http://download.oracle.com/javase/6/docs/technotes/guides/security/jgss/tutorials/SampleClient.java

SampleClient.java

/*
 * @(#)SampleClient.java
 *
 * Copyright (c) 2001, 2002, Oracle and/or its affiliates. All rights reserved.
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 * without modification, are permitted provided that the following 
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 * the documentation and/or other materials provided with the 
 * distribution.
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 * from this software without specific prior written permission.
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import org.ietf.jgss.*;
import java.net.Socket;
import java.io.IOException;
import java.io.DataInputStream;
import java.io.DataOutputStream;

/**
 * A sample client application that uses JGSS to do mutual authentication
 * with a server using Kerberos as the underlying mechanism. It then
 * exchanges data securely with the server.
 *
 * Every message sent to the server includes a 4-byte application-level 
 * header that contains the big-endian integer value for the number
 * of bytes that will follow as part of the JGSS token.
 *
 * The protocol is:
 *    1.  Context establishment loop:
 *         a. client sends init sec context token to server
 *         b. server sends accept sec context token to client
 *         ....
 *    2. client sends a wrap token to the server.
 *    3. server sends a MIC token to the client for the application
 *       message that was contained in the wrap token.
 */

public class SampleClient {

    public static void main(String[] args) 
       throws IOException, GSSException  {

	// Obtain the command-line arguments and parse the port number
	
	if (args.length < 3) {
	    System.err.println("Usage: java <options> Login SampleClient "
			       + " <server> <hostName> <port>");
	    System.exit(-1);
	}

	String server = args[0];
	String hostName = args[1];
	int port = Integer.parseInt(args[2]);

	Socket socket = new Socket(hostName, port);
	DataInputStream inStream = 
	  new DataInputStream(socket.getInputStream());
	DataOutputStream outStream = 
	  new DataOutputStream(socket.getOutputStream());

	System.out.println("Connected to server " 
			   + socket.getInetAddress());

	/*
	 * This Oid is used to represent the Kerberos version 5 GSS-API
	 * mechanism. It is defined in RFC 1964. We will use this Oid
	 * whenever we need to indicate to the GSS-API that it must
	 * use Kerberos for some purpose.
	 */
	Oid krb5Oid = new Oid("1.2.840.113554.1.2.2");

	GSSManager manager = GSSManager.getInstance();

	/*
	 * Create a GSSName out of the server's name. The null
	 * indicates that this application does not wish to make
	 * any claims about the syntax of this name and that the
	 * underlying mechanism should try to parse it as per whatever
	 * default syntax it chooses.
	 */
	GSSName serverName = manager.createName(server, null);

	/*
	 * Create a GSSContext for mutual authentication with the
	 * server.
	 *    - serverName is the GSSName that represents the server.
	 *    - krb5Oid is the Oid that represents the mechanism to
	 *      use. The client chooses the mechanism to use.
	 *    - null is passed in for client credentials
	 *    - DEFAULT_LIFETIME lets the mechanism decide how long the
	 *      context can remain valid.
	 * Note: Passing in null for the credentials asks GSS-API to
	 * use the default credentials. This means that the mechanism
	 * will look among the credentials stored in the current Subject
	 * to find the right kind of credentials that it needs.
	 */
	GSSContext context = manager.createContext(serverName,
					krb5Oid,
					null,
					GSSContext.DEFAULT_LIFETIME);

	// Set the desired optional features on the context. The client
	// chooses these options.

	context.requestMutualAuth(true);  // Mutual authentication
	context.requestConf(true);  // Will use confidentiality later
	context.requestInteg(true); // Will use integrity later

	// Do the context eastablishment loop

	byte[] token = new byte[0];
	
	while (!context.isEstablished()) {

	    // token is ignored on the first call
	    token = context.initSecContext(token, 0, token.length);

	    // Send a token to the server if one was generated by
	    // initSecContext
	    if (token != null) {
		System.out.println("Will send token of size "
				   + token.length
				   + " from initSecContext.");
		outStream.writeInt(token.length);
		outStream.write(token);
		outStream.flush();
	    }

	    // If the client is done with context establishment
	    // then there will be no more tokens to read in this loop
	    if (!context.isEstablished()) {
		token = new byte[inStream.readInt()];
		System.out.println("Will read input token of size "
				   + token.length
				   + " for processing by initSecContext");
		inStream.readFully(token);
	    }
	}
	
	System.out.println("Context Established! ");
	System.out.println("Client is " + context.getSrcName());
	System.out.println("Server is " + context.getTargName());

	/*
	 * If mutual authentication did not take place, then only the
	 * client was authenticated to the server. Otherwise, both
	 * client and server were authenticated to each other.
	 */
	if (context.getMutualAuthState())
	    System.out.println("Mutual authentication took place!");

	byte[] messageBytes = "Hello There!\0".getBytes();
	
	/*
	 * The first MessageProp argument is 0 to request
	 * the default Quality-of-Protection.
	 * The second argument is true to request
	 * privacy (encryption of the message).
	 */
	MessageProp prop =  new MessageProp(0, true);

	/*
	 * Encrypt the data and send it across. Integrity protection
	 * is always applied, irrespective of confidentiality
	 * (i.e., encryption).
	 * You can use the same token (byte array) as that used when 
	 * establishing the context.
	 */

	token = context.wrap(messageBytes, 0, messageBytes.length, prop);
	System.out.println("Will send wrap token of size " + token.length);
	outStream.writeInt(token.length);
	outStream.write(token);
	outStream.flush();

	/*
	 * Now we will allow the server to decrypt the message,
	 * calculate a MIC on the decrypted message and send it back
	 * to us for verification. This is unnecessary, but done here
	 * for illustration.
	 */

	token = new byte[inStream.readInt()];
	System.out.println("Will read token of size " + token.length);
	inStream.readFully(token);
	context.verifyMIC(token, 0, token.length, 
			  messageBytes, 0, messageBytes.length,
			  prop);
	
	System.out.println("Verified received MIC for message.");

	System.out.println("Exiting...");
	context.dispose();
	socket.close();
    }
}