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Topic: Need a measure of goodness of mixing two n-bit computer words
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Mok-Kong Shen

Posts: 629
Registered: 12/8/04
Need a measure of goodness of mixing two n-bit computer words
Posted: Mar 20, 2014 7:28 PM
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Given two n-bit computer words, each containing bits that are not
perfectly random (i.e. the entropy is less than n bits), I like to find
experimentally some ways of processing to combine them into one single
n-bit word that is better in the sense of having higher entropy. What
could be a reasonable and practically not difficult to compute good
measure of the goodness of a procedure of mixing such pairs of words?

Perhaps I may say what I myself have thought up till now about the
issue: Let z = f(x,y) mod 2**n be the result of mixing x and y. Let
a bit of x be flipped to become xr, compute zr = f(xr,y) and then
c = z xor zr. bc = bitcount(c) gives then the "avalanche" for that bit,
i.e. the number of bits of z that get changed as consequence of the
bit flipping of x. Thus a good f() should provide a good (large, near
n/2) mean value of the mean value (over all n bits of x that get
flipped) of bc as well as a good (small) standard deviation. Could
this thinking of mine be correct?

Thanks in advance.

M. K. Shen

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