----- Original Message ----- 
From: "Andy Isaacson" <[EMAIL PROTECTED]>
To: "Florian Weimer" <[EMAIL PROTECTED]>
Cc: <cryptography@metzdowd.com>
Sent: Saturday, December 25, 2004 4:56 AM
Subject: Re: SSL/TLS passive sniffing

> On Wed, Dec 22, 2004 at 07:43:13PM +0100, Florian Weimer wrote:
> > > Actually reasoning along these lines is why Lutz Jaenicke
> > > implemented PRNGD, it is strongly recommended (at least by me)
> > > that mail servers use PRNGD or similar.  PRNGD delivers
> > > psuedo-random numbers mixing in real entropy periodically.
> That's basically what /dev/urandom does, no?  (Except that it has the
> undesirable side-effect of depleting the entropy estimate maintained
> inside the kernel.)

This "entropy depletion" issue keeps coming up every now and then, but I
still don't understand how it is supposed to happen. If the PRNG uses a
really non-invertible algorithm (or one invertible only with intractable
complexity), its output gives no insight whatsoever on its internal state.
As entropy is a measure of the information we don't have about the
internal state of a system, it seems to me that in a good PRNGD its value
cannot be reduced just by extracting output bits. If there is an entropy
estimator based on the number of bits extracted, that estimator must be


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