title: Java Error: Input length must be multiple of 16 when decrypting with padded cipher
body: <p>Ello mates! I am attempting to make a chat program that uses AES encryption but I have ran into a problem. When I try to run the program, it gives the error that you can see in the title. I will give you the code for both classes because it seems that, that is the only way to fully diagnose the problem. There are many duplicates of this but none seem to answer my specific problem. Thanks for all your help!
  Client:</p>
  
  <pre><code>package Chat.Application;
  
  import java.awt.event.ActionEvent;
  import java.awt.event.ActionListener;
  import java.io.BufferedReader;
  import java.io.IOException;
  import java.io.InputStreamReader;
  import java.io.PrintWriter;
  import java.net.Socket;
  import java.security.InvalidAlgorithmParameterException;
  import java.security.InvalidKeyException;
  import java.security.NoSuchAlgorithmException;
  
  import javax.swing.JFrame;
  import javax.swing.JOptionPane;
  import javax.swing.JScrollPane;
  import javax.swing.JTextArea;
  import javax.swing.JTextField;
  
  import java.security.spec.AlgorithmParameterSpec;
  import java.util.logging.Level;
  import java.util.logging.Logger;
  import javax.crypto.BadPaddingException;
  import javax.crypto.Cipher;
  import javax.crypto.IllegalBlockSizeException;
  import javax.crypto.NoSuchPaddingException;
  import javax.crypto.spec.IvParameterSpec;
  import javax.crypto.spec.SecretKeySpec;
  import sun.misc.BASE64Encoder;
  
  /**
   * A simple Swing-based client for the chat server. Graphically it is a frame
   * with a text field for entering messages and a textarea to see the whole
   * dialog.
   *
   * The client follows the Chat Protocol which is as follows. When the server
   * sends "SUBMITNAME" the client replies with the desired screen name. The
   * server will keep sending "SUBMITNAME" requests as long as the client submits
   * screen names that are already in use. When the server sends a line beginning
   * with "NAMEACCEPTED" the client is now allowed to start sending the server
   * arbitrary strings to be broadcast to all chatters connected to the server.
   * When the server sends a line beginning with "MESSAGE " then all characters
   * following this string should be displayed in its message area.
   */
  public class ChatClient {
  
      BufferedReader in;
      PrintWriter out;
      JFrame frame = new JFrame("ELECTRON Chatroom");
      JTextField textField = new JTextField(40);
      JTextArea messageArea = new JTextArea(8, 40);
          Cipher cipher;
          // password for encryption
          final String strPassword = "1234567890123456";
          // put this as key in AES
          final SecretKeySpec key = new SecretKeySpec(strPassword.getBytes(), "AES");
      /**
       * Constructs the client by laying out the GUI and registering a listener
       * with the textfield so that pressing Return in the listener sends the
       * textfield contents to the server. Note however that the textfield is
       * initially NOT editable, and only becomes editable AFTER the client
       * receives the NAMEACCEPTED message from the server.
       */
      public ChatClient() {
  
          // Layout GUI
          textField.setEditable(false);
          messageArea.setEditable(false);
          messageArea.setWrapStyleWord(true);
          messageArea.setLineWrap(true);
          frame.getContentPane().add(textField, "North");
          frame.getContentPane().add(new JScrollPane(messageArea), "Center");
          frame.pack();
  
  
          // Add Listeners
          textField.addActionListener(new ActionListener() {
              /**
               * Responds to pressing the enter key in the textfield by sending
               * the contents of the text field to the server. Then clear the text
               * area in preparation for the next message.
               */
              @Override
              public void actionPerformed(ActionEvent e) {
                  try {
                      String input = (textField.getText());
                      //ENCRYPTION
                      // Parameter specific algorithm
                      AlgorithmParameterSpec paramSpec = new IvParameterSpec(strPassword.getBytes());
                      //Whatever you want to encrypt/decrypt
                      Cipher cipher = Cipher.getInstance("AES/CBC/PKCS5Padding");
  
                      // You can use ENCRYPT_MODE (ENCRYPTunderscoreMODE)  or DECRYPT_MODE (DECRYPT underscore MODE) 
  
                      cipher.init(Cipher.ENCRYPT_MODE, key, paramSpec);
  
                      // encrypt data 
                      byte[] encrypted = cipher.doFinal(input.getBytes());
  
