RandomGuid.java

/**
 * Copyright 2005-2010 hdiv.org
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * 	http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.hdiv.idGenerator;

/*
 * 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;

/**
 * Globally unique identifier generator.
 * <p>
 * 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.
 * <p>
 * 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.
 * <p>
 * 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.
 * <p>
 * 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.
 * <p>
 * <b>Security of RandomGuid</b>: 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.
 * <p>
 * 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.
 * <p>
 * Using the secure random option, this class complies with the statistical
 * random number generator tests specified in FIPS 140-2, Security Requirements
 * for Cryptographic Modules, secition 4.9.1.
 * <p>
 * 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.
 * <p>
 * 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.
 * 
 * @version 1.2.1 11/05/02
 * @author Marc A. Mnich
 */
public class RandomGuid {

	private static Random random;

	private static SecureRandom secureRandom;

	private static String id;

	private String guid;

	/*
	 * 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 {
		secureRandom = new SecureRandom();
		long secureInitializer = secureRandom.nextLong();
		random = new Random(secureInitializer);
		try {
			id = InetAddress.getLocalHost().toString();
		} catch (UnknownHostException e) {
			throw new RuntimeException(e);
		}
	}

	/**
	 * Default constructor. With no specification of security option, this
	 * constructor defaults to lower security, high performance.
	 */
	public RandomGuid() {
		getRandomGuid(false);
	}

	/**
	 * 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) {
		getRandomGuid(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) {
			throw new RuntimeException(e);
		}

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

		if (secure) {
			rand = secureRandom.nextLong();
		} else {
			rand = random.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!
		sbValueBeforeMD5.append(id);
		sbValueBeforeMD5.append(":");
		sbValueBeforeMD5.append(Long.toString(time));
		sbValueBeforeMD5.append(":");
		sbValueBeforeMD5.append(Long.toString(rand));

		String valueBeforeMD5 = sbValueBeforeMD5.toString();
		md5.update(valueBeforeMD5.getBytes());

		byte[] array = md5.digest();
		StringBuffer sb = new StringBuffer();
		for (int j = 0; j < array.length; ++j) {
			int b = array[j] & 0xFF;
			if (b < 0x10)
				sb.append('0');
			sb.append(Integer.toHexString(b));
		}
		guid = sb.toString();
	}

	/**
	 * Convert to the standard format for GUID (Useful for SQL Server
	 * UniqueIdentifiers, etc).
	 */
	public String toString() {
		return guid.toUpperCase();
	}
}