Bit of a noob question and a fair bit of Googling didn't help. I understand that the current encryption standard is AES256. A few questions
1) Is the point of this (and other encryption techniques) to reuse the same private key for multiple messages? If we're using one-time keys, then a simple addition is enough and the complicated AES256 protocol is unnecessary yes?
2) If we are using the key multiple times, how many times is it okay to use it before the message is no longer secure?
3) How do we measure the strength or weakness of such a protocol? I assume the objective of an attacker is to guess the key but how is it quantified, how easily someone can guess the key? And in this context, why is AES thought to be very good (is it that an attacker can do no better than a brute force guessing?)?
Apologies if this is too basic but all the explanations about AES focussed on the details of the protocol, not the objectives.