MD5的全称是Message-Digest Algorithm 5（信息-摘要算法），是基于消息摘要原理的，消息摘要的基本特征就是很难根据摘要推算出消息报文。

消息摘要（Message Digest）又称为数字摘要(Digital Digest)。它是一个唯一对应一个消息或文本的固定长度的值，它由一个单向Hash加密函数对消息进行作用而产生。如果消息在途中改变了，则接收者通过对收到消息的新产生的摘要与原摘要比较，就可知道消息是否被改变了。因此消息摘要保证了消息的完整性。 消息摘要采用单向Hash 函数将需加密的明文"摘要"成一串128bit的密文，这一串密文亦称为数字指纹(Finger Print)，它有固定的长度，且不同的明文摘要成密文，其结果总是不同的，而同样的明文其摘要必定一致。这样这串摘要便可成为验证明文是否是"真身"的"指纹"了。 

MD5算法加密~16位、32位、64位。

MD5加密后所得到的通常是32位的编码，而在不少地方会用到16位的编码
它们有什么区别呢？
16位加密就是从32位MD5散列中把中间16位提取出来！
其实破解16位MD5散列要比破解32位MD5散列还慢
因为他多了一个步骤,就是使用32位加密后再把中间16位提取出来, 然后再进行对比
而破解32位的则不需要,加密后直接对比就可以了

admin 的加密代码：

16位加密：7a57a5a743894a0e

32位加密：21232f297a57a5a743894a0e4a801fc3

package net.test.util;

import java.io.UnsupportedEncodingException;  
import java.security.MessageDigest;  
import java.security.NoSuchAlgorithmException;  
import sun.misc.BASE64Encoder;

public class MD5 {
	/**Determine encrypt algorithm MD5*/  
    private static final String ALGORITHM_MD5 = "MD5";  
    /**UTF-8 Encoding*/  
    private static final String UTF_8 = "UTF-8";  
      
    /** 
     * MD5 16bit Encrypt Methods. 
     * @param readyEncryptStr ready encrypt string 
     * @return String encrypt result string 
     * @throws NoSuchAlgorithmException  
     * */  
    public static final String MD5_16bit(String readyEncryptStr) throws NoSuchAlgorithmException{  
        if(readyEncryptStr != null){  
            return MD5.MD5_32bit(readyEncryptStr).substring(8, 24);  
        }else{  
            return null;  
        }  
    }  
      
    /** 
     * MD5 32bit Encrypt Methods. 
     * @param readyEncryptStr ready encrypt string 
     * @return String encrypt result string 
     * @throws NoSuchAlgorithmException  
     * */  
    public static final String MD5_32bit(String readyEncryptStr) throws NoSuchAlgorithmException{  
        if(readyEncryptStr != null){  
            //Get MD5 digest algorithm's MessageDigest's instance.  
            MessageDigest md = MessageDigest.getInstance(ALGORITHM_MD5);  
            //Use specified byte update digest.  
            md.update(readyEncryptStr.getBytes());  
            //Get cipher text  
            byte [] b = md.digest();  
            //The cipher text converted to hexadecimal string  
            StringBuilder su = new StringBuilder();  
            //byte array switch hexadecimal number.  
            for(int offset = 0,bLen = b.length; offset < bLen; offset++){  
                String haxHex = Integer.toHexString(b[offset] & 0xFF);  
                if(haxHex.length() < 2){  
                    su.append("0");  
                }  
                su.append(haxHex);  
            }  
            return su.toString();  
        }else{  
            return null;  
        }  
    }  
      
    /** 
     * MD5 32bit Encrypt Methods. 
     * @param readyEncryptStr ready encrypt string 
     * @return String encrypt result string 
     * @throws NoSuchAlgorithmException  
     * */  
    public static final String MD5_32bit1(String readyEncryptStr) throws NoSuchAlgorithmException{  
        if(readyEncryptStr != null){  
            //The cipher text converted to hexadecimal string  
            StringBuilder su = new StringBuilder();  
            //Get MD5 digest algorithm's MessageDigest's instance.  
            MessageDigest md = MessageDigest.getInstance(ALGORITHM_MD5);  
            byte [] b = md.digest(readyEncryptStr.getBytes());  
            int temp = 0;  
            //byte array switch hexadecimal number.  
            for(int offset = 0,bLen = b.length; offset < bLen; offset++){  
                temp = b[offset];  
                if(temp < 0){  
                    temp += 256;  
                }  
                int d1 = temp / 16;  
                int d2 = temp % 16;  
                su.append(Integer.toHexString(d1) + Integer.toHexString(d2)) ;  
            }  
            return su.toString();  
        }else{  
            return null;  
        }  
    }  
      
    /** 
     * MD5 32bit Encrypt Methods. 
     * @param readyEncryptStr ready encrypt string 
     * @return String encrypt result string 
     * @throws NoSuchAlgorithmException  
     * */  
    public static final String MD5_32bit2(String readyEncryptStr) throws NoSuchAlgorithmException{  
        if(readyEncryptStr != null){  
            //The cipher text converted to hexadecimal string  
            StringBuilder su = new StringBuilder();  
            //Get MD5 digest algorithm's MessageDigest's instance.  
            MessageDigest md = MessageDigest.getInstance(ALGORITHM_MD5);  
            //Use specified byte update digest.  
            md.update(readyEncryptStr.getBytes());  
            byte [] b = md.digest();  
            int temp = 0;  
            //byte array switch hexadecimal number.  
            for(int offset = 0,bLen = b.length; offset < bLen; offset++){  
                temp = b[offset];  
                if(temp < 0){  
                    temp += 256;  
                }  
                if(temp < 16){  
                    su.append("0");  
                }  
                su.append(Integer.toHexString(temp));  
            }  
            return su.toString();  
        }else{  
            return null;  
        }  
    }  
      
    /** 
     * MD5 16bit Encrypt Methods. 
     * @param readyEncryptStr ready encrypt string 
     * @return String encrypt result string 
     * @throws NoSuchAlgorithmException  
     * @throws UnsupportedEncodingException  
     * */  
    public static final String MD5_64bit(String readyEncryptStr) throws NoSuchAlgorithmException, UnsupportedEncodingException{  
        MessageDigest md = MessageDigest.getInstance(ALGORITHM_MD5);  
        BASE64Encoder base64Encoder = new BASE64Encoder();  
        return base64Encoder.encode(md.digest(readyEncryptStr.getBytes(UTF_8)));  
    }  
      
    public static void main(String[] args) {  
        try {
        	String s = "admin";
            String md516 = MD5.MD5_16bit(s);
            System.out.println("16bit-md5:\n" + md516); //  
//            String md532 = MD5.MD5_32bit(s);  
//            String md5321 = MD5.MD5_32bit1(s);  
//            String md5322 = MD5.MD5_32bit2(s);  
//            System.out.println("32bit-md5:"); //5d052f1e32af4e4ac2544a5fc2a9b992  
//            System.out.println("1:  " + md532);  
//            System.out.println("2:  " + md5321);  
//            System.out.println("3:  " + md5322);  
//            String md564 = MD5.MD5_64bit(s);  
//            System.out.println("64bit-md5:\n" + md564); //  
        } catch (Exception e) {  
            e.printStackTrace();  
        }  
    }
}