Make a unique filesystem path, without creating the file

Paul Rubin no.email at nospam.invalid
Tue Feb 23 02:18:21 EST 2016


Marko Rauhamaa <marko at pacujo.net> writes:
> It is also correct that /dev/urandom depletes the entropy pool as
> effectively as /dev/random. 

I think see what's confusing you: the above is a misconception that is
probably held by lots of people.  Entropy is not water and from a
cryptographic standpoint there is essentially no such thing as
"depleting" an entropy pool.  There is either enough entropy (say 256
bits or more) in the PRNG or else there isn't.  If there's not enough,
urandom can misbehave by giving you bad output because it doesn't block
until more is gathered.  If there is enough, /dev/random misbehaves by
blocking under this bogus concept of "depletion".  If you have a seed
with 256 bits of entropy and you generate a gigabyte of random numbers
from it, you have not increased the predictability of the seed in any
significant way.

So once /dev/random unblocks, it should never again block, the behavior
of getrandom.  There used to be an article on David Wagner's web site
(cs.berkeley.edu/~daw) about the concept of "depleting" entropy by
iterated hashing, but I can't find it now.  That's unfortunate since it
might help cast light on the subject.

>> http://www.2uo.de/myths-about-urandom/
> Already addressed.

No really, all you've done is repeat bad advice.  The people cited in
that article are very knowledgeable and the stuff they say makes good
mathematical sense.  The stuff you say makes no sense and you haven't
given any convincing reason for anyone to listen to you.


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