Is a natural noise source such as a reverse biased diode, radioactive decay,the sound generated by turbulent flow etc and that noise sampled and fed into message digest 5 (MD5) or RC4 or some other function which spreads the entropy.We used the current through a reverse biased diode and then processed in that way in order to produce a noise source which passed all statistical tests you could throw at it.In fact part of the generators function was to run the same tests which are in diehard on the resulting noise in order to make sure it was well conditioned.We did try radioactive decay from the alpha source in a smoke detector but desided that the health and safety issues were insurmountable.It wasnt the manufavture that stopped us but working on development in the lab.