that's unrelated - AES-256 for example can be executed just fine on either a 32- or 64-bit machine. in theory there's nothing stopping you from running it on an 8-bit or 16-bit CPU (although other considerations related to the size of AES's lookup tables make this unlikely). from some random googling, here is an implementation of Chacha20, another 256-bit encryption algorithm, for 8-bit microcontrollers.

I started out programming a 6502a in 1980, 680X0 a little later in 87, so I get that bit, but it's easier doing operations on a larger register. I remember writing code for 8 bit multiplication of floating points.

I enjoyed and understood the rest of your prose though. Didn't do much/any programming/low level after say 2005, and regret it now. Trying to re-learn but things have moved on so much.

I take that there isn't much motivation in moving to 128 because it's big enough; it's only 8 cycles (?) to fill a 512 (that can't be right?).