From: "Timothy C. May" <tcmay@got.net>
Date: Mon, 2 Dec 1996 23:49:42 -0800 (PST)
To: cypherpunks@toad.com
Subject: Modulating the FM noise spectrum considered infeasible
In-Reply-To: <849204012.65013.0@fatmans.demon.co.uk>
Message-ID: <v03007802aec98b7eae7e@[207.167.93.63]>
MIME-Version: 1.0
Content-Type: text/plain


At 12:30 PM +0100 11/29/96, Pavel Korensky wrote:
>paul@fatmans.demon.co.uk wrote:
>>
>> I`m not entirely sure about the possibility of correlations in any of
>> these sources but they aren`t really a good idea because all can be
>> accessed by someone else. eg. Steal your random noise audio tape,
>> sample dolby decoded sound at the same time as you etc.
>
>And what if I will use FM receiver, tuned on some channel where is the noise.
>If the potentional attacker don't know the what channel was tuned, he is not
>able to reproduce this IMHO. Of course, maybe it is possible to record the
>whole
>spectrum to tapes, but I think that the noise will change when recorded and
>played back from tape.

Not to mention the point that an external attacker--say, the NSA van parked
across the street--will under no circumstances be able to measure "the"
spectrum: his antennas cannot possibly measure the signals (at the lower
bits) seen by the FM receiver, noise source local to the computer, whatever.

(Obviously other SIGINT/TEMPEST methods might be able to read the bits in
other ways, but this is a different issue. Ditto for black bag
cryptanalysis, which is usually the most cost-effective approach.)

Likewise, feeding the FM antennas with "special hiss" is implausible in the
extreme. Again, the attackers don't know antenna responses, atmospheric and
room geometry variations, etc. Even if the FM hiss were to be somewhat
biased, the LSBs would have a fair amount of entropy....collecting and
distilling this entropy would still be trivial.

Yes, this is all obvious stuff. But it keeps coming up. Not even the NSA
has the super powers to modulate the FM noise spectrum in a cost-effective
way.

Having said this, Johnson noise makes a superior noise source, if a
physical source is desired.

--Tim May

Just say "No" to "Big Brother Inside"
We got computers, we're tapping phone lines, I know that that ain't allowed.
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