Calculate time taken to break AES key

One of my favourite gems on encryption is from Bruce Schneier in his book Applied Cryptography.

One of the consequences of the second law of thermodynamics is that a certain amount of energy is necessary to represent information. To record a single bit by changing the state of a system requires an amount of energy no less than kT, where T is the absolute temperature of the system and k is the Boltzman constant. (Stick with me; the physics lesson is almost over.)

Given that k = 1.38×10^-16 erg/K, and that the ambient temperature of the universe is 3.2 K, an ideal computer running at 3.2 K would consume 4.4×10^-16 ergs every time it set or cleared a bit. To run a computer any colder than the cosmic background radiation would require extra energy to run a heat pump.

Now, the annual energy output of our Sun is about 1.21×10⁴¹ ergs. This is enough to power about 2.7×10⁵⁶ single bit changes on our ideal computer; enough state changes to put a 187-bit counter through all its values. If we built a Dyson sphere around the sun and captured all its energy for 32 years, without any loss, we could power a computer to count up to 2¹⁹². Of course, it wouldn't have the energy left over to perform any useful calculations with this counter.

But that's just one star, and a measly one at that. A typical supernova releases something like 10⁵¹ ergs. (About a hundred times as much energy would be released in the form of neutrinos, but let them go for now.) If all of this energy could be channeled into a single orgy of computation, a 219-bit counter could be cycled through all of its states.

These numbers have nothing to do with the technology of the devices; they are the maximums that thermodynamics will allow. And they strongly imply that brute-force attacks against 256-bit keys will be infeasible until computers are built from something other than matter and occupy something other than space.

See also https://www.schneier.com/blog/archives/2009/09/the_doghouse_cr.html

Well, using simple math: If checking one key takes 1000 clock cycles, and the computer has 2,000,000,000 cycles per second, it checks 2 million keys per second. The best case is that the first key you try is correct: total time is half a microsecond. The worst case is that the last key you try is correct: you have 2²⁵⁶ keys divided by around 2²¹ checked a second (that's more like 2.1 million, but close enough), which is 2²³⁵ seconds, which according to Wolfram Alpha is around 1.75 vigintillion (that's 1.75*10⁶³) years, or around 1.3*10⁵³ times the age of the universe.

If running your computer for as long as the universe has existed lets you check A keys (A is absurdly huge), and a magical computer that can check A keys per second (this is running a desktop computer for 14 billion years every second) running for as long as the universe has existed would have checked B keys in that time, then if you happen to have a super-duper magical computer that checks B keys per second (keep in mind that this is as fast as a desktop running every second for 14 billion years, per second) and has been running since the Big Bang, you would only be around 68% done with your brute-force.

To put it another way: The Sun will die out in a paltry 5*10⁹ years. In that time, the ratio of the progress you've made to the total amount of work you have to do is within a couple orders of magnitude of the ratio of the mass of one hydrogen atom to the mass of the supermassive black hole at the center of the galaxy. However, Wikipedia lists the heat death of the universe as occurring at earliest in 10¹⁰⁰ years, so you will crack it by then.

The average case is half of the worst case.

This sounds like homework to me, but try WolframAlpha;

Worst case, you have to try all 2^256 possible values it takes 1836000000000000000000000000000000000000000000000000000000000000 years

Best Case, you try one value and it's correct it takes 0.0000005 seconds.