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After becoming frustrated with the lack of a standalone, portable,
decent random number generator, I decided to make one based on a
cryptographic one-way hash function. I chose MD5 since it is fast
and free source was readily available. More cryptographically
secure hash functions are available (e.g. SHA-1), but for the
purposes of a rand/random/erand48 replacement, MD5 should be more
than sufficient.
MD5 takes an arbitrary amount of input and yields a 16 byte hash.
This RNG continually MD5's a 16 byte digest, and uses the bottom N
bits as the random number yielded, where N is just large enough to
include the largest random number desired.
To yield a random number between 0 and r:
create mask which has enough bits to include all of r
(for example, if r is 100, mask would be 0x7F)
do {
digest = MD5(digest)
number = digest & mask
} while (number > r)
The digest should be loaded and saved to a disk file between
invocations of a program using the RNG.
Random functions appear after the included MD5 code.
Send comments to: skrenta@pbm.com (Rich Skrenta)
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The Amiga 68k/PPC version comes with both, makefiles for 68k and PPC,
and test programs for both CPUs.
A few changes had to be made for the Amiga port (#ifdef AMIGA) - and
since this random number generator has a "brain", the most important
change perhaps was, that the location of this brain now is "s:randseed".
In case you want to run multiple copies of it, you'll perhaps like
to choose a different (process-dependent) name. You also could fetch
the "digest" from somewhere else (i.e. using a conventional random
number generator).
--
ARK, 30/May/2000
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