The IV isn't secret, but in CBC mode it does need to be securely random, otherwise you open yourself up to a class of attacks based on IV prediction. In other m

The IV isn't secret, but in CBC mode it does need to be securely random, otherwise you open yourself up to a class of attacks based on IV prediction. In other modes (CTR, for instance) you can use any nonce you like, so long as you're not reusing it.