.AEADBadTagException: Tag mismatch exception