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

Do not use hashed password as a single source for key and IV. As a rule of thumb, you should generate random IV EVERY TIME you update encrypted data and store IV with this data. Key can be reused multiple times, but use salted hashing and store salt with data too.

If you just hash user passwords and use it as encryption keys, users with same passwords will have same keys. Depending on your database structure and intruder access rights there could be some unfortunate cases when users with same passwords can be detected. Add at least unique username to this hash.

If you do not change IV for every data update, information about data changes can be leaked. With CBC or CFB mode identical first plaintext blocks will be encrypted to identical ciphertext until first plaintext change, so position of this change can be determined.