As many of these comments show, it's easy to come up with creative and probably effective sources of entropy.
The problem is knowing when you've collected enough to (re)seed your random bit generator.
Entropy sources usually have failure modes that result in predictable data in the output. The entropy source has to work even when someone is using an arc welder nearby or puts popcorn in the microwave.
Assuming kernel memory stays secret, collecting entropy should only be a problem during boot.
But distro maintainers aggressively optimize boot times, so there's a little time to spend collecting entropy at boot. Systems usually save a bit of entropy on the hard drive across boots, but that has its own issues. Unfortunately, first boot is when the SSH keys are generated, so that's kind of the worst-case scenario.
The problem is knowing when you've collected enough to (re)seed your random bit generator.
Entropy sources usually have failure modes that result in predictable data in the output. The entropy source has to work even when someone is using an arc welder nearby or puts popcorn in the microwave.
Assuming kernel memory stays secret, collecting entropy should only be a problem during boot.
But distro maintainers aggressively optimize boot times, so there's a little time to spend collecting entropy at boot. Systems usually save a bit of entropy on the hard drive across boots, but that has its own issues. Unfortunately, first boot is when the SSH keys are generated, so that's kind of the worst-case scenario.