[dsacco]

I’m not sure if you mean to be disagreeing here or simply adding color, but what you’re saying is the same as the parent comment. Grover’s algorithm allows symmetric key recovery for n bits in 2^(n/2) steps; as the parent commenter said, symmetric algorithm key sizes need to be doubled. A break in 2^64 steps is the same as 64-bit security, so changing the key size to 256-bit will offer 128 bits of security.

[sampo]

Just wanted to clarify that in this case, 64-bit quantum computer is not enough to break said 64-bit security, still need 128 bits of memory.