There also seem to be some fundamental limits on computation in physical systems. The Landauer limit is a famous one. Even with quantum computers, you quickly s

There also seem to be some fundamental limits on computation in physical systems. The Landauer limit is a famous one. Even with quantum computers, you quickly start needing ridiculous amounts of energy, on the order of "build a dyson swarm". Any symmetric system with a 512-bit key will be secure against solar-system sized quantum computers for many human lifetimes.