Java - Encrypt part of the file(audio & image) instead of the whole file?

I read something about this here Android - Increas开发者_如何学Ce speed of encryption

I need to only encrypt part of the file(audio & image). Is this possible? Can somebody provide snippet of both encryption and decryption if it's possible?

Thanks in advance =)

You could define an encrypted member and a non-encrypted members of your class. Then, just serialize them out appropriately. For instance, your class may be something like this:

public Class partiallyEncrypted {
  private transient byte[] imagePart1Decrypted; // transient member will not be serialized by default
  private byte[] imagePart1Encrypted;
  private byte[] imagePart2;
  private static final int BYTES_TO_ENCRYPT = 2048;
  // other non-encrypted fields
  ...

  public setImage(byte[] imageBytes) {
    // calc bytes to encrypt and those left over, initialize arrays
    int bytesToEncrypt = Math.min(imageBytes.length, BYTES_TO_ENCRYPT);
    int bytesLeftOver = Math.max(imageBytes.length - bytesToEncrypt, 0);
    imagePart1Decrypted = new byte[bytesToEncrypt];
    imagePart2 = new byte[bytesLeftOver];

    // copy data into local arrays
    System.arraycopy(imageBytes, 0, imagePart1Decrypted, 0, bytesToEncrypt);
    if (bytesLeftOver > 0)
        System.arraycopy(imageBytes, bytesToEncrypt, imagePart2, 0, bytesLeftOver);

    imagePart1Encrypted = encrypt(bytesToEncrypt);
  }

  public byte[] getImage() {
    if (imagePart1Decrypted == null)
      imagePart1Decrypted = decrypt(imagePart1Encrypted);

    byte[] fullImage = new byte[imagePart1Decrypted.length + imagePart2.length];
    System.arraycopy(imagePart1Decrypted, 0, fullImage, 0, imagePart1Decrypted.length);
    if (imagePart2 != null && imagePart2.length > 0)
        System.arraycopy(imagePart2, 0, fullImage, imagePart1Decrypted.length, imagePart2.length);

    return fullImage;
  }
}