Digitally Signing Physical Objects

[tcmay@netcom.com (Timothy C. May)]

[In this post I'll explore an interesting use of P-K methods to allow
physical objects to be digitally signed.]


Alan (wex@media.mit.edu) forwards to us a clipping and bets that no
encryption is used:

> [The following was snipped from EDUPAGE, the online summary service.  Anyone
> have access to more information?  --AW]
> 
> 
> AND OPTICAL FINGERPRINTS. The National Registry uses an optical scanning
> technology to create and compare digital maps of the finger surface. The
> map can be converted to a modified bar code for inclusion on a wallet ID
> card. The information can also be encoded on circuitry inside a credit or
> debit card. (Tampa Tribune 2/20/94 B&F1)

I'm not familiar with this, but I'll bet public key crypto is used. Or
it certainly _could_ be used. How it could be used is an interesting
example of using public key methods for authentication.

So, even if these National Registry folks are not using P-K crypto,
here's an example of how this could work:

Imagine that one has a object--a fingerprint, a photograph, a dollar
bill, a factory-made automobile part--which one wants to "sign," to
protect against forgery. (The application to manufactured goods is
obvious: lower-quality parts are often made by forgers and sold as
factory parts. The counterfeiting application is also obvious. Ditto
for the photo, even with digital scanning....for reasons that will
become clearer. The application to fingerprints I'll leave for you to
think about.)

Call this thing "the original object." It will have variations in
surface appearance (or deeper, in principle, but I'll just stick to
surface features). For example:

- dollar bills: variations in paper fibers, in flecks of particles, in
surface roughness, etc.

- auto parts: scratches and tool marks on smooth surfaces, metal
grains, etc.

- fingerprints: whorls

- photos: grains will vary from photo to photo

How can one "sign" these objects in an unforgeable way?

1. Scan some part of the object, e.g., a linescan between two
reference points. An intensity variation of reflected light, for
example, will produce a vector of intensity variations.

(What resolution, how many points taken, the location of reference
marks, etc, are all details of vendor implementation.)

2. Take this feature vector and encrypt it to the *private* key of the
factory or other authenticating agent. The resulting number is stamped
on the object.

(In the case of "Light Signatures," an L.A.-based company which was
proposing this scheme several years ago, the manufacturer of parts
would stamp the resulting number on the finished part--and perhaps
include it with the paperwork for the part. Harley-Davidson was
supposedly considering the use of this, as they were having big
problems with counterfeit replacement parts. Jim Omura, Presidende of
Cylink, a Public Key Partner, told me this in 1988. I haven't heard
any more about "Light Signatures.")

3. The shop or customer wishing to authenticate the part takes the
number stamped on the part, runs it through the *public* key of the
manufacturer (widely available, not kept secret, of course) and gets
back the feature vector, which he can then compare to what he actually
sees on the object.

(This clearly requires similar hardware to what was originally used by
the manufacturer. And some tolerance for variations in intensity
caused by equipment variations, wear, new scratches, etc., is needed.
Not a really big problem, fortunately. You can fill in the details of
what would be needed for fingerprints, for phots, for lottery tickets,
for currency, etc.)

4. A would-be forger cannot generate a "digital object signature" that
correctly decrypts through the published public key. 

Thus, the manufacturer or authenticator (whoever knows the private key
corresponding to the public key) can "sign" his work and no one else
can.

This has obvious applications for authenticating paintings, original
photographs (physical photos, not digital ones!), money, objects, etc.
Anything in which natural variations can be converted into a
very-hard-to-duplicate feature vector.

This issue is also related to "is-a-person" credentialling in various
ways. One might imagine Big Brother issuing ID cards in which
fingerprints, retinal scans, facial features, etc., are encrypted with
a private key. Any local cop or "checkpoint" (border, random stops,
whatever) could then do the authentication locally.

As with other uses of public key encryption--in contrast to symmetric
ciphers, as we discussed earlier today--the private key is held safely
and securely (presumably) and the field users don't run the risk of
compromising security.

I've wondered why this technology has not appeared in the six years
since I first heard about it. Seems like a wonderful market niche.

And I wonder how this fits in with Clipper and Capstone. After all, if
the government holds escrowed digital signature keys as well, they
could forge these items as well. Maybe that's what they want.

--Tim May


-- 
..........................................................................
Timothy C. May         | Crypto Anarchy: encryption, digital money,  
tcmay@netcom.com       | anonymous networks, digital pseudonyms, zero
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W.A.S.T.E.: Aptos, CA  | black markets, collapse of governments.
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"National borders are just speed bumps on the information superhighway."