- #1

Adrian B

- 21

- 5

While attempting to learn something about cryptography, I have repeatedly encountered a commonly quoted argument about the minimum energy required to cycle a 256 bit counter through all its states. It says that the absolute minimum energy required to change the state of a bit is kT and that if you multiply that by the number of bit toggles required to cycle through all the values of a 256 bit counter you'd need more energy than you could obtain by building a Dyson sphere around a supernova. See

**Schneier on Security**for the argument as he originally stated it.

Here's what I'm wondering: If I build a counter such that it requires energy "E" to change the state from 0 to 1, can't I design the counter such that I

__recover__energy up to E when the state returns from 1 to 0? If so, wouldn't that mean that the maximum energy invested in an n-bit counter would have a lower bound of nE, with the greatest energy investment occurring when the counter reaches its final state with all bits equal to 1? If minimum E is kT, then the minimum investment would be nkT, no? For a 256-bit counter, that's a lot less than the energy expended by a supernova.

Thanks in advance for any comments.