Password Authentication

Example 19-5. A GUI authenticator

import javax.mail.*;
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
public class MailAuthenticator extends Authenticator {
 private JDialog passwordDialog = new JDialog(new JFrame( ), true); private JLabel mainLabel = new JLabel(
 "Please enter your user name and password: ");
 private JLabel userLabel = new JLabel("User name: ");
 private JLabel passwordLabel = new JLabel("Password: ");
 private JTextField usernameField = new JTextField(20);
 private JPasswordField passwordField = new JPasswordField(20);
 private JButton okButton = new JButton("OK");
 public MailAuthenticator( ) {
 this("");
 }
 public MailAuthenticator(String username) {
 Container pane = passwordDialog.getContentPane( );
 pane.setLayout(new GridLayout(4, 1));
 pane.add(mainLabel);
 JPanel p2 = new JPanel( );
 p2.add(userLabel);
 p2.add(usernameField);
 usernameField.setText(username);
 pane.add(p2);
 JPanel p3 = new JPanel( );
 p3.add(passwordLabel);
 p3.add(passwordField);
 pane.add(p3);
 JPanel p4 = new JPanel( );
 p4.add(okButton);
 pane.add(p4); passwordDialog.pack( );
 ActionListener al = new HideDialog( );
 okButton.addActionListener(al);
 usernameField.addActionListener(al);
 passwordField.addActionListener(al);
 }
 class HideDialog implements ActionListener {
 public void actionPerformed(ActionEvent e) {
 passwordDialog.hide( );
 } }
 public PasswordAuthentication getPasswordAuthentication( ) {
 passwordDialog.show( );
 // getPassword( ) returns an array of chars for security reasons.
 // We need to convert that to a String for // the PasswordAuthentication( ) constructor.
 String password = new String(passwordField.getPassword( ));
 String username = usernameField.getText( );
 // Erase the password in case this is used again.
 // The provider should cache the password if necessary.
 passwordField.setText("");
 return new PasswordAuthentication(username, password);
 }
}

Most of this code is just for handling the GUI. Screenshot-4 shows the rather simple dialog box this produces.

Screenshot-4. An authentication dialog

Interestingly, JPasswordField takes more pains to be secure than PasswordAuthentication does. JPasswordField stores passwords as an array of chars so that when you're done with the password, you can overwrite it with nulls. This means the password exists in memory for less time and the virtual memory system is less likely to swap the program out to disk and leave the password there in clear text. However, PasswordAuthentication stores passwords as strings, which are immutable and therefore may be unintentionally stored on the disk. Modifying the POP client to support this style of authentication is straightforward, as Example 19-6 demonstrates. We replace the hardcoded username and password with nulls and pass an instance of MailAuthenticator as the second argument to connect( ). The only other change is that we call System.exit( ) at the end of the main( ) method, since the program will no longer exit when the main( ) method returns once the AWT thread has been started.

Example 19-6. A POP client that asks the user for the password as necessary

import javax.mail.*;
import javax.mail.internet.*;
import java.util.*;
import java.io.*;
public class SecurePOP3Client {
 public static void main(String[] args) {
 Properties props = new Properties( ); String host = "utopia.poly.edu";
 String provider = "pop3";
 try {
 // Connect to the POP3 server
 Session session = Session.getDefaultInstance(props, new MailAuthenticator( ));
 Store store = session.getStore(provider);
 store.connect(host, null, null);
 // Open the folder
 Folder inbox = store.getFolder("INBOX");
 if (inbox == null) {
 System.out.println("No INBOX");
 System.exit(1);
 } inbox.open(Folder.READ_ONLY);
 // Get the messages from the server
 Message[] messages = inbox.getMessages( );
 for (int i = 0; i < messages.length; i++) {
 System.out.println("------------ Message " + (i+1) + " ------------");
 messages[i].writeTo(System.out);
 } // Close the connection // but don't remove the messages from the server
 inbox.close(false);
 store.close( );
 } catch (Exception ex) {
 ex.printStackTrace( );
 } // since we brought up a GUI returning from main( ) won't exit
 System.exit(0); }
}