Fixing getrandom()

I think you can get a decent amount of entropy even if there is no light on the camera, because of thermal noise in the sensor. That's probably safer than receiving significant light, because if the sensor gets saturated then you'll get a pure white image with no entropy. Phone cameras don't have physical shutters but the sensors often have some non-exposed pixels around the edge (for black level calibration etc) and you could probably use those.

> you need a way to get the raw data (transformation into JPEG usually tries to get rid of the noise that we want for the RNG)

It's not just the JPEG compression - the Android camera API is happy to give you uncompressed YUV but that still wouldn't be raw enough. You'd want the (typically) 10-bit Bayer data directly from the sensor, before the ISP has tried to make it look pretty (doing noise reduction, adjusting levels in a way that might saturate the noise out of existence, smoothing the image, etc). And you probably want to manually configure the sensor to maximise noise (long exposure, high gain, disable binning, etc). Android provides enough control to let applications request that, but I don't know how many of the camera drivers implement it fully, so it's probably not a very portable approach.