I read a reference someone posted somewhere once (sorry to not be more specific) that sounded pretty convincing. Apparently it takes a minimum amount of energy

I read a reference someone posted somewhere once (sorry to not be more specific) that sounded pretty convincing. Apparently it takes a minimum amount of energy to flip one bit in a conventional (non quantum) computer. In order to brute force a 128 bit key, you'd need some sort of 128 bit register. Multiply the amount of energy needed per bit flip and the result implies there are never likley to be enough power plants on Earth to iterate through all the combinations (let alone perform the computations required to test the candidate key). Hope this helps, perhaps this is enough for you to find the original reference. Edit: perhaps it was http://en.wikipedia.org/wiki/Landauer%27s_principle http://en.wikipedia.org/wiki/Landauer%27s_principle as mentioned below.