                      // encode data using standard encoder
                      //String output = new BASE64Encoder().encode(encrypted);
  
                      System.out.println("Orginal tring: " + input);
                      System.out.println("Encrypted string: " + encrypted);
  
                      textField.setText("");
                  } catch (NoSuchAlgorithmException | NoSuchPaddingException | IllegalBlockSizeException | BadPaddingException | InvalidKeyException | InvalidAlgorithmParameterException ex) {
                      Logger.getLogger(ChatClient.class.getName()).log(Level.SEVERE, null, ex);
                  }
              }
          });
      }
  
      /**
       * Prompt for and return the address of the server.
       */
      private String getServerAddress() {
          return JOptionPane.showInputDialog(
                  frame,
                  "Enter IP Address of the Server:",
                  "ELECTRON Chatroom",
                  JOptionPane.QUESTION_MESSAGE);
      }
  
      /**
       * Prompt for and return the desired screen name.
       */
      private String getName() {
          return JOptionPane.showInputDialog(
                  frame,
                  "Choose a screen name:",
                  "Screen name selection",
                  JOptionPane.PLAIN_MESSAGE);
      }
  
      /**
       * Connects to the server then enters the processing loop.
       */
      private void run() throws IOException, NoSuchAlgorithmException, NoSuchPaddingException, IllegalBlockSizeException, BadPaddingException, InvalidKeyException, InvalidAlgorithmParameterException {
  
          // Make connection and initialize streams
          String serverAddress = getServerAddress();
          Socket socket = new Socket(serverAddress, 9001);
          in = new BufferedReader(new InputStreamReader(
                  socket.getInputStream()));
          out = new PrintWriter(socket.getOutputStream(), true);
          // Process all messages from server, according to the protocol.
          while (true) {
              String line = in.readLine();
              if (line.startsWith("SUBMITNAME")) {
                  out.println(getName());
              } else if (line.startsWith("NAMEACCEPTED")) {
                  textField.setEditable(true);
              } else if (line.startsWith("MESSAGE")) {
                  //DECRYPTION
                  line = line.substring(8);
                  System.out.println(line);
                  // Parameter specific algorithm
                  AlgorithmParameterSpec paramSpec = new IvParameterSpec(strPassword.getBytes());
                  //Whatever you want to encrypt/decrypt
                  Cipher cipher = Cipher.getInstance("AES/CBC/PKCS5Padding");
  
                  // You can use ENCRYPT_MODE (ENCRYPTunderscoreMODE)  or DECRYPT_MODE (DECRYPT underscore MODE) 
                  cipher.init(Cipher.DECRYPT_MODE, key, paramSpec);
                  byte messageByte[] = cipher.doFinal(line.getBytes());
                  String message = new String(messageByte, "UTF-8");
                  messageArea.append(message.substring(8) + "\n");
                  System.out.println(message);
              }
          }
      }
  
      /**
       * Runs the client as an application with a closeable frame.
       */
      public static void main(String[] args) throws Exception {
          ChatClient client = new ChatClient();
          client.frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
          client.frame.setVisible(true);
          client.run();
      }
  }
  </code></pre>
  
  <p>Server: </p>
  
  <pre><code>package Chat.Application;
  
  import java.io.BufferedReader;
  import java.io.IOException;
  import java.io.InputStreamReader;
  import java.io.PrintWriter;
  import java.net.ServerSocket;
  import java.net.Socket;
  import java.util.HashSet;
  
  import java.security.GeneralSecurityException;
  import java.security.MessageDigest;
  import javax.crypto.Cipher;
  import javax.crypto.spec.SecretKeySpec;
  import org.omg.PortableInterceptor.SYSTEM_EXCEPTION;
  
  /**
   * A multi-threaded chat room server.  When a client connects the
   * server requests a screen name by sending the client the
   * text "SUBMITNAME", and keeps requesting a name until
   * a unique one is received.  After a client submits a unique
   * name, the server acknowledges with "NAMEACCEPTED".  Then
   * all messages from that client will be broadcast to all other
   * clients that have submitted a unique screen name.  The
   * broadcast messages are prefixed with "MESSAGE ".
   *
   * Because this is just a teaching example to illustrate a simple
   * chat server, there are a few features that have been left out.
   * Two are very useful and belong in production code:
   *
   *     1. The protocol should be enhanced so that the client can
   *        send clean disconnect messages to the server.
   *
   *     2. The server should do some logging.
   */
  public class ChatServer {
  
