From: Derek Upham <upham@cs.ubc.ca>
Date: Sun, 10 Apr 94 18:16:13 PDT
To: Cypherpunks mailing list <cypherpunks@toad.com>
Subject: Re: Zero Knowledge, Hamiltonian Cycles, and Passwords
Message-ID: <199404110115.AA23628@grolsch.cs.ubc.ca>
MIME-Version: 1.0
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> A potential use for such systems is for passwords--one can prove one
> has the knowledge without actually producing it (by typing in a
> password, for example). I don't know that anyone is actually
> exploring this application, yet, but I expect it'll come.

Look at "Strongbox: A System for Self-Securing Programs" by J. D.
Tygar and B. S. Yee in the "CMU Computer Science 25th Anniversary
Commemorative" proceedings (from 1991).  As the paper describes:

    ``Strongbox uses an authentication protocol derived from Rabin's
    observation about the square root operation: if one can extract
    square roots modulo  n  where  n=p*q ,  p  and  q  primes, then
    one can factor  n .  [That should be `if and only if', i.e.,
    finding the square roots is too hard unless you created  n  in the
    first place.]  Both our protocol and FFS are *zero-knowledge
    authentication protocols_*  [. . .]  And in contrast to Needham
    and Schroeder's authentication protocol, zero-knowledge
    authentication protocols require no central authentication server
    and thus there is no single point of failure that would cripple
    the entire system.''

In addition to zero-knowledge authentication, the paper provides an
algorithm for the secure exchange of sessional symmetric encryption
keys, and ways of combining authentication and key-exchange steps.

I managed to get the key-exchange working some months back (in C++,
using GMP to handle the number-crunching), but it was hampered by my
incredibly slow 386 on one side and odd bugs in the Sun4 environment
on the other.  Contact me if you want to hack around on it.  I also
know where to find unreleased GMP 1.9 sources for some additional,
probably more reliable, functions for calculating the Legendre symbol
(which the whole system depends upon).

Derek