Java AES without padding

Question

What are some of the simplest ways to AES encrypt and decrypt a 16 byte array without the automatic padding? I have found solutions that use external libraries, but I want to avoid that if possible.

My current code is

SecretKeySpec skeySpec = new SecretKeySpec(getCryptoKeyByteArray(length=16)); // 128 bits
Cipher encryptor = Cipher.getInstance("AES");
encryptor.init(Cipher.ENCRYPT_MODE, skeySpec);
byte[] encrypted = encryptor.doFinal(plain);

How can I prevent the padding? The plain data is always fixed length and includes its own padding. How can I allow plain to be 16 bytes without causing encrypted to become 32 bytes?

Answer 1

See my comment. Sorry I probably should have taken a closer look the first time.

1. Change "AES" to "AES/CBC/NoPadding"
2. Change decryptor.init(Cipher.DECRYPT_MODE, skeySpec); to decryptor.init(Cipher.DECRYPT_MODE, skeySpec, encryptor.gerParameters());

To encrypt only 16 bytes of data, fixed length, using a method that requires no initialization vector to be saved, Change "AES" to "AES/ECB/NoPadding"

I pick ECB because that is the default.

If you need to encrypt more than 16 bytes, consider using something other than ECB, which suffers a certain repetition detection flaw

In this bitmap example, this image has repeated white blocks, so you can deduce the outline of the image simply by looking for where the blocks become different.

If you are only encrypting one block, it doesn't really matter though, only if you are encrypting multiple blocks that are combined does ECB become revealing.

Related: https://security.stackexchange.com/questions/15740/what-are-the-variables-of-aes

Answer 2

Agree with @rossum, but there's more to it:

CTR mode needs an initialisation vector (IV). This is a "counter" (which is what "CTR" refers to). If you can store the IV separately (it doesn't need to be protected) that would work. You'll need the same IV value when you decrypt the data.

If you don't want to store the IV and you can guarantee that no two values will be encrypted with the same key, it's fine to use a fixed IV (even an array of 0s).

The above is very important because encrypting more than one message with the same key/IV combination destroys security. See the Initialization vector (IV) section in this Wikipedia article: http://en.wikipedia.org/wiki/Block_cipher_modes_of_operation

An AES CTR implementation of your code might be:

SecretKeySpec skeySpec = new SecretKeySpec(getCryptoKeyByteArray(length=16)); 
Cipher encryptor = Cipher.getInstance("AES/CTR/NoPadding");

// Initialisation vector:
byte[] iv = new byte[encryptor.getBlockSize()];
SecureRandom.getInstance("SHA1PRNG").nextBytes(iv); // If storing separately
IvParameterSpec ivParameterSpec = new IvParameterSpec(iv);

encryptor.init(Cipher.ENCRYPT_MODE, skeySpec, ivParameterSpec); 
byte[] encrypted = encryptor.doFinal(plain);