building a true RNG

[Matt Crawford]

    2) I can't prove that a standard hash function such as SHA1
    generates all possible codes, but I consider it likely.  It would 
    be quite shocking if a strong hash function such as SHA1 generated
    fewer codes than a weak function such as H0.

I think you could do a probabilistic proof similar to the "DES is not
a group" quasi-proof.  To test a hash function h() whose range is S,
let F be the set of "balanced" functions from S -> {0, 1}.  (Balanced
meaning that each f in F maps exactly half of S to 0 and half to 1.)
If you can contrive to choose many members of F at random, and compose
them with h for many arguments of h, you should be able to set
confidence limits on how much of S is covered by h.


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