> On top of that, I don't remember the specific assumptions but my Crypto professor proved that just to increment a counter to 2^256 would take more energy than

> On top of that, I don't remember the specific assumptions but my Crypto professor proved that just to increment a counter to 2^256 would take more energy than exists in the observable universe. This proof is present in Schneier's "Applied Cryptography" if someone wants source.