I\'m implementing an entropy gathering program. For that I collect 2 bits from t

Question

I'm implementing an entropy gathering program.

For that I collect 2 bits from two gpio-pins connected to a random source. I whiten this stream of bits using the von Neumann decorrelation algorithm. Now 2 out of 4 cases I can't use the bits because of that whitening algorithm (e.g. when both bits are 0 or both are 1). The timestamp when I have to discard bits is random.

My question now is: I use the value of the two bits, can I now also use that bit-discard-timestamp as an entropy source? Or are these (the values and the timestamps) correlated? I think they are not neccessarily correlated but I'm not entirely sure.

Explanation / Answer

Assuming the random source is a Bernoulli process such that the von Neumann corrector can make it perfectly random, the bits are, by definition, uncorrelated with anything else, including your timestamps.

In the real world that assumption may not hold, but in that case your existing von Neumann corrected random stream is also faulty.

That said, the timestamps may still not be a good entropy source, even if they are uncorrelated with your random source. There is a widely held belief in the Linux kernel circles, for example, that clock jitter is just a black box PRNG that someone might analyze and break.

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