Where to specify key and IV value when doing AES encryption/decryption in Android?

Question

I am working on AES encryption and decryption in android. I have an Audio file which is encrypted using AES/CBC. I have the key and IV (initialization vector).

I have read some links. From this

SecretKeySpec skeySpec = new SecretKeySpec(raw, "AES");

The SecretkeySpec class used. Where should I use my key and IV values?

And I need to decrypt only first 256 bytes data only. How can I achieve this?

Answer 1

In your code snippet, raw is your key (as a byte array):

SecretKeySpec skeySpec = new SecretKeySpec(raw, "AES");
//                                         ^^^ this is your key

Of course, if you have your key as a hex string, you'll need to convert it to bytes.

When encrypting, you can either specify an IV yourself or let a random one be generated. If you want to specify yourself, use:

Cipher cipher = Cipher.getInstance("AES/CBC/PKCS5Padding"); // adjust padding
cipher.init(Cipher.ENCRYPT_MODE, skeySpec, new IvParameterSpec(...));

if you want to use an automatic one, be sure to record which one was chosen by calling

cipher.getIV();

For decryption, you must specify the IV and you do so using an IvParameterSpec, just like with encryption.

And I need to decrypt only first 256 bytes data only. How can I achieve this?

Consider using a CipherInputStream and read only 256 bytes from it.