[* Until Linux kernel 3.6 the person maintaining urandom was busily
turning off interrupts as a source of entropy, I think because he didn't
know how much entropy he was getting so better not to get it at all
(huh?). In 3.6 this was changed to use all interrupts as entropy
sources, which is good. This means earlier kernels aren't so
good--though I notice that Ubuntu's kernel has the 3.6 improvement in
their version of 3.2, so individual distributions will vary.]
I'll also observe that on new mobile platforms, there are typically a flotilla of physical-world sensors. The low-level drivers for
these should be contributing entropy to the pool in the kernel. At the apps layer, typically, the "raw" sensor values have been
filtered by application-specific algorithms, so that they're less useful as entropy sources at that level.
For example, low-G accelerometers are quite noisy -- these are typically used as multi-axis rotation sensors (they use the gravity-field orientation to sense rotation).
Any physical-world sensor driver, where the sensor inherently has a bit of noise, I think has a "moral obligation" to contribute bits to the kernel entopy pool.
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