Re: The Elevator Problem

At 05:29 PM 12/11/95 -0500, it was written:
>[...]
>> but since the contents of the key were never 
>> touched on DIRECTLY, no eavesdfropper could determine the key, 
>
>No, the integrity of the system really depends upon the quantum properties
>of light. Among other things, Heisenberg's Uncertainty Principle makes it
>impossible for Eve to "eavesdrop" on the photons without affecting them
>measurably.
>
Disregarding the "quantum properties of light" issue, if Eve can listen
in on every communication between Alice and Bob about their keys, then
doesn't she have enough information to reconstruct the key herself?

The way I understand the system, in order for Alice to have Bob's key (and
vice versa) they each have to transmit a considerable amount of data about
their keys.  Even if those data are in the form of "twenty questions," neither
knows anything about the other's key at the start.  Is there something
painfully obvious that I'm missing?

>In this situation Eve can read the email traffic without modifying it, so the
>game is up.
Exactly.

Dave

----- David E. Smith, c/o Southeast Missouri State University
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Dave writes:
> Disregarding the "quantum properties of light" issue, if Eve can listen
> in on every communication between Alice and Bob about their keys, then
> doesn't she have enough information to reconstruct the key herself?

By "disregarding the quantum properties", I assume you are asking about
deterministic cryptosystems. Eve does not have sufficient information if 
Alice and Bob use, say, Diffie-Hellman key exchange (DH). As I said, that's
why DH is so clever. :)  (cf. any good crypto text)  But that's patented
(for now), and we were asked about a patent-free solution.

-Futplex <futplex@pseudonym.com>
"We live in a time when...individual rights are constantly expanded" -Sen. 
Orrin Hatch (R-UT), speaking in support of an anti-flag-desecration 
Constitutional amendment