Create an initialization vector without a good source of randomness

Question

To store passwords (written in C) for Rockbox I need to generate initialization vectors.

The problem is that I do not have a good source of randomness. Rockbox supplied random () is not a cryptographic RNG. And I have almost no sources of randomness with which I can access (no mouse movements, ... on the iPod running Rockbox).

The key is currently output via PBKDF2 from the user-provided password and salt (which is a constant prefix + some data from random ()). I think pseudo-random data should be good enough for a salt with 10,000 iterations of PBKDF2.

However, where to get my initialization vector? Is it okay if I take some semi-random data (time + random ()) and SHA, which, say, 10,000 times? Should I take arc4random with a seed taken from random ()?

Do I even need an IV if I will never use the same key twice (is the salt recounted every time the stored data changes)? What is the best way to deal with such situations?

Edit: Just one user (I who owns an iPod) encryption algorithm: AES-CBC 256 bit. The file simply stores a list of sites / accounts / passwords for various sites. It rarely changes (when I create a new account on the website), when this happens, a new salt and a new IV are created.

Answer 1

, CBC IV MUST . " " . , CBC , , -. , , , IV . 128- , AES, , , , - , IV 128- , IV (, IV, ) , , .

: , IV. "".

" IV" CBC. , IV. , EAX GCM, , IV PRNG, ; IV (, " " ) -, . PRNG! , .

Answer 2

IV , (, IV CBC).

, () .

Answer 3

GREAT NEWS! The initialization vector does not have to be random, it just has to be different for each encryption. This way you can use the username as salt. If you use the username and time, the attacker will not be able to detect password reuse.