you reverse bias the diode so the current is mostly random shot noise caused by electron- hole recombination and then you collect a large amount of the random input from the diode and put it through a hash function called md5,then you put it through several statistical tests to see what the distribution looks like and if it looks ok youve got some random noise.
We did try using the alpha particle source from smoke detectors but the health and safety people went nuts,blinking nanny state.
the radiation levels are only just over background.