Java加解密Class使用 - 對稱加密

稍微記錄一下Java中加密類別的使用方法，要使用加解密會用到下列三個Class：KeyGenerator、SecretKey、Cipher。  KeyGenerator主要是負責產生加解密用的Key
例如

KeyGenerator keygen = KeyGenerator.getInstance("DES");//參數代表使用的加解密演算法
SecretKey key = keygen.generateKey(); //產生key

在產生完Key後，利用Cipher來對資料加密。

String str ="example";
byte[] src = str.getBytes();
Cipher cipher = Cipher.getInstance("DES");
cipher.init(Cipher.ENCRYPT_MODE, key);//決定是何種模式有  ENCRYPT_MODE, DECRYPT_MODE, WRAP_MODE, UNWRAP_MODE
byte [] result = cipher.doFinal(src);//依據設定的Mode來開始動作，result即為加密後之密文

如果要解密的話，則改用DECRYPT_MODE

cipher.init(Cipher.ENCRYPT_MODE, key);
byte [] src = cipher.doFinal(result);//取回明文，在轉換回對應之type即可
String str = new String(src);

因為對稱加密，加解密都是用同一把Key所以會需要保存Key，最簡單的方法就是使用檔案來存放，例如

//存成檔案
byte [] bytekey = key.getEncoded();
int len = bytekey.length;
FileOutputStream fos;
fos = new FileOutputStream(FILEPATH)
fos.write(bytekey);
fos.close();
//讀檔
byte [] buf = new byte[len]
FileInputStream fis = new FileInputStream(FILEPATH); 
int i = fis.read(buf);
fis.close();

SecretKey resultKey = new SecretKeySpec(buf, "DES");//從byte array轉回key

當然也可用ObjectInputStream、ObjectOutputStream拉，如此一來就可以少去byte轉換的動作。

LocalBroadcastManager 使用

LocalBroadcastManager 簡單的說就是intra-app的Broadcast 發送，用來避免Broadcast  發送到外部所造成的security issue(當然你也可以指定target或是用Permission拉...)。 下面是Android 上所提到的優點：
   

• You know that the data you are broadcasting won't leave your app, so don't need to worry about leaking private data.
• It is not possible for other applications to send these broadcasts to your app, so you don't need to worry about having security holes they can exploit.
• It is more efficient than sending a global broadcast through the system. 

 簡單的說就是app內部的 Broadcast系統，不會發送到外部也不會被外部的Broadcast影響。唯一一個小小的缺點就是要自行import  Android Compatibility package， 要記得把 android-support-v4.jar 放到libs下即可。  可以看一下官方的文件LocalBroadcastManager  ，就幾個簡單的API而已。
範例

LocalBroadcastManager mLocalBroadcastManager;
public void onCreate(Bundle savedInstanceState) {
    mLocalBroadcastManager = LocalBroadcastManager.getInstance(getApplicationContext());   //要先getInstance
}

protected void onPause() {
    mLocalBroadcastManager.unregisterReceiver(TTLNotifyReceiver);
}

protected void onResume() {  
    IntentFilter filter = new IntentFilter("com.test.hello");
    mLocalBroadcastManager.registerReceiver(receiver, filter);
}

private BroadcastReceiver receiver = new BroadcastReceiver() {
    public void onReceive(Context context, Intent intent) {
 //rece the Intent
    }
};