Random GUID : UUID « Development « Java Tutorial

 * RandomGUID from http://www.javaexchange.com/aboutRandomGUID.html
 * @version 1.2.1 11/05/02
 * @author Marc A. Mnich
 * From www.JavaExchange.com, Open Software licensing
 * 11/05/02 -- Performance enhancement from Mike Dubman.  
 *             Moved InetAddr.getLocal to static block.  Mike has measured
 *             a 10 fold improvement in run time.
 * 01/29/02 -- Bug fix: Improper seeding of nonsecure Random object
 *             caused duplicate GUIDs to be produced.  Random object
 *             is now only created once per JVM.
 * 01/19/02 -- Modified random seeding and added new constructor
 *             to allow secure random feature.
 * 01/14/02 -- Added random function seeding with JVM run time

import java.net.InetAddress;
import java.net.UnknownHostException;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
import java.security.SecureRandom;
import java.util.Random;

 * In the multitude of java GUID generators, I found none that
 * guaranteed randomness.  GUIDs are guaranteed to be globally unique
 * by using ethernet MACs, IP addresses, time elements, and sequential
 * numbers.  GUIDs are not expected to be random and most often are
 * easy/possible to guess given a sample from a given generator.
 * SQL Server, for example generates GUID that are unique but
 * sequencial within a given instance.
 * GUIDs can be used as security devices to hide things such as
 * files within a filesystem where listings are unavailable (e.g. files
 * that are served up from a Web server with indexing turned off).
 * This may be desireable in cases where standard authentication is not
 * appropriate. In this scenario, the RandomGUIDs are used as directories.
 * Another example is the use of GUIDs for primary keys in a database
 * where you want to ensure that the keys are secret.  Random GUIDs can
 * then be used in a URL to prevent hackers (or users) from accessing
 * records by guessing or simply by incrementing sequential numbers.
 * There are many other possiblities of using GUIDs in the realm of
 * security and encryption where the element of randomness is important.
 * This class was written for these purposes but can also be used as a
 * general purpose GUID generator as well.
 * RandomGUID generates truly random GUIDs by using the system's
 * IP address (name/IP), system time in milliseconds (as an integer),
 * and a very large random number joined together in a single String
 * that is passed through an MD5 hash.  The IP address and system time
 * make the MD5 seed globally unique and the random number guarantees
 * that the generated GUIDs will have no discernable pattern and
 * cannot be guessed given any number of previously generated GUIDs.
 * It is generally not possible to access the seed information (IP, time,
 * random number) from the resulting GUIDs as the MD5 hash algorithm
 * provides one way encryption.
 * ----> Security of RandomGUID: <-----
 * RandomGUID can be called one of two ways -- with the basic java Random
 * number generator or a cryptographically strong random generator
 * (SecureRandom).  The choice is offered because the secure random
 * generator takes about 3.5 times longer to generate its random numbers
 * and this performance hit may not be worth the added security
 * especially considering the basic generator is seeded with a
 * cryptographically strong random seed.
 * Seeding the basic generator in this way effectively decouples
 * the random numbers from the time component making it virtually impossible
 * to predict the random number component even if one had absolute knowledge
 * of the System time.  Thanks to Ashutosh Narhari for the suggestion
 * of using the static method to prime the basic random generator.
 * Using the secure random option, this class compies with the statistical
 * random number generator tests specified in FIPS 140-2, Security
 * Requirements for Cryptographic Modules, secition 4.9.1.
 * I converted all the pieces of the seed to a String before handing
 * it over to the MD5 hash so that you could print it out to make
 * sure it contains the data you expect to see and to give a nice
 * warm fuzzy.  If you need better performance, you may want to stick
 * to byte[] arrays.
 * I believe that it is important that the algorithm for
 * generating random GUIDs be open for inspection and modification.
 * This class is free for all uses.
 * - Marc

public class RandomGUID  {

  public String valueBeforeMD5 = "";
  public String valueAfterMD5 = "";
  private static Random myRand;
  private static SecureRandom mySecureRand;

  private static String s_id;

   * Static block to take care of one time secureRandom seed.
   * It takes a few seconds to initialize SecureRandom.  You might
   * want to consider removing this static block or replacing
   * it with a "time since first loaded" seed to reduce this time.
   * This block will run only once per JVM instance.
  static {
    mySecureRand = new SecureRandom();
    long secureInitializer = mySecureRand.nextLong();
    myRand = new Random(secureInitializer);
    try {
      s_id = InetAddress.getLocalHost().toString();
    } catch ( UnknownHostException e ) {


   * Default constructor.  With no specification of security option,
   * this constructor defaults to lower security, high performance.
  public RandomGUID() {

   * Constructor with security option.  Setting secure true
   * enables each random number generated to be cryptographically
   * strong.  Secure false defaults to the standard Random function seeded
   * with a single cryptographically strong random number.
  public RandomGUID(boolean secure) {

   * Method to generate the random GUID
  private void getRandomGUID(boolean secure) {
    MessageDigest md5 = null;
    StringBuffer sbValueBeforeMD5 = new StringBuffer();

    try {
      md5 = MessageDigest.getInstance("MD5");
    } catch ( NoSuchAlgorithmException e ) {
      System.out.println("Error: " + e);

    try {
      long time = System.currentTimeMillis();
      long rand = 0;

      if ( secure ) {
        rand = mySecureRand.nextLong();
      } else {
        rand = myRand.nextLong();

      // This StringBuffer can be a long as you need; the MD5
      // hash will always return 128 bits.  You can change
      // the seed to include anything you want here.
      // You could even stream a file through the MD5 making
      // the odds of guessing it at least as great as that
      // of guessing the contents of the file!

      valueBeforeMD5 = sbValueBeforeMD5.toString();

      byte[] array = md5.digest();
      StringBuffer sb = new StringBuffer();
      for ( int j = 0; j < array.length; ++j ) {
        int b = array[j] & 0xFF;
        if ( b < 0x10 ) {

      valueAfterMD5 = sb.toString();

    } catch ( Exception e ) {
      System.out.println("Error:" + e);

   * Convert to the standard format for GUID
   * (Useful for SQL Server UniqueIdentifiers, etc.)
   * Example: C2FEEEAC-CFCD-11D1-8B05-00600806D9B6
  public String toString() {
    String raw = valueAfterMD5.toUpperCase();
    StringBuffer sb = new StringBuffer();
    sb.append(raw.substring(0, 8));
    sb.append(raw.substring(8, 12));
    sb.append(raw.substring(12, 16));
    sb.append(raw.substring(16, 20));

    return sb.toString();

   * Demonstraton and self test of class
  public static void main(String args[]) {
    for ( int i = 0; i < 100; i++ ) {
      RandomGUID myGUID = new RandomGUID();
      System.out.println("Seeding String=" + myGUID.valueBeforeMD5);
      System.out.println("rawGUID=" + myGUID.valueAfterMD5);
      System.out.println("RandomGUID=" + myGUID.toString());

6.54.1.Get a unique identifier Using java.rmi.dgc.VMID
6.54.2.Using java.util.UUID
6.54.3.Using java.util.concurrent.AtomicLong: A numerical id, start at zero and increment by one.
6.54.4.Create your own basic UUID
6.54.5.Random GUID
6.54.6.Session ID generator
6.54.7.Generates a UUID
6.54.8.ID generator
6.54.9.Generator for Globally unique Strings
6.54.10.Generates random UUIDs
6.54.11.UUID generator