为每一个用户，生成一个唯一的验证码

[zhangfengsummer]

在java里，如何实现，用什么算法比较好。

[wangxian00]

package com.trulytech.mantis.util;

/*
* 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
*/

/**
*
* <p>
* Title: Mantis
* </p>
*
* <p>
* Description: 获得GUID
* </p>
*
* <p>
* Copyright: Copyright (c) 2002
* </p>
*
* <p>
* Company:
* </p>
*
* @author Wang Xian
* @version 1.0
*/
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().toLowerCase();
}

/*
* 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()); } }
*/
}

[如一宝宝]

http://blog.csdn.net/zjcxc/archive/2006/08/20/1099215.aspx
参考