Today I learned that the hardware rng, even though its driver was loaded,
wasn't being used at all other than to create /dev/hwrng which isn't used
by anything. The problem is that the driver neglects to declare a "quality"
parameter that indicates the quality of the entropy it generates, which
causes it to be ignored for the purposes of filling the kernel's entropy
pool.
The fix was easy: add rng_core.default_quality=1024 to the kernel
parameters ("cmdline" variable in /boot/uEnv.txt) and reboot. Voila, you
can now read more than 100 KB/s from /dev/random. (But note that you
shouldn't be reading from /dev/random in the first place, use /dev/urandom
or the getrandom() system call instead.)
If you don't fully trust the hwrng and want to be paranoid, you can set the
quality parameter to a lower value. For example setting it to 100 will
cause ten times as much data to be drawn from the hwrng and hashed together.
Matthijs
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