Newsgroups: sci.crypt Path: cactus.org!ritter From: ritter@cactus.org (Terry Ritter) Subject: Re: IBM-PC random generator, source included Message-ID: <1992Jun25.201031.19945@cactus.org> Organization: Capital Area Central Texas UNIX Society, Austin, Tx References: <2673@accucx.cc.ruu.nl> <1992Jun23.080147.15804@cactus.org> + <1992Jun25.033711.26770@massey.ac.nz> Date: Thu, 25 Jun 1992 20:10:31 GMT In <1992Jun25.033711.26770@massey.ac.nz> T.Drawneek@massey.ac.nz (Ted Drawneek) writes: >The phase noise present even in crystal oscillators is nondeterministic. >Suppose you have two oscillators in phase at the same frequency, each >driving some sort of trigger so that nominally they both trigger at the >same time. Then would they actually both trigger at the same time? >Probably not - you would find that sometimes one would be before the other, >at other times it would be later, if you looked closely enough. The issue follows from "if you looked closely enough." I do not deny that the capability exists in a laboratory to perceive and measure phase noise in crystal oscillators. I simply deny that this could possibly be the effect which is measured by Nico's design. On the other hand, jitter arising from the background refresh process could scarcely be missed, and its effect will be overwhelming. Presumably, if we could predict that effect, we would then be set to measure events of smaller magnitude. I see no attempt to eliminate or compensate for this effect. Actually, if we were to get down to it, it may be possible for a laboratory to measure event-to-event "jitter" differences in digital logic itself. These would arise from thermal and junction noise in what is fundamentally an analog process; transistors are not inherently digital. The question is whether such an effect, in the magnitude in which it could exist, could possibly be important in the results. Perhaps Mr. Drawneek would care to speculate on the magnitude of crystal oscillator nondeterministic phase shifts, and on the rate of such occurrences. And then on the possibility of measuring such shifts by any method on an ordinary PC (after the normal digital division). And then on the ability to draw 128 bits per second of random information from such a source. A 20 Mhz crystal has a period of 50 nsec. A good AT-cut would vary by +/- 0.005% (50 parts per million) over full temperature range (-55 deg C to +105 deg C). If we use this difference as an indication of the magnitude of possible phase jitter, we could have jitter on the order of 0.0025 nsec. Measure *that* on a PC with software and timers at 128 bits per second. The quartz resonator itself is a physical material and in operation the surface pulls about 1,000,000 G's. This is stored resonant energy, and a typical crystal will have a Q in excess of 100,000. In steady-state operation the circuit supplies only replacement energy, so we have a mechanical integration of about 100,000 cycles worth of supplied energy stored in the rock. (Because of this, it can take a 32 kHz clock crystal up to a second to reach steady state.) The crystal might wiggle a little, but my guess is that a good proportion of any measured "phase noise" is the simple effect of thermal noise at the node which converts the beautiful sine wave into harsh digital form. Of course, the source of the randomness wouldn't matter at all, if we could just measure it. I am open to the possibility. --- Terry Ritter ritter@cactus.org