java - Need solution for wrong IV length in AES

Question

I'm trying to implement AES in Java and this is the code I use:

 byte[] sessionKey = {00000000000000000000000000000000};
 byte[] iv = {00000000000000000000000000000000};
 byte[] plaintext = "6a84867cd77e12ad07ea1be895c53fa3".getBytes();
 Cipher cipher = Cipher.getInstance("AES/CBC/PKCS5Padding");

 cipher.init(Cipher.ENCRYPT_MODE, new SecretKeySpec(sessionKey, "AES"), new IvParameterSpec(iv));
 byte[] ciphertext = cipher.doFinal(plaintext);

 cipher.init(Cipher.DECRYPT_MODE, new SecretKeySpec(sessionKey, "AES"), new IvParameterSpec(iv));
 byte[] deciphertext = cipher.doFinal(ciphertext);

I need this fixed key and IV for test purpose but I get the following exception:

Exception in thread "main"
java.security.InvalidAlgorithmParameterException: 
  Wrong IV length: must be 16 bytes long    at
com.sun.crypto.provider.SunJCE_h.a(DashoA12275)     at
com.sun.crypto.provider.AESCipher.engineInit(DashoA12275)   at
javax.crypto.Cipher.a(DashoA12275)  at
javax.crypto.Cipher.a(DashoA12275)  at
javax.crypto.Cipher.init(DashoA12275)   at
javax.crypto.Cipher.init(DashoA12275)

How can I use this fixed IV with this implementation of AES? Is there any way?

Answer 1

Firstly,

byte[] iv = {00000000000000000000000000000000};

creates a byte array of size 1 and not a byte array of size 32 (if that is your intention).

Secondly, the IV size of AES should be 16 bytes or 128 bits (which is the block size of AES-128). If you use AES-256, the IV size should be 128 bits large, as the AES standard allows for 128 bit block sizes only. The original Rijndael algorithm allowed for other block sizes including the 256 bit long block size.

Thirdly, if you are intending to use a AES-256, this does not come out of the box. You need to download and install the JCE Unlimited Strength Jurisdiction Policy Files (scroll to the bottom of the page); I would also recommend reading the accompanying license.

This would result in the following change to your code:

byte[] iv = {0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0};

Finally, the initialization vector is meant to be unique and unpredictable. A sequence of 16 bytes, with each byte represented by a value of 0, is not a suitable candidate for an IV. If this is production code, consider getting help.