From: Stanton McCandlish <mech@eff.org>
Date: Fri, 22 Oct 93 07:23:16 PDT
To: cypherpunks@toad.com
Subject: Clipper chip stuff (fwd)
Message-ID: <199310221419.AA02752@eff.org>
MIME-Version: 1.0
Content-Type: text/plain


Forwarded message:

> Could you forward this to cypherpunks for me?  Walter is a technoid and
> happened to be one of the plaintiffs in Steve Jackson Games (e-mail user on
> Illuminati).  I'm hoping one of the 'punks might be able to comment to him
> on his theory -- it sounds interesting to me, but I'm not technical enough
> to know if its got any merit.  Thanks.
> 
> >Date: Thu, 21 Oct 93 18:05 EDT
> >From: Walter Milliken <milliken@BBN.COM>
> >Subject: Clipper chip stuff
> >To: ssteele@eff.org
> >
> >Hi Shari -- it's been a while....
> >
> >I know you're probably not the right person to talk to about Clipper
> >chip stuff, but I figured you could forward this and tell me who is.
> >
> >Basically, I think I see NSA's backdoor in the whole scheme.  Probably
> >other people have noticed this too, but in case they haven't (and I
> >haven't seen any comment on these lines on comp.org.eff.talk), I figured
> >I'd point it out.
> >
> >The trick isn't necessarily in the algorithm (which I don't know, of
> >course).  I think it's in the key generation process.  I read Dorothy
> >Denning's description as posted to comp.org.eff.talk the other day, and
> >decided it sounded fishy.  Why are the secret per-chip keys generated
> >from the chip serial number (which is observable by anyone with the
> >law-enforcement key)?  To escrow keys, all you need is to 1) associate a chip
> >serial number with a secret key and 2) split the secret key into two,
> >unusuable parts for the escrow agents.  I can't see any reason why the
> >secret key isn't just a random number (or rather the XOR of two random
> >numbers, one for each escrow agent).
> >
> >Instead, we've got this complex algorithm for converting the chip serial
> >number into the chip secret key.  Thus, if you know the chip serial
> >number, the key generating algorithm, and the initialization states for
> >the key generator (provided by the two escrow agents) *you can compute
> >the chip secret key*!  (Or so it appears on a very superficial reading.)
> >
> >I suspect you could dispense with knowing the initialization states, if
> >you were NSA and could obtain any chip from the same batch -- you open
> >the chip up (I'm sure *they* know how to do that), extract its secret
> >key, and reverse-engineer the initialization data.  (This last step is
> >non-trivial, but I'd be surprised if NSA couldn't do it -- it's a
> >variant of known-plaintext attack.)  It's also possible that NSA may
> >*supply* the initialization states, or an algorithm for generating them,
> >or at least advice on how to pick them.  After all, it's their game,
> >they get to make the rules....
> >
> >In any case, I can see no way that using the *observable* chip serial
> >number to help generate the chip secret key can in *any* way improve the
> >security of the system.  You'd be much better off just sticking any old
> >random number into the chip as a secret key, and just noting it down
> >with the associated chip serial number in the escrow files.  Personally,
> >I think I'd use a non-algorithmic mechanism for generating random keys
> >-- perhaps a truly random number source, such as atomic decay processes.
> >There's only one of these things -- it can afford to get fancy.
> >
> >
> >Disclaimer: I'm not a cryptography expert, so it's possible I'm missing
> >something here.  Possibly factoring in the chip serial number makes it
> >harder to crack secret keys if you somehow manage to obtain a few chips
> >from the same batch and open them.  But it certainly seems suspicious to
> >me....
> >
> >---Walter


-- 
-=> mech@eff.org <=-
Stanton McCandlish     Electronic Frontier Foundation Online Activist & SysOp
"A nation that is afraid to let its people judge the truth and falsehood of
ideas in an open market is a nation that is afraid of its people." -JFK
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