消息摘要的概念

对消息通过一个单向Hash算法进行计算，得到一个唯一的结果。每个消息对应一个唯一结果，所有结果的长度固定。

消息摘要的分类

（1）MD5（Message Digest）消息摘要算法
（2）SHA（Secure Hash AlgorIThm）安全散列算法
（3）MAC（Message Authentication Code）消息认证码算法，结合了MD5和SHA的优势，同时用密钥对摘要进行加密，是一种更为安全的消息摘要算法。

日常应用

校验数据的完整性，防止在传输途中被篡改

编程使用

import java.io.File; import java.io.FileinputStream; import java.security.DigestInputStream; import java.security.MessageDigest;  import javax.crypto.keygenerator; import javax.crypto.Mac; import javax.crypto.SecretKey; import javax.crypto.sPEc.SecretKeySpec;  public class MessageDigestUtil {    /**    * 计算MD5值    */   public static String encryptMD5(byte[] data) throws Exception {     MessageDigest messageDigest = MessageDigest.getInstance("MD5");//MD5,MD2     messageDigest.update(data);     return BytestoHex.FromBytesToHex(messageDigest.digest());   }    /**    * 计算文件的MD5值（指纹签名）    */   public static String getFileMD5(String path) throws Exception {     FileInputStream fis = new FileInputStream(new File(path));     DigestInputStream dis = new DigestInputStream(fis, MessageDigest.getInstance("MD5"));     try {       byte[] buffer = new byte[1024];       int read = dis.read(buffer, 0, 1024);       while (read != -1) {         read = dis.read(buffer, 0, 1024);       }       MessageDigest md = dis.getMessageDigest();       return BytesToHex.fromBytesToHex(md.digest());     } finally {       dis.close();       fis.close();     }    }    /**    * 计算SHA值    */   public static String encryptSHA(byte[] data) throws Exception {     MessageDigest messageDigest = MessageDigest.getInstance("SHA");//SHA,SHA1,SHA-1,SHA-256,SHA-384,SHA-512     messageDigest.update(data);     return BytesToHex.fromBytesToHex(messageDigest.digest());   }    /**    * 初始化HMAC密钥    */   public static byte[] initHmacKey() throws Exception {     KeyGenerator keyGenerator = KeyGenerator.getInstance("HmacSHA512");//HmacMD5,HmacSHA1,HmacSHA256,HmacSHA384,HmacSHA512     return keyGenerator.generateKey().getencoded();   }    /**    * 使用Hmac生成的密钥对数据进行加密    */   public static String encryptHmac(byte[] data, byte[] key) throws Exception {     SecretKey secretKey = new SecretKeySpec(key, "HmacSHA512");     Mac mac = Mac.getInstance("HmacSHA512");     mac.init(secretKey);     return BytesToHex.fromBytesToHex(mac.doFinal(data));   }    public static void main(String[] args) throws Exception {     String data = "123456";     String path = "/Users/admin/Downloads/httpd-2.4.20.tar.gz";          System.out.PRintln(data + " 的MD5值： " + MessageDigestUtil.encryptMD5(data.getBytes()));     System.out.println(path + " 的文件MD5签名： " + MessageDigestUtil.getFileMD5(path));     System.out.println(data + " 的SHA值： " + MessageDigestUtil.encryptSHA(data.getBytes()));      byte[] hmacKey = initHmacKey();     System.out.println("初始化Hmac密钥：" + BytesToHex.fromBytesToHex(hmacKey));     System.out.println(data + " 是的Hmac加密值为： " + MessageDigestUtil.encryptHmac(data.getBytes(), hmacKey));   }  }

字节码数组转成16进制字符串工具类

public class BytesToHex {          public static String fromBytesToHex(byte[] resultBytes) {         StringBuilder builder = new StringBuilder();         for (int i = 0; i < resultBytes.length; i++) {             if (Integer.toHexString(0xFF &amp; resultBytes[i]).length() == 1) {                 builder.append("0").append(                         Integer.toHexString(0xFF & resultBytes[i]));             } else {                 builder.append(Integer.toHexString(0xFF & resultBytes[i]));             }         }         return builder.toString();     }  }