[contravariant]

This is probably a decent estimate, but there's a couple of routes of attack it fails to account for.

First it uses the current average temperature of the universe. Lowering the temperature can be done by just waiting a while before turning the machine on. I assume that powering a sufficiently powerful fridge is not an option, given the origin of the theoretical limit, but I can't quite point out why it wouldn't work.

Secondly it assumes that an unsuccessful attempt must flip at least some bits in an semi-permanent manner. This is obviously true of all current computers, but doesn't have to be true for all possible apparatuses. A specialized hyper-efficient password cracking system should be expected to get below this limit. Will we ever build one? Who knows.

Arguably this latter 'loop-hole' is just pointing out that quantum computers or more efficient algorithms could do better, so maybe we should absorb it into the definition of 'brute-force'.

[skykooler]

If you wait for the temperature to drop, and the universe is expanding, distant galaxies will recede to the point that they are now moving away faster than the speed of light and their matter is no longer available to contribute.

[Jerrrrrrry]

Intersection of light cones, the expansion of the universe, and the Margolus–Levitin limit of computation is truly the ultimate secret.

5 picoseconds to respond, then you are disconnected from the system by the laws of physics.

No amount of frame-jacking via time dilation using black holes can cheat that captcha, or brute force that keyspace.

By the time you find the key, we are no longer connected in space-time/causality

[mattigames]

Thank it is expanding now really means that it will expand forever? Are there not other physics at play that could stop that at some point, yeah I don't know much about any of this just wondering out loud.

[sushibowl]

This is generally unknown, of course. However it currently appears that the expansion of the universe is accelerating, i.e. it is expanding faster and faster.

Obviously it can't be ruled at that at some point this would stop and/or reverse. But there's no reason to think so, and if we're considering arbitrary future changes then we may as well consider that the universe might suddenly start heating up again in the future, or more mass will start appearing out of nowhere. Or god appears and hands out free decryption keys to everyone.

[contravariant]

Nothing says you have to wait with gathering, just wait a bit for the CPU to cool down before you start computing.

[skykooler]

Gathering matter into one place heats it up, and prevents it from cooling (more or less - the outside can cool but thanks to the square-cube law that's basically negligible on universe-sized masses.)