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.
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
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.