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RE: /dev/random is probably not
From: "David Schwartz" <davids () webmaster com>
Date: Thu, 7 Jul 2005 13:46:54 -0700

At the last place at which I worked, a few years ago, a "random
number" was generated, and used in a FIPS 140-1 compliant
encryption device, by capturing 128 ethernet frames in sequence
from the local in-house network, gathering the LSB from the
arrival time of each frame, and using those values to generate
an encryption key. This was part of the "activation sequence"
which had to be done, once, on each such device.

Any studies out there on the randomness of such a number?
At first glance a non-deterministic network would seem to be
able to generate a useful number for the key.

It doesn't look like a good source of entropy. At least it wouldn't
withstand an active attack during this activation phase.

- Bob Foxworth, GSEC, CISSP

        What "active attack" allows an attacker to predict the jitter between the
network card's quartz oscillator and the frequency multiplier that generates
the CPU clock? The low order bit of the TSC at the time a packet is received
is believed to be almost purely a function of this jitter, for typical x86
CPUs at normal temperatures.


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