[thesz]

I think, you meant "rand() + rand() mod 2".

This is used to get more randomness from not-so-random source.

Just an example. If our rand() returns 0 2/3 of time and 1 1/3 of time, our rand() + rand() mod 2 will return:

0 + 0 mod 2 = 0 2/32/3 = 4/9 times, 1 + 0 mod 2 = 1 1/32/3 = 2/9 times, 0 + 1 mod 2 = 1 2/31/3 = 2/9 times and 1 + 1 mod 2 = 0 1/31/3 = 1/9 times.

So we will get 0 5/9 times and 1 4/9 times. Now we get random source that is much more uniform.

I think it is possible to obtain random bits even from source with correlated samples.