You also need to understand whether it is safe to reuse the keys and under what conditions that is safe. Information may leak if there are repeated patterns. T

You also need to understand whether it is safe to reuse the keys and under what conditions that is safe. Information may leak if there are repeated patterns. Things can also be problematic if there are conditions under which you will encrypt data provided by you adversary as they could trick you into decoding the message for them. Finally opponents may be able to tamper with the message even if they can't decode it unless there is a MAC to protect it.