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How good is security via hashing

Paul Rubin no.email at nospam.invalid
Wed Jun 8 03:40:33 EDT 2011 
Nobody <nobody at nowhere.com> writes:
>>> If you're going to read from /dev/urandom, limit it to a few bytes per
>>> minute, not per request.
>> That's really not going to help you.
> In what way?
> If I need security, I'll use /dev/random or /dev/urandom. If I don't, I'll
> save the real entropy for something which needs it.

I just mean that if /dev/urandom has enough internal state then within
practical bounds, its output is effectively random no matter how much
you read from it.  Did you look at the paper I linked?  "Saving" the
"real entropy" isn't feasible since the maximum capacity of the two
"real" entropy pools is 4096 bits each.  They will both fill pretty
quickly on an active system.  Reading /dev/urandom will empty the
primary pool but /dev/random is fed by the secondary pool, which
receives entropy from both the primary pool and physical sources.  If
you read too fast from /dev/urandom, the worst that happens (if I
understand correctly) is that the rate you can read from /dev/random is
cut in half and it will block more often.  If that's a serious issue for
your application, you should probably rethink your approach and get an
HSM.
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