方法一：只加密

import java.security.*;

import java.security.spec.*;

class MD5_test {

public final static String MD5(String s) {

char hexDigits[] = { '0', '1', '2', '3', '4', '5', '6', '7', '8', '9',

'a', 'b', 'c', 'd', 'e', 'f' };

try {

byte[] strTemp = s.getBytes();

MessageDigest mdTemp = MessageDigest.getInstance("MD5");

mdTemp.update(strTemp);

byte[] md = mdTemp.digest();

int j = md.length;

char str[] = new char[j * 2];

int k = 0;

for (int i = 0; i < j; i++) {

byte byte0 = md[i];

str[k++] = hexDigits[byte0 >>> 4 & 0xf];

str[k++] = hexDigits[byte0 & 0xf];

}

return new String(str);

} catch (Exception e) {

return null;

}

}

public static void main(String[] args) {

// MD5_Test aa = new MD5_Test();

System.out.print(MD5_test.MD5("b"));

}

}

方法二：加密与解密

import java.security.MessageDigest;

public class MD5andKL {

// MD5加码。32位

public static String MD5(String inStr) {

MessageDigest md5 = null;

try {

md5 = MessageDigest.getInstance("MD5");

} catch (Exception e) {

System.out.println(e.toString());

e.printStackTrace();

return "";

}

char[] charArray = inStr.toCharArray();

byte[] byteArray = new byte[charArray.length];

for (int i = 0; i < charArray.length; i++)

byteArray[i] = (byte) charArray[i];

byte[] md5Bytes = md5.digest(byteArray);

StringBuffer hexValue = new StringBuffer();

for (int i = 0; i < md5Bytes.length; i++) {

int val = ((int) md5Bytes[i]) & 0xff;

if (val < 16)

hexValue.append("0");

hexValue.append(Integer.toHexString(val));

}

return hexValue.toString();

}

// 可逆的加密算法

public static String KL(String inStr) {

// String s = new String(inStr);

char[] a = inStr.toCharArray();

for (int i = 0; i < a.length; i++) {

a[i] = (char) (a[i] ^ 't');

}

String s = new String(a);

return s;

}

// 加密后解密

public static String JM(String inStr) {

char[] a = inStr.toCharArray();

for (int i = 0; i < a.length; i++) {

a[i] = (char) (a[i] ^ 't');

}

String k = new String(a);

return k;

}

// 测试主函数

public static void main(String args[]) {

String s = new String("a");

System.out.println("原始：" + s);

System.out.println("MD5后：" + MD5(s));

System.out.println("MD5后再加密：" + KL(MD5(s)));

System.out.println("解密为MD5后的：" + JM(KL(MD5(s))));

}

}

参考自：http://happymen001.javaeye.com/blog/576388