[ILUG] gpg question...

[Tiarnan O Corrain]

On Wed, May 23, 2001 at 09:02:34AM +0100, kevin lyda wrote:
> On Wed, May 23, 2001 at 12:58:06AM +0100, Niall O Broin wrote:
> > What do you mean exactly ? Is this a bit like the captain and the XO on the
> > submarine with the keys to the nuclear missile launcher i.e. do two people
> > have to agree to decrypt ? If so, who gets to get the plain text ?
> 
> the two people sitting at the terminal.  the files in question contain
> valuable data, and it would seem wise to me to offer some security in
> handling them.  not just for the people you purchased the files, but
> for those that handle them on their behalf.

Don't know if you can do it securely with GPG, but the protocol is
called secret sharing. There's plenty of information on it in _Applied
Cryptography_, as well as numerous bibliographical references.

--- begin excerpt ---

...if you want to create a (3,n)-threshold scheme (three shadows are
necessary to reconstruct M), generate a quadratic polynomial
        (ax^2 + bx + M)mod p
where p is a random prime largerthan any of the coefficients. The
coefficients a and b are chosen randomly; they are kept secret and
discarded after the shadows are handed out. M is the message. The prime
must be made public.
        The shadows are obtained by evaluating the polynomial at n
 different points:
        k_i = F(x_i)
In other words, the first shadow could be the polynomial evaluated at
x=1, the second shadow could be the polynomial evaluated at x = 2, and
so forth.
        Since the quadratic polynomial has three unknown coefficients,
a, b, and M, any three shadows can be used to create three equations.
Two shadows cannot. One shadow cannot. Four or five shadows are
redundant.

--- end excerpt --
 From Applied Cryptography, p528ff

Seems like gpg is excessive for the problem.
--
Tiarnan O Corrain 
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Gold is shiny.  Shiny stuff is mad. Being mad is cool.  You
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