Java實現MD5加密以及解密類，附帶測試類，具體見代碼。

MD5加密解密類——MyMD5Util,代碼如下

package com.zyg.security.md5;

import java.io.UnsupportedEncodingException;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
import java.security.SecureRandom;
import java.util.Arrays;

public class MyMD5Util {
 
 private static final String HEX_NUMS_STR="0123456789ABCDEF";
 private static final Integer SALT_LENGTH = 12;
 
 /** 
  * 將16進制字符串轉換成字節數組 
  * @param hex 
  * @return 
  */
 public static byte[] hexStringToByte(String hex) {
  int len = (hex.length() / 2);
  byte[] result = new byte[len];
  char[] hexChars = hex.toCharArray();
  for (int i = 0; i < len; i++) {
   int pos = i * 2;
   result[i] = (byte) (HEX_NUMS_STR.indexOf(hexChars[pos]) << 4 
       | HEX_NUMS_STR.indexOf(hexChars[pos + 1]));
  }
  return result;
 }

 
 /**
  * 將指定byte數組轉換成16進制字符串
  * @param b
  * @return
  */
 public static String byteToHexString(byte[] b) {
  StringBuffer hexString = new StringBuffer();
  for (int i = 0; i < b.length; i++) {
   String hex = Integer.toHexString(b[i] & 0xFF);
   if (hex.length() == 1) {
    hex = "0" + hex;
   }
   hexString.append(hex.toUpperCase());
  }
  return hexString.toString();
 }
 
 /**
  * 驗證口令是否合法
  * @param password
  * @param passwordInDb
  * @return
  * @throws NoSuchAlgorithmException
  * @throws UnsupportedEncodingException
  */
 public static boolean validPassword(String password, String passwordInDb)
   throws NoSuchAlgorithmException, UnsupportedEncodingException {
  //將16進制字符串格式口令轉換成字節數組
  byte[] pwdInDb = hexStringToByte(passwordInDb);
  //聲明鹽變量
  byte[] salt = new byte[SALT_LENGTH];
  //將鹽從數據庫中保存的口令字節數組中提取出來
  System.arraycopy(pwdInDb, 0, salt, 0, SALT_LENGTH);
  //創建消息摘要對象
  MessageDigest md = MessageDigest.getInstance("MD5");
  //將鹽數據傳入消息摘要對象
  md.update(salt);
  //將口令的數據傳給消息摘要對象
  md.update(password.getBytes("UTF-8"));
  //生成輸入口令的消息摘要
  byte[] digest = md.digest();
  //聲明一個保存數據庫中口令消息摘要的變量
  byte[] digestInDb = new byte[pwdInDb.length - SALT_LENGTH];
  //取得數據庫中口令的消息摘要
  System.arraycopy(pwdInDb, SALT_LENGTH, digestInDb, 0, digestInDb.length);
  //比較根據輸入口令生成的消息摘要和數據庫中消息摘要是否相同
  if (Arrays.equals(digest, digestInDb)) {
   //口令正確返回口令匹配消息
   return true;
  } else {
   //口令不正確返回口令不匹配消息
   return false;
  }
 }


 /**
  * 獲得加密后的16進制形式口令
  * @param password
  * @return
  * @throws NoSuchAlgorithmException
  * @throws UnsupportedEncodingException
  */
 public static String getEncryptedPwd(String password)
   throws NoSuchAlgorithmException, UnsupportedEncodingException {
  //聲明加密后的口令數組變量
  byte[] pwd = null;
  //隨機數生成器
  SecureRandom random = new SecureRandom();
  //聲明鹽數組變量
  byte[] salt = new byte[SALT_LENGTH];
  //將隨機數放入鹽變量中
  random.nextBytes(salt);

  //聲明消息摘要對象
  MessageDigest md = null;
  //創建消息摘要
  md = MessageDigest.getInstance("MD5");
  //將鹽數據傳入消息摘要對象
  md.update(salt);
  //將口令的數據傳給消息摘要對象
  md.update(password.getBytes("UTF-8"));
  //獲得消息摘要的字節數組
  byte[] digest = md.digest();

  //因為要在口令的字節數組中存放鹽，所以加上鹽的字節長度
  pwd = new byte[digest.length + SALT_LENGTH];
  //將鹽的字節拷貝到生成的加密口令字節數組的前12個字節，以便在驗證口令時取出鹽
  System.arraycopy(salt, 0, pwd, 0, SALT_LENGTH);
  //將消息摘要拷貝到加密口令字節數組從第13個字節開始的字節
  System.arraycopy(digest, 0, pwd, SALT_LENGTH, digest.length);
  //將字節數組格式加密后的口令轉化為16進制字符串格式的口令
  return byteToHexString(pwd);
 }
}

測試類——Client,代碼如下：

package com.zyg.security.md5;

import java.io.UnsupportedEncodingException;
import java.security.NoSuchAlgorithmException;
import java.util.HashMap;
import java.util.Map;

public class Client {
 private static Map users = new HashMap();
 
 public static void main(String[] args){
  String userName = "zyg";
  String password = "123";
  registerUser(userName,password);
  
  userName = "changong";
  password = "456";
  registerUser(userName,password);
  
  String loginUserId = "zyg";
  String pwd = "1232";
  try {
   if(loginValid(loginUserId,pwd)){
    System.out.println("歡迎登陸！??！");
   }else{
    System.out.println("口令錯誤，請重新輸入?。?！");
   }
  } catch (NoSuchAlgorithmException e) {
   // TODO Auto-generated catch block
   e.printStackTrace();
  } catch (UnsupportedEncodingException e) {
   // TODO Auto-generated catch block
   e.printStackTrace();
  } 
 }
 
 /**
  * 注冊用戶
  * 
  * @param userName
  * @param password
  */
 public static void registerUser(String userName,String password){
  String encryptedPwd = null;
  try {
   encryptedPwd = MyMD5Util.getEncryptedPwd(password);
   
   users.put(userName, encryptedPwd);
   
  } catch (NoSuchAlgorithmException e) {
   // TODO Auto-generated catch block
   e.printStackTrace();
  } catch (UnsupportedEncodingException e) {
   // TODO Auto-generated catch block
   e.printStackTrace();
  }
 }
 
 /**
  * 驗證登陸
  * 
  * @param userName
  * @param password
  * @return
  * @throws UnsupportedEncodingException 
  * @throws NoSuchAlgorithmException 
  */
 public static boolean loginValid(String userName,String password) 
    throws NoSuchAlgorithmException, UnsupportedEncodingException{
  String pwdInDb = (String)users.get(userName);
  if(null!=pwdInDb){ // 該用戶存在
    return MyMD5Util.validPassword(password, pwdInDb);
  }else{
   System.out.println("不存在該用戶?。。?);
   return false;
  }
 }
}