Encryption with AES-256 and the Initialization Vector

Question

I have a question relating to the use of an Initialization Vector in AES encryption. I am referencing the following articles / posts to build encryption into my program:

[1] Java 256-bit AES Password-Based Encryption
[2] http://gmailassistant.sourceforge.net/src/org/freeshell/zs/common/Encryptor.java.html

I was originally following erickson's solution from the first link but, from what I can tell, PBKDF2WithHmacSHA1 is not supported on my implementation. So, I turned to the second link to get an idea for my own iterative SHA-256 hash creation.

My question comes in how the IV is created. One implementation ([1]) uses methods from the Cypher class to derive the IV where are the other ([2]) uses the second 16 bytes of the hash as the IV. Quite simply, why the difference and which is better from a security standpoint? I am kinda confused to the derivation and use of IVs as well (I understand what they are used for, just not the subtler differences), so any clarification is also very welcome.

I noticed that the second link uses AES-128 rather than AES-256 which would suggest to me that I would have to go up to SHA-512 is I wanted to use this method. This seems like it would be an unfortunate requirement as the user's password would have to be 16 characters longer to ensure a remotely secure hash and this app is destined for a cell phone.

Source is available on request, though it is still incomplete.

Thank you in advance.

Answer 1

The IV should not be generated from the password alone.

The point of the IV that even with the same key and plaintext is re-used, a different ciphertext will be produced. If the IV is deterministically produced from the password only, you'd get the same ciphertext every time. In the cited example, a salt is randomly chosen, so a new key is generated even with the same password.

Just use a random number generator to choose an IV. That's what the cipher is doing internally.

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I want to stress that you have to store either the IV (if you use the first method) or a salt (if you use the second method) together with the ciphertext. You won't have good security if everything is derived from the password; you need some randomness in every message.