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> You can never straight up brute force a full-strength 512-bit key. That's just a fact of the universe.

If there is some number of bits n < 512 where brute forcing an n-bit key is not a "fact of the universe", does it stand that cracking 512 bit keys is also not a fact of the universe?

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No? That's like saying "if my glass can fit a drop of water, then it can also fit an ocean". There is an upper bound to how much processing power there can be in the universe, and 512 bit keys need more than that to be cracked.

That's assuming classical computers though, right? What about future quantum computing developments?

I guess it depends on the algorithm they chose, HChaCha20 is the symmetric encryption algorithm for libsodium.js which is quantum resistant.

https://doc.libsodium.org/secret-key_cryptography/secretstre...

https://crypto.stackexchange.com/questions/79518/is-xchacha2...

Most if not all symmetric encryption is quantum resistant though (with a large enough key). Quantum computing only causes problems for asymmetric algorithms that involve factorisation iirc

The key words here are "brute force" -- there might be some [possibly quantum] techniques around it, but probabilities around guessing and checking an arbitrary number remain constant.

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