I've looked at NIST documents and done many Google searches, but I can't find an answer to this seemingly simple question.
I am storing data on harddrives, encrypted with AES-XTS 256 bit. Some of this data has a long lifetime, i.e. will still be valuable to an attacker and not good to lose into the future.
My scenario is that a harddrive could be lost today, and while the AES encryption is unbreakable today, how about in 20 years? 30 years? An attacker could keep it or find it in the future and use an not-yet-known attack to break it. Of course this is guesswork, my question is if there is an "official guess" from the NIST or someone else that I can quote and that says "we consider AES-256 secure until at least 2030" or some such.