Limiting the energy source to a single star is weak. If you use a whole galaxy (or several) you can easily bring that up to 256 bits. For example, the Milky Wa

Limiting the energy source to a single star is weak. If you use a whole galaxy (or several) you can easily bring that up to 256 bits. For example, the Milky Way has on the order of 10^11 stars; most of those aren't supernova-sized, but if we use a few million galaxies we can get that many. That gives you an extra 37 bits, bringing the counter to 256. So, if you have terrifically huge enemies, who will burn galaxies to get at your secrets, you might want to use a few more bits. You might worry about quantum computers, too, since afaik Grover's algorithm halves effective key size.