Re: generating keys by using only part of longer random number

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Kim Schulz wrote:
On Jun 10, 11:59 am, David Eather <eat...@xxxxxxxxxx> wrote:
Kristian Gjøsteen wrote:
Kim Schulz <kimu...@xxxxxxxxx> wrote:
If I have a random number generator that e.g. generates 512bit long
keys but I only need 160bits of them. Would the quality of the
randomness then be compromised if I always just uses the first 160bits
of the key?
If the generator samples 512 bit strings from the uniform distribution
(or something that looks like the uniform distribution), then it is
ok. Otherwise, it depends on the exact distribution.
The OP might want to look for BBS stream generatorhttp://en.wikipedia.org/wiki/Blum_Blum_Shub
Which may be useful for the job

Thanks for the input guys! appreciate it. The BBS seems to be
something I can actually use for another project I have so thanks for
that input too.
You can't lose any more entropy than the data you remove. If you have
512 bits of high-entropy random data, for example, then sample anything
from it and have your sample be high-entropy too. The only danger is if
your prng gives 512 bits of input and some ranges are more random than
others.

In general, many prngs will not output very high entropy at first, so
most applications discard the first several bits of data. I would
actually recommend using the last 160 bits instead of the first 160
bits, although whether that actually matters depends on the algorithm.

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