*             解密后存放的文件名 如 c:/ test/ 解密后文件 .txt 
      */  
    public void decryptFile(String file, String dest) throws Exception {  
       Cipher cipher = Cipher.getInstance ( "DES" );  
       cipher.init(Cipher. DECRYPT_MODE , this . key );  
       InputStream is = new FileInputStream(file);  
       OutputStream out = new FileOutputStream(dest);  
       CipherOutputStream cos = new CipherOutputStream(out, cipher);  
       byte [] buffer = new byte [1024];  
       int r;  
       while ((r = is.read(buffer)) >= 0) {  
           cos.write(buffer, 0, r);  
       }  
       cos.close();  
       out.close();  
       is.close();  
    }