Encryption: Use of initialization vector vs key?

Question

I am using PHP\'s mcrypt library and the AES-256 (rijndael) algorithm, which requires both a key + initialization vector to run.

My logical br

Answer 1

Initialization Vector (IV) is not a key at all, and is not secret. In fact, it is often exposed (e.g. prepended to the encrypted data). It is used as an additional random input to the encryption algorithm so that the result of encrypting the same clear data is different each time you use a different IV. This way, statistics cannot be gathered on the encrypted data. It does not "improve" the encryption strength by itself.

You can look here for nice diagrams showing how and why IV is used.