Comment by dist-epoch 17 hours ago
> My layman's understanding is that 128 bit AES is known to be broken by a quantum computer
Weakened, not broken. Quantum computers turns 128 bit AES into 64 bit equivalent. Which will still be extremely difficult for quantum computers due to the large computer size/number of steps required.
Well, you get sqrt(n / N) as a result. It works like any other parallel computation.
E.g. if you have 256 quantum computers, then each one of them needs to search only 60 bits of the key space to crack a 128-bit key (each one of them will only need to search 2^120 keys).
It's not really going to make much difference with near-future quantum computers. Especially since Grover's algorithm _has_ to complete all the 2^60 steps to produce a reliable result, you can't just run a quantum computer for a while, stop it, and then restart it.
And it's 64-bit equivalent in a way that's inherently impossible to parallelize, so 2^64 sequential quantum operations. Those operations are much, much slower than classical ones.