Follow Fang!: Java AES

這篇主要參考『Java AES Encrypt & Decrypt Example(加解密)』，不過要將他變自己的Know-how還是得去看一下官方文件

先用KeyGenerator去選擇演算法，並且生成Key。可以參考KeyGenerator有提到一些概要

This class provides the functionality of a secret (symmetric) key generator.

Key generators are constructed using one of the getInstance class methods of this class.

KeyGenerator objects are reusable, i.e., after a key has been generated, the same KeyGenerator object can be re-used to generate further keys.

There are two ways to generate a key: in an algorithm-independent manner, and in an algorithm-specific manner. The only difference between the two is the initialization of the object:

在透過SecretKey去取得Key，SecretKey本身是Interface，且可以觀察到實做類別為Key, Serializable

可以看得到Key被實做的類別分別有KerberosKey及SecretKeySpec，這兩個類別分別有實做出
getEncoded()。

getEncoded()這個方法可以取得到Key的位元組，因為SecretKey繼承Serializable，所以我們又可以透過ObjectOutputStream和ObjectInputStream將Key儲存成一個檔案。

在用SecretKeySpec去檢查Key的規範是否有符合標準，如果沒有則會噴錯
Cipher這個類別提供加解密的功能，可以運用Cipher的init去選擇要加密還是解密
加密是用C ipher.ENCRYPT_MODE，解密是用Cipher.DECRYPT_MODE

解碼的時候在透過ObjectInputStream將檔案讀近來，不過在這邊我尚未實做
僅只於分析而已，源碼都是來自於『Java AES Encrypt & Decrypt Example(加解密)』

import java.security.InvalidKeyException;
import java.security.NoSuchAlgorithmException;

import javax.crypto.BadPaddingException;
import javax.crypto.Cipher;
import javax.crypto.IllegalBlockSizeException;
import javax.crypto.KeyGenerator;
import javax.crypto.NoSuchPaddingException;
import javax.crypto.SecretKey;
import javax.crypto.spec.SecretKeySpec;

public class AES {
 public static void main(String[] args) throws NoSuchAlgorithmException,
   NoSuchPaddingException, InvalidKeyException,
   IllegalBlockSizeException, BadPaddingException {
  // 欲加密的字串
  String msg = "This is a message.";
  System.out.println("原始字串：" + new String(msg));
  // 設定要使用的加密演算法
  KeyGenerator keyG = KeyGenerator.getInstance("AES");
  // 設定key的長度
  keyG.init(128);
  // 產生SecretKey
  SecretKey secuK = keyG.generateKey();
  // 取得要用來加密的key(解密也需使用這把key)
  byte[] key = secuK.getEncoded();
  System.out.println("key：" + new String(key));
  SecretKeySpec spec = new SecretKeySpec(key, "AES");
  Cipher cipher = Cipher.getInstance("AES");
  // 設定為加密模式
  cipher.init(Cipher.ENCRYPT_MODE, spec);
  // 將字串加密，並取得加密後的資料
  byte[] encryptData = cipher.doFinal(msg.getBytes());
  System.out.println("加密後字串：" + new String(encryptData));

  // 使用剛剛用來加密的key進行解密
  spec = new SecretKeySpec(key, "AES");
  cipher = Cipher.getInstance("AES");
  // 設定為解密模式
  cipher.init(Cipher.DECRYPT_MODE, spec);
  byte[] original = cipher.doFinal(encryptData);
  System.out.println("解密後字串：" + new String(original));

 }
}

參考資料：
http://cooking-java.blogspot.tw/2010/03/java-aes-encrypt.html
http://docs.oracle.com/javase/6/docs/api/javax/crypto/KeyGenerator.html

2013/05/23

Java AES