If this is used as input to a randomness pool and filtered through a hash function, ideally together with mouse, keyboard and network event timing as in the linux random device, this should be very hard to break, even with some biases. I would not have used the output of such a device directly, even if the output appeared random on statistical tests, just becuase the biases you mentioned. There will still usually be some randomness in the output, even when it is biased.
Atmel has an MC you can get for about 4 bucks that does AES encryption onboard, and if you pushed the data through that, the harmonics wouldn't matter.
Re:USB Random Number Generator
by tlarkworthy on Monday September 30, 2013 @11:47AM
(#6470196)
My USB microphone agrees
did one of these in 1993, working with pxn when we were both at Fringeware. It turned out to be nearly impossible to eliminate a 60Hz signal without going to pure (D cell) battery power, and a metal chassis.
I'd expect there to be all sorts of harmonics on a USB device.