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출처 : http://javaexchange.com/aboutRandomGUID.html

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Random GUID generator in Java


Download RandomGUID. -- generates truly random GUIDs in the standard format.


RandomGUID.java


/* * RandomGUID * @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.*; import java.util.*; import java.security.*; /* * 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 extends Object { 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) { e.printStackTrace(); } } /* * 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) { 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! sbValueBeforeMD5.append(s_id); sbValueBeforeMD5.append(":"); sbValueBeforeMD5.append(Long.toString(time)); sbValueBeforeMD5.append(":"); sbValueBeforeMD5.append(Long.toString(rand)); 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)); } 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("-"); sb.append(raw.substring(8, 12)); sb.append("-"); sb.append(raw.substring(12, 16)); sb.append("-"); sb.append(raw.substring(16, 20)); sb.append("-"); sb.append(raw.substring(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()); } } }

Download RandomGUID. -- generates truly random GUIDs in the standard format.
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