import java.security.MessageDigest;
public class MD5 {
	//公鹽
	private static final String PUBLIC_SALT = "demo" ;
	//十六進制下數字到字符的映射數組 
	private final static String[] hexDigits = {"0", "1", "2", "3", "4", 
	  "5", "6", "7", "8", "9", "a", "b", "c", "d", "e", "f"};
	/**
  * 用戶密碼加密，鹽值為 ：私鹽+公鹽
  * @param password 密碼
  * @param salt 私鹽
  * @return MD5加密字符串
  */
	public static String encryptPassword(String password,String salt){
		return encodeByMD5(PUBLIC_SALT+password+salt);
	}
	/**
  * md5加密算法
  * @param originString
  * @return 
  */
	private static String encodeByMD5(String originString){
		if (originString != null){
			try{
				//創建具有指定算法名稱的信息摘要 
				MessageDigest md = MessageDigest.getInstance("MD5");
				//使用指定的字節數組對摘要進行最后更新，然后完成摘要計算 
				byte[] results = md.digest(originString.getBytes());
				//將得到的字節數組變成字符串返回 
				String resultString = byteArrayToHexString(results);
				return resultString.toUpperCase();
			}
			catch(Exception ex){
				ex.printStackTrace();
			}
		}
		return null;
	}
	/** 
  * 轉換字節數組為十六進制字符串 
  * @param  字節數組 
  * @return 十六進制字符串 
  */
	private static String byteArrayToHexString(byte[] b){
		StringBuffer resultSb = new StringBuffer();
		for (int i = 0; i < b.length; i++){
			resultSb.append(byteToHexString(b[i]));
		}
		return resultSb.toString();
	}
	/** 將一個字節轉化成十六進制形式的字符串  */
	private static String byteToHexString(byte b){
		int n = b;
		if (n < 0) 
		   n = 256 + n;
		int d1 = n / 16;
		int d2 = n % 16;
		return hexDigits[d1] + hexDigits[d2];
	}
}