From: Bill Sommerfeld <sommerfeld@orchard.medford.ma.us>
Date: Wed, 20 Sep 95 05:25:33 PDT
To: jsw@neon.netscape.com (Jeff Weinstein)
Subject: Re: My Day
In-Reply-To: <43oquc$70f@tera.mcom.com>
Message-ID: <199509201218.MAA00433@orchard.medford.ma.us>
MIME-Version: 1.0
Content-Type: text/plain


A couple comments on using the time as a seed:

Any system running NTP will let you know its clock to within a couple
ms; some folks have gotten NTP accuracy down to the high hundred
microseconds on real-time systems..

Any entropy you get from sampling the system clock will have to come
from the low-order bits of the tv_usec, or equivalent, and you'll only
get a few bits per sample.

Getting real entropy from mouse movements under X may be tricky,
because the X server goes out of its way to compress mouse movement
reporting and to buffer events sent to the client ("X is an exercise
in avoiding system calls").  You'll probably get less entropy than you
might think.

>   the second 32bit seed is the "tick count", which I'm told is the number of
> milliseconds since windows started.

A 32-bit ms-resolution counter wraps roughly every 50 days.  Very few
Windoze PC's stay up that long :-).

In a long-term active attack, the tick count can be estimated by
periodically pinging the system under attack, noticing when it goes
off the air and then back on again, and using that as a base value for
the tick count search, so the tick count probably only adds a factor
of somewhat less than 2**10 to the keyspace, not 2**32..

					- Bill