      /**
       * The port that the server listens on.
       */
      private static final int PORT = 9001;
  
      /**
       * The set of all names of clients in the chat room.  Maintained
       * so that we can check that new clients are not registering name
       * already in use.
       */
      private static HashSet&lt;String&gt; names = new HashSet&lt;String&gt;();
  
      /**
       * The set of all the print writers for all the clients.  This
       * set is kept so we can easily broadcast messages.
       */
      private static HashSet&lt;PrintWriter&gt; writers = new HashSet&lt;PrintWriter&gt;();
  
      /**
       * The application main method, which just listens on a port and
       * spawns handler threads.
       */
      public static void main(String[] args) throws Exception {
          System.out.println("Chat Server Activated");
          ServerSocket listener = new ServerSocket(PORT);
          try {
              while (true) {
                  new Handler(listener.accept()).start();
              }
          } finally {
              listener.close();
          }
      }
  
      /**
       * A handler thread class.  Handlers are spawned from the listening
       * loop and are responsible for a dealing with a single client
       * and broadcasting its messages.
       */
      private static class Handler extends Thread {
          private String name;
          private Socket socket;
          private BufferedReader in;
          private PrintWriter out;
          private Integer length;
  
          /**
           * Constructs a handler thread, squirreling away the socket.
           * All the interesting work is done in the run method.
           */
          public Handler(Socket socket) {
              this.socket = socket;
          }
  
          /**
           * Services this thread's client by repeatedly requesting a
           * screen name until a unique one has been submitted, then
           * acknowledges the name and registers the output stream for
           * the client in a global set, then repeatedly gets inputs and
           * broadcasts them.
           */
          public void run() {
              try {
  
                  // Create character streams for the socket.
                  in = new BufferedReader(new InputStreamReader(
                      socket.getInputStream()));
                  out = new PrintWriter(socket.getOutputStream(), true);
                  // Request a name from this client.  Keep requesting until
                  // a name is submitted that is not already used.  Note that
                  // checking for the existence of a name and adding the name
                  // must be done while locking the set of names.
                  while (true) {
                      out.println("SUBMITNAME");
  
                  /*Supposed to change anybody who tries to login as Admin into different name... "Supposed" to....
                      if (name.equals("Admin")) {
                          out.println("SUBMITNAME");
                          return;
                      }*/
  
                      name = in.readLine();
                      length = name.length();
                      if (length == 0) {
                          out.println("SUBMITNAME");
                          return;
                      }
                      if (name == "null") {
                          out.println("SUBMITNAME");
                          return;
                      }
                          synchronized (names) {
                          if (!names.contains(name)) {
                              names.add(name);
                              break;
                          }
                      }
                  }
                  // Now that a successful name has been chosen, add the
                  // socket's print writer to the set of all writers so
                  // this client can receive broadcast messages.
                  out.println("NAMEACCEPTED");
                  //Announces that user is Online
                  out.println("MESSAGE " + name + " is now Online");
  
                  for (PrintWriter writer : writers) {
                  writer.println("MESSAGE " + name + " is now Online");
                  }
                  writers.add(out);
                  // Accept messages from this client and broadcast them.
                  // Ignore other clients that cannot be broadcasted to.
                  while (true) {
                      String input = in.readLine();
                      System.out.println(input);
                      if (input == null) {
                          return;
                      }
                      for (PrintWriter writer : writers) {
                          writer.println("MESSAGE " + name + ": " + input);
                      }
                  }
              } catch (IOException e) {
                  System.out.println(e);
              } finally {
                  // This client is going down!  Remove its name and its print
                  // writer from the sets, and close its socket.
                  for (PrintWriter writer : writers) {
                  writer.println("MESSAGE " + name + " is now Offline");
                  }                if (name != null) {
                      names.remove(name);
                  }
                  if (out != null) {
                      writers.remove(out);
                  }
                  try {
                      socket.close();
                  } catch (IOException e) {
                  }
              }
          }
      }
  }
  </code></pre>
  
  <p>Thanks so much for all your help in saving a complete noob at programming! </p>
  
  <p>-Silver</p>