android - Speed up encryption/decryption?

Question

I have an encryption and decryption code which I use to encrypt and decrypt video files (mp4). I'm trying to speed up the decryption process as the encryption one is not that relevant for my case. This is the code that I have for the decryption process:

private static void  decryptFile() throws IOException, ShortBufferException, IllegalBlockSizeException, BadPaddingException
    {

        //int blockSize = cipher.getBlockSize();
        int blockSize = cipher.getBlockSize();
        int outputSize = cipher.getOutputSize(blockSize);
        System.out.println("outputsize: " + outputSize);
        byte[] inBytes = new byte[blockSize];
        byte[] outBytes = new byte[outputSize];
        in= new FileInputStream(inputFile);
        out=new FileOutputStream(outputFile);

        BufferedInputStream inStream = new BufferedInputStream(in);
        int inLength = 0;;
        boolean more = true;
        while (more)
          {
             inLength = inStream.read(inBytes);
             if (inLength == blockSize)
             {
                int outLength 
                   = cipher.update(inBytes, 0, blockSize, outBytes);
                out.write(outBytes, 0, outLength);

             }
             else more = false;         
          }
          if (inLength > 0)
             outBytes = cipher.doFinal(inBytes, 0, inLength);
          else
             outBytes = cipher.doFinal();

          out.write(outBytes);

}

My question is how to speed up the decryption process in this code. I've tried decrypting a 10MB mp4 file and it decrypts in 6-7 seconds. However, I'm aiming for < 1 seconds. Another thing I would like to know is if my writing to the FileOutputStream out is actually slowing the process down rather than the decryption process itself. Any suggestions on how to go about speeding things up here.

I'm using AES for encryption/decryption.

Until I find a solution, I will be using a ProgressDialog which tells the user to wait until the video has been decrypted (Obviously, I'm not going to use the word: decrypted).

Answer 1

Why are you decrypting data only by blockSize increments ? You do not show what type of object cipher is, but I am guessing this is a javax.crypto.Cipher instance. It can handle update() calls over arrays of arbitrary length, and you will have much less overhead if you use longer arrays. You should process data by blocks of, say, 8192 bytes (that's the traditional length for a buffer, it interacts reasonably well with CPU inner caches).