https://pthree.org/2016/06/19/the-physics-of-brute-force/

The total energy output of a supernova is enough to count up to 2^220, making a lot of generous (invalid) assumptions. You'd need 2^36 supernovas to just count up to 2^256, never mind actually running SHA256. At minimum.

There are circa 2^38 stars in the Milky Way, and they're not going to all go supernova. Add in the actual cost of compute and it just isn't happening, not in this galaxy.

For 128-bit crypto we can look at a more earthly calculation. Using the same math as that article, but at room temperature, and taking the total solar irradiance on the Earth as an energy source, it would take this long to count up to 128 bits (calculated using Google):

((2^128) * (1.38064852 * ((10^(−16)) (ergs / K))) * (298 K)) / ((1361 (W / (m^2))) * (pi * (radius of Earth^2))) = 8.04911615 seconds

Definitely more on the plausible side, but we're already moving away from 128-bit crypto and there's a staggering number of generous assumptions being made here; we aren't going to be getting thermodynamically ideal computers using a significant fraction of the total solar irradiance on the Earth any time soon.

Just to give you an idea of how far away we are from that, looking at actual SHA256 calculations:

https://www.iea.org/data-and-statistics/charts/efficiency-of...

22222 MH/J is the highest, or 4.5 × 10^-12 joules per hash. That's a factor of 2^30 worse, so if we used the total solar irradiance of the Earth to power Bitcoin miner style ASICs, it would take about 272 years to go through 128 bits' worth of brute forcing something of similar complexity to SHA-256.

The factor is more like 2^36 relative to the first "temperature of outer space" calculation. That happens to be about the number of galaxies in the observable universe, so using current technology, it would take somewhere on the order of all the stars in the observable universe going supernova to power through a single SHA-256 brute